4653 lines
190 KiB
Plaintext
4653 lines
190 KiB
Plaintext
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libpng-manual.txt - A description on how to use and modify libpng
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libpng version 1.5.10 - March 29, 2012
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Updated and distributed by Glenn Randers-Pehrson
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<glennrp at users.sourceforge.net>
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Copyright (c) 1998-2011 Glenn Randers-Pehrson
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This document is released under the libpng license.
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For conditions of distribution and use, see the disclaimer
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and license in png.h
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Based on:
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libpng versions 0.97, January 1998, through 1.5.10 - March 29, 2012
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Updated and distributed by Glenn Randers-Pehrson
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Copyright (c) 1998-2011 Glenn Randers-Pehrson
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libpng 1.0 beta 6 version 0.96 May 28, 1997
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Updated and distributed by Andreas Dilger
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Copyright (c) 1996, 1997 Andreas Dilger
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libpng 1.0 beta 2 - version 0.88 January 26, 1996
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For conditions of distribution and use, see copyright
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notice in png.h. Copyright (c) 1995, 1996 Guy Eric
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Schalnat, Group 42, Inc.
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Updated/rewritten per request in the libpng FAQ
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Copyright (c) 1995, 1996 Frank J. T. Wojcik
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December 18, 1995 & January 20, 1996
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I. Introduction
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This file describes how to use and modify the PNG reference library
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(known as libpng) for your own use. There are five sections to this
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file: introduction, structures, reading, writing, and modification and
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configuration notes for various special platforms. In addition to this
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file, example.c is a good starting point for using the library, as
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it is heavily commented and should include everything most people
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will need. We assume that libpng is already installed; see the
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INSTALL file for instructions on how to install libpng.
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For examples of libpng usage, see the files "example.c", "pngtest.c",
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and the files in the "contrib" directory, all of which are included in
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the libpng distribution.
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Libpng was written as a companion to the PNG specification, as a way
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of reducing the amount of time and effort it takes to support the PNG
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file format in application programs.
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The PNG specification (second edition), November 2003, is available as
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a W3C Recommendation and as an ISO Standard (ISO/IEC 15948:2003 (E)) at
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<http://www.w3.org/TR/2003/REC-PNG-20031110/
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The W3C and ISO documents have identical technical content.
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The PNG-1.2 specification is available at
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<http://www.libpng.org/pub/png/documents/>. It is technically equivalent
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to the PNG specification (second edition) but has some additional material.
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The PNG-1.0 specification is available
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as RFC 2083 <http://www.libpng.org/pub/png/documents/> and as a
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W3C Recommendation <http://www.w3.org/TR/REC.png.html>.
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Some additional chunks are described in the special-purpose public chunks
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documents at <http://www.libpng.org/pub/png/documents/>.
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Other information
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about PNG, and the latest version of libpng, can be found at the PNG home
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page, <http://www.libpng.org/pub/png/>.
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Most users will not have to modify the library significantly; advanced
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users may want to modify it more. All attempts were made to make it as
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complete as possible, while keeping the code easy to understand.
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Currently, this library only supports C. Support for other languages
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is being considered.
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Libpng has been designed to handle multiple sessions at one time,
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to be easily modifiable, to be portable to the vast majority of
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machines (ANSI, K&R, 16-, 32-, and 64-bit) available, and to be easy
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to use. The ultimate goal of libpng is to promote the acceptance of
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the PNG file format in whatever way possible. While there is still
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work to be done (see the TODO file), libpng should cover the
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majority of the needs of its users.
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Libpng uses zlib for its compression and decompression of PNG files.
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Further information about zlib, and the latest version of zlib, can
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be found at the zlib home page, <http://www.info-zip.org/pub/infozip/zlib/>.
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The zlib compression utility is a general purpose utility that is
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useful for more than PNG files, and can be used without libpng.
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See the documentation delivered with zlib for more details.
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You can usually find the source files for the zlib utility wherever you
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find the libpng source files.
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Libpng is thread safe, provided the threads are using different
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instances of the structures. Each thread should have its own
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png_struct and png_info instances, and thus its own image.
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Libpng does not protect itself against two threads using the
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same instance of a structure.
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II. Structures
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There are two main structures that are important to libpng, png_struct
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and png_info. Both are internal structures that are no longer exposed
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in the libpng interface (as of libpng 1.5.0).
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The png_info structure is designed to provide information about the
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PNG file. At one time, the fields of png_info were intended to be
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directly accessible to the user. However, this tended to cause problems
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with applications using dynamically loaded libraries, and as a result
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a set of interface functions for png_info (the png_get_*() and png_set_*()
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functions) was developed, and direct access to the png_info fields was
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deprecated..
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The png_struct structure is the object used by the library to decode a
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single image. As of 1.5.0 this structure is also not exposed.
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Almost all libpng APIs require a pointer to a png_struct as the first argument.
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Many (in particular the png_set and png_get APIs) also require a pointer
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to png_info as the second argument. Some application visible macros
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defined in png.h designed for basic data access (reading and writing
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integers in the PNG format) don't take a png_info pointer, but it's almost
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always safe to assume that a (png_struct*) has to be passed to call an API
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function.
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You can have more than one png_info structure associated with an image,
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as illustrated in pngtest.c, one for information valid prior to the
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IDAT chunks and another (called "end_info" below) for things after them.
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The png.h header file is an invaluable reference for programming with libpng.
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And while I'm on the topic, make sure you include the libpng header file:
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#include <png.h>
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and also (as of libpng-1.5.0) the zlib header file, if you need it:
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#include <zlib.h>
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Types
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The png.h header file defines a number of integral types used by the
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APIs. Most of these are fairly obvious; for example types corresponding
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to integers of particular sizes and types for passing color values.
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One exception is how non-integral numbers are handled. For application
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convenience most APIs that take such numbers have C (double) arguments,
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however internally PNG, and libpng, use 32 bit signed integers and encode
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the value by multiplying by 100,000. As of libpng 1.5.0 a convenience
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macro PNG_FP_1 is defined in png.h along with a type (png_fixed_point)
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which is simply (png_int_32).
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All APIs that take (double) arguments also have a matching API that
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takes the corresponding fixed point integer arguments. The fixed point
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API has the same name as the floating point one with "_fixed" appended.
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The actual range of values permitted in the APIs is frequently less than
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the full range of (png_fixed_point) (-21474 to +21474). When APIs require
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a non-negative argument the type is recorded as png_uint_32 above. Consult
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the header file and the text below for more information.
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Special care must be take with sCAL chunk handling because the chunk itself
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uses non-integral values encoded as strings containing decimal floating point
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numbers. See the comments in the header file.
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Configuration
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The main header file function declarations are frequently protected by C
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preprocessing directives of the form:
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#ifdef PNG_feature_SUPPORTED
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declare-function
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#endif
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...
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#ifdef PNG_feature_SUPPORTED
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use-function
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#endif
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The library can be built without support for these APIs, although a
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standard build will have all implemented APIs. Application programs
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should check the feature macros before using an API for maximum
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portability. From libpng 1.5.0 the feature macros set during the build
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of libpng are recorded in the header file "pnglibconf.h" and this file
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is always included by png.h.
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If you don't need to change the library configuration from the default, skip to
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the next section ("Reading").
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Notice that some of the makefiles in the 'scripts' directory and (in 1.5.0) all
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of the build project files in the 'projects' directory simply copy
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scripts/pnglibconf.h.prebuilt to pnglibconf.h. This means that these build
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systems do not permit easy auto-configuration of the library - they only
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support the default configuration.
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The easiest way to make minor changes to the libpng configuration when
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auto-configuration is supported is to add definitions to the command line
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using (typically) CPPFLAGS. For example:
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CPPFLAGS=-DPNG_NO_FLOATING_ARITHMETIC
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will change the internal libpng math implementation for gamma correction and
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other arithmetic calculations to fixed point, avoiding the need for fast
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floating point support. The result can be seen in the generated pnglibconf.h -
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make sure it contains the changed feature macro setting.
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If you need to make more extensive configuration changes - more than one or two
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feature macro settings - you can either add -DPNG_USER_CONFIG to the build
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command line and put a list of feature macro settings in pngusr.h or you can set
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DFA_XTRA (a makefile variable) to a file containing the same information in the
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form of 'option' settings.
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A. Changing pnglibconf.h
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A variety of methods exist to build libpng. Not all of these support
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reconfiguration of pnglibconf.h. To reconfigure pnglibconf.h it must either be
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rebuilt from scripts/pnglibconf.dfa using awk or it must be edited by hand.
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Hand editing is achieved by copying scripts/pnglibconf.h.prebuilt to
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pnglibconf.h and changing the lines defining the supported features, paying
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very close attention to the 'option' information in scripts/pnglibconf.dfa
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that describes those features and their requirements. This is easy to get
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wrong.
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B. Configuration using DFA_XTRA
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Rebuilding from pnglibconf.dfa is easy if a functioning 'awk', or a later
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variant such as 'nawk' or 'gawk', is available. The configure build will
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automatically find an appropriate awk and build pnglibconf.h.
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The scripts/pnglibconf.mak file contains a set of make rules for doing the
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same thing if configure is not used, and many of the makefiles in the scripts
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directory use this approach.
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When rebuilding simply write a new file containing changed options and set
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DFA_XTRA to the name of this file. This causes the build to append the new file
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to the end of scripts/pnglibconf.dfa. The pngusr.dfa file should contain lines
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of the following forms:
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everything = off
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This turns all optional features off. Include it at the start of pngusr.dfa to
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make it easier to build a minimal configuration. You will need to turn at least
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some features on afterward to enable either reading or writing code, or both.
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option feature on
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option feature off
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Enable or disable a single feature. This will automatically enable other
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features required by a feature that is turned on or disable other features that
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require a feature which is turned off. Conflicting settings will cause an error
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message to be emitted by awk.
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setting feature default value
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Changes the default value of setting 'feature' to 'value'. There are a small
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number of settings listed at the top of pnglibconf.h, they are documented in the
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source code. Most of these values have performance implications for the library
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but most of them have no visible effect on the API. Some can also be overridden
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from the API.
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This method of building a customized pnglibconf.h is illustrated in
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contrib/pngminim/*. See the "$(PNGCONF):" target in the makefile and
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pngusr.dfa in these directories.
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C. Configuration using PNG_USR_CONFIG
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If -DPNG_USR_CONFIG is added to the CFLAGS when pnglibconf.h is built the file
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pngusr.h will automatically be included before the options in
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scripts/pnglibconf.dfa are processed. Your pngusr.h file should contain only
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macro definitions turning features on or off or setting settings.
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Apart from the global setting "everything = off" all the options listed above
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can be set using macros in pngusr.h:
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#define PNG_feature_SUPPORTED
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is equivalent to:
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option feature on
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#define PNG_NO_feature
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is equivalent to:
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option feature off
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#define PNG_feature value
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is equivalent to:
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setting feature default value
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Notice that in both cases, pngusr.dfa and pngusr.h, the contents of the
|
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pngusr file you supply override the contents of scripts/pnglibconf.dfa
|
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|
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If confusing or incomprehensible behavior results it is possible to
|
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|
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examine the intermediate file pnglibconf.dfn to find the full set of
|
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|
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dependency information for each setting and option. Simply locate the
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feature in the file and read the C comments that precede it.
|
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This method is also illustrated in the contrib/pngminim/* makefiles and
|
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pngusr.h.
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III. Reading
|
||
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We'll now walk you through the possible functions to call when reading
|
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in a PNG file sequentially, briefly explaining the syntax and purpose
|
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of each one. See example.c and png.h for more detail. While
|
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|
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progressive reading is covered in the next section, you will still
|
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need some of the functions discussed in this section to read a PNG
|
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file.
|
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Setup
|
||
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You will want to do the I/O initialization(*) before you get into libpng,
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so if it doesn't work, you don't have much to undo. Of course, you
|
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will also want to insure that you are, in fact, dealing with a PNG
|
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file. Libpng provides a simple check to see if a file is a PNG file.
|
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To use it, pass in the first 1 to 8 bytes of the file to the function
|
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png_sig_cmp(), and it will return 0 (false) if the bytes match the
|
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corresponding bytes of the PNG signature, or nonzero (true) otherwise.
|
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Of course, the more bytes you pass in, the greater the accuracy of the
|
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prediction.
|
||
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If you are intending to keep the file pointer open for use in libpng,
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you must ensure you don't read more than 8 bytes from the beginning
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of the file, and you also have to make a call to png_set_sig_bytes_read()
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with the number of bytes you read from the beginning. Libpng will
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then only check the bytes (if any) that your program didn't read.
|
||
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(*): If you are not using the standard I/O functions, you will need
|
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to replace them with custom functions. See the discussion under
|
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|
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Customizing libpng.
|
||
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|
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FILE *fp = fopen(file_name, "rb");
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if (!fp)
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{
|
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return (ERROR);
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}
|
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fread(header, 1, number, fp);
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is_png = !png_sig_cmp(header, 0, number);
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if (!is_png)
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{
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return (NOT_PNG);
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}
|
||
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|
||
|
|
|
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|
|
Next, png_struct and png_info need to be allocated and initialized. In
|
||
|
|
order to ensure that the size of these structures is correct even with a
|
||
|
|
dynamically linked libpng, there are functions to initialize and
|
||
|
|
allocate the structures. We also pass the library version, optional
|
||
|
|
pointers to error handling functions, and a pointer to a data struct for
|
||
|
|
use by the error functions, if necessary (the pointer and functions can
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|
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be NULL if the default error handlers are to be used). See the section
|
||
|
|
on Changes to Libpng below regarding the old initialization functions.
|
||
|
|
The structure allocation functions quietly return NULL if they fail to
|
||
|
|
create the structure, so your application should check for that.
|
||
|
|
|
||
|
|
png_structp png_ptr = png_create_read_struct
|
||
|
|
(PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
|
||
|
|
user_error_fn, user_warning_fn);
|
||
|
|
|
||
|
|
if (!png_ptr)
|
||
|
|
return (ERROR);
|
||
|
|
|
||
|
|
png_infop info_ptr = png_create_info_struct(png_ptr);
|
||
|
|
|
||
|
|
if (!info_ptr)
|
||
|
|
{
|
||
|
|
png_destroy_read_struct(&png_ptr,
|
||
|
|
(png_infopp)NULL, (png_infopp)NULL);
|
||
|
|
return (ERROR);
|
||
|
|
}
|
||
|
|
|
||
|
|
If you want to use your own memory allocation routines,
|
||
|
|
use a libpng that was built with PNG_USER_MEM_SUPPORTED defined, and use
|
||
|
|
png_create_read_struct_2() instead of png_create_read_struct():
|
||
|
|
|
||
|
|
png_structp png_ptr = png_create_read_struct_2
|
||
|
|
(PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
|
||
|
|
user_error_fn, user_warning_fn, (png_voidp)
|
||
|
|
user_mem_ptr, user_malloc_fn, user_free_fn);
|
||
|
|
|
||
|
|
The error handling routines passed to png_create_read_struct()
|
||
|
|
and the memory alloc/free routines passed to png_create_struct_2()
|
||
|
|
are only necessary if you are not using the libpng supplied error
|
||
|
|
handling and memory alloc/free functions.
|
||
|
|
|
||
|
|
When libpng encounters an error, it expects to longjmp back
|
||
|
|
to your routine. Therefore, you will need to call setjmp and pass
|
||
|
|
your png_jmpbuf(png_ptr). If you read the file from different
|
||
|
|
routines, you will need to update the longjmp buffer every time you enter
|
||
|
|
a new routine that will call a png_*() function.
|
||
|
|
|
||
|
|
See your documentation of setjmp/longjmp for your compiler for more
|
||
|
|
information on setjmp/longjmp. See the discussion on libpng error
|
||
|
|
handling in the Customizing Libpng section below for more information
|
||
|
|
on the libpng error handling. If an error occurs, and libpng longjmp's
|
||
|
|
back to your setjmp, you will want to call png_destroy_read_struct() to
|
||
|
|
free any memory.
|
||
|
|
|
||
|
|
if (setjmp(png_jmpbuf(png_ptr)))
|
||
|
|
{
|
||
|
|
png_destroy_read_struct(&png_ptr, &info_ptr,
|
||
|
|
&end_info);
|
||
|
|
fclose(fp);
|
||
|
|
return (ERROR);
|
||
|
|
}
|
||
|
|
|
||
|
|
Pass (png_infopp)NULL instead of &end_info if you didn't create
|
||
|
|
an end_info structure.
|
||
|
|
|
||
|
|
If you would rather avoid the complexity of setjmp/longjmp issues,
|
||
|
|
you can compile libpng with PNG_NO_SETJMP, in which case
|
||
|
|
errors will result in a call to PNG_ABORT() which defaults to abort().
|
||
|
|
|
||
|
|
You can #define PNG_ABORT() to a function that does something
|
||
|
|
more useful than abort(), as long as your function does not
|
||
|
|
return.
|
||
|
|
|
||
|
|
Now you need to set up the input code. The default for libpng is to
|
||
|
|
use the C function fread(). If you use this, you will need to pass a
|
||
|
|
valid FILE * in the function png_init_io(). Be sure that the file is
|
||
|
|
opened in binary mode. If you wish to handle reading data in another
|
||
|
|
way, you need not call the png_init_io() function, but you must then
|
||
|
|
implement the libpng I/O methods discussed in the Customizing Libpng
|
||
|
|
section below.
|
||
|
|
|
||
|
|
png_init_io(png_ptr, fp);
|
||
|
|
|
||
|
|
If you had previously opened the file and read any of the signature from
|
||
|
|
the beginning in order to see if this was a PNG file, you need to let
|
||
|
|
libpng know that there are some bytes missing from the start of the file.
|
||
|
|
|
||
|
|
png_set_sig_bytes(png_ptr, number);
|
||
|
|
|
||
|
|
You can change the zlib compression buffer size to be used while
|
||
|
|
reading compressed data with
|
||
|
|
|
||
|
|
png_set_compression_buffer_size(png_ptr, buffer_size);
|
||
|
|
|
||
|
|
where the default size is 8192 bytes. Note that the buffer size
|
||
|
|
is changed immediately and the buffer is reallocated immediately,
|
||
|
|
instead of setting a flag to be acted upon later.
|
||
|
|
|
||
|
|
If you want CRC errors to be handled in a different manner than
|
||
|
|
the default, use
|
||
|
|
|
||
|
|
png_set_crc_action(png_ptr, crit_action, ancil_action);
|
||
|
|
|
||
|
|
The values for png_set_crc_action() say how libpng is to handle CRC errors in
|
||
|
|
ancillary and critical chunks, and whether to use the data contained
|
||
|
|
therein. Note that it is impossible to "discard" data in a critical
|
||
|
|
chunk.
|
||
|
|
|
||
|
|
Choices for (int) crit_action are
|
||
|
|
PNG_CRC_DEFAULT 0 error/quit
|
||
|
|
PNG_CRC_ERROR_QUIT 1 error/quit
|
||
|
|
PNG_CRC_WARN_USE 3 warn/use data
|
||
|
|
PNG_CRC_QUIET_USE 4 quiet/use data
|
||
|
|
PNG_CRC_NO_CHANGE 5 use the current value
|
||
|
|
|
||
|
|
Choices for (int) ancil_action are
|
||
|
|
PNG_CRC_DEFAULT 0 error/quit
|
||
|
|
PNG_CRC_ERROR_QUIT 1 error/quit
|
||
|
|
PNG_CRC_WARN_DISCARD 2 warn/discard data
|
||
|
|
PNG_CRC_WARN_USE 3 warn/use data
|
||
|
|
PNG_CRC_QUIET_USE 4 quiet/use data
|
||
|
|
PNG_CRC_NO_CHANGE 5 use the current value
|
||
|
|
|
||
|
|
Setting up callback code
|
||
|
|
|
||
|
|
You can set up a callback function to handle any unknown chunks in the
|
||
|
|
input stream. You must supply the function
|
||
|
|
|
||
|
|
read_chunk_callback(png_structp png_ptr,
|
||
|
|
png_unknown_chunkp chunk);
|
||
|
|
{
|
||
|
|
/* The unknown chunk structure contains your
|
||
|
|
chunk data, along with similar data for any other
|
||
|
|
unknown chunks: */
|
||
|
|
|
||
|
|
png_byte name[5];
|
||
|
|
png_byte *data;
|
||
|
|
png_size_t size;
|
||
|
|
|
||
|
|
/* Note that libpng has already taken care of
|
||
|
|
the CRC handling */
|
||
|
|
|
||
|
|
/* put your code here. Search for your chunk in the
|
||
|
|
unknown chunk structure, process it, and return one
|
||
|
|
of the following: */
|
||
|
|
|
||
|
|
return (-n); /* chunk had an error */
|
||
|
|
return (0); /* did not recognize */
|
||
|
|
return (n); /* success */
|
||
|
|
}
|
||
|
|
|
||
|
|
(You can give your function another name that you like instead of
|
||
|
|
"read_chunk_callback")
|
||
|
|
|
||
|
|
To inform libpng about your function, use
|
||
|
|
|
||
|
|
png_set_read_user_chunk_fn(png_ptr, user_chunk_ptr,
|
||
|
|
read_chunk_callback);
|
||
|
|
|
||
|
|
This names not only the callback function, but also a user pointer that
|
||
|
|
you can retrieve with
|
||
|
|
|
||
|
|
png_get_user_chunk_ptr(png_ptr);
|
||
|
|
|
||
|
|
If you call the png_set_read_user_chunk_fn() function, then all unknown
|
||
|
|
chunks will be saved when read, in case your callback function will need
|
||
|
|
one or more of them. This behavior can be changed with the
|
||
|
|
png_set_keep_unknown_chunks() function, described below.
|
||
|
|
|
||
|
|
At this point, you can set up a callback function that will be
|
||
|
|
called after each row has been read, which you can use to control
|
||
|
|
a progress meter or the like. It's demonstrated in pngtest.c.
|
||
|
|
You must supply a function
|
||
|
|
|
||
|
|
void read_row_callback(png_structp png_ptr,
|
||
|
|
png_uint_32 row, int pass);
|
||
|
|
{
|
||
|
|
/* put your code here */
|
||
|
|
}
|
||
|
|
|
||
|
|
(You can give it another name that you like instead of "read_row_callback")
|
||
|
|
|
||
|
|
To inform libpng about your function, use
|
||
|
|
|
||
|
|
png_set_read_status_fn(png_ptr, read_row_callback);
|
||
|
|
|
||
|
|
When this function is called the row has already been completely processed and
|
||
|
|
the 'row' and 'pass' refer to the next row to be handled. For the
|
||
|
|
non-interlaced case the row that was just handled is simply one less than the
|
||
|
|
passed in row number, and pass will always be 0. For the interlaced case the
|
||
|
|
same applies unless the row value is 0, in which case the row just handled was
|
||
|
|
the last one from one of the preceding passes. Because interlacing may skip a
|
||
|
|
pass you cannot be sure that the preceding pass is just 'pass-1', if you really
|
||
|
|
need to know what the last pass is record (row,pass) from the callback and use
|
||
|
|
the last recorded value each time.
|
||
|
|
|
||
|
|
As with the user transform you can find the output row using the
|
||
|
|
PNG_ROW_FROM_PASS_ROW macro.
|
||
|
|
|
||
|
|
Unknown-chunk handling
|
||
|
|
|
||
|
|
Now you get to set the way the library processes unknown chunks in the
|
||
|
|
input PNG stream. Both known and unknown chunks will be read. Normal
|
||
|
|
behavior is that known chunks will be parsed into information in
|
||
|
|
various info_ptr members while unknown chunks will be discarded. This
|
||
|
|
behavior can be wasteful if your application will never use some known
|
||
|
|
chunk types. To change this, you can call:
|
||
|
|
|
||
|
|
png_set_keep_unknown_chunks(png_ptr, keep,
|
||
|
|
chunk_list, num_chunks);
|
||
|
|
keep - 0: default unknown chunk handling
|
||
|
|
1: ignore; do not keep
|
||
|
|
2: keep only if safe-to-copy
|
||
|
|
3: keep even if unsafe-to-copy
|
||
|
|
|
||
|
|
You can use these definitions:
|
||
|
|
PNG_HANDLE_CHUNK_AS_DEFAULT 0
|
||
|
|
PNG_HANDLE_CHUNK_NEVER 1
|
||
|
|
PNG_HANDLE_CHUNK_IF_SAFE 2
|
||
|
|
PNG_HANDLE_CHUNK_ALWAYS 3
|
||
|
|
|
||
|
|
chunk_list - list of chunks affected (a byte string,
|
||
|
|
five bytes per chunk, NULL or '\0' if
|
||
|
|
num_chunks is 0)
|
||
|
|
|
||
|
|
num_chunks - number of chunks affected; if 0, all
|
||
|
|
unknown chunks are affected. If nonzero,
|
||
|
|
only the chunks in the list are affected
|
||
|
|
|
||
|
|
Unknown chunks declared in this way will be saved as raw data onto a
|
||
|
|
list of png_unknown_chunk structures. If a chunk that is normally
|
||
|
|
known to libpng is named in the list, it will be handled as unknown,
|
||
|
|
according to the "keep" directive. If a chunk is named in successive
|
||
|
|
instances of png_set_keep_unknown_chunks(), the final instance will
|
||
|
|
take precedence. The IHDR and IEND chunks should not be named in
|
||
|
|
chunk_list; if they are, libpng will process them normally anyway.
|
||
|
|
If you know that your application will never make use of some particular
|
||
|
|
chunks, use PNG_HANDLE_CHUNK_NEVER (or 1) as demonstrated below.
|
||
|
|
|
||
|
|
Here is an example of the usage of png_set_keep_unknown_chunks(),
|
||
|
|
where the private "vpAg" chunk will later be processed by a user chunk
|
||
|
|
callback function:
|
||
|
|
|
||
|
|
png_byte vpAg[5]={118, 112, 65, 103, (png_byte) '\0'};
|
||
|
|
|
||
|
|
#if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
|
||
|
|
png_byte unused_chunks[]=
|
||
|
|
{
|
||
|
|
104, 73, 83, 84, (png_byte) '\0', /* hIST */
|
||
|
|
105, 84, 88, 116, (png_byte) '\0', /* iTXt */
|
||
|
|
112, 67, 65, 76, (png_byte) '\0', /* pCAL */
|
||
|
|
115, 67, 65, 76, (png_byte) '\0', /* sCAL */
|
||
|
|
115, 80, 76, 84, (png_byte) '\0', /* sPLT */
|
||
|
|
116, 73, 77, 69, (png_byte) '\0', /* tIME */
|
||
|
|
};
|
||
|
|
#endif
|
||
|
|
|
||
|
|
...
|
||
|
|
|
||
|
|
#if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
|
||
|
|
/* ignore all unknown chunks: */
|
||
|
|
png_set_keep_unknown_chunks(read_ptr, 1, NULL, 0);
|
||
|
|
|
||
|
|
/* except for vpAg: */
|
||
|
|
png_set_keep_unknown_chunks(read_ptr, 2, vpAg, 1);
|
||
|
|
|
||
|
|
/* also ignore unused known chunks: */
|
||
|
|
png_set_keep_unknown_chunks(read_ptr, 1, unused_chunks,
|
||
|
|
(int)sizeof(unused_chunks)/5);
|
||
|
|
#endif
|
||
|
|
|
||
|
|
User limits
|
||
|
|
|
||
|
|
The PNG specification allows the width and height of an image to be as
|
||
|
|
large as 2^31-1 (0x7fffffff), or about 2.147 billion rows and columns.
|
||
|
|
Since very few applications really need to process such large images,
|
||
|
|
we have imposed an arbitrary 1-million limit on rows and columns.
|
||
|
|
Larger images will be rejected immediately with a png_error() call. If
|
||
|
|
you wish to change this limit, you can use
|
||
|
|
|
||
|
|
png_set_user_limits(png_ptr, width_max, height_max);
|
||
|
|
|
||
|
|
to set your own limits, or use width_max = height_max = 0x7fffffffL
|
||
|
|
to allow all valid dimensions (libpng may reject some very large images
|
||
|
|
anyway because of potential buffer overflow conditions).
|
||
|
|
|
||
|
|
You should put this statement after you create the PNG structure and
|
||
|
|
before calling png_read_info(), png_read_png(), or png_process_data().
|
||
|
|
|
||
|
|
When writing a PNG datastream, put this statement before calling
|
||
|
|
png_write_info() or png_write_png().
|
||
|
|
|
||
|
|
If you need to retrieve the limits that are being applied, use
|
||
|
|
|
||
|
|
width_max = png_get_user_width_max(png_ptr);
|
||
|
|
height_max = png_get_user_height_max(png_ptr);
|
||
|
|
|
||
|
|
The PNG specification sets no limit on the number of ancillary chunks
|
||
|
|
allowed in a PNG datastream. You can impose a limit on the total number
|
||
|
|
of sPLT, tEXt, iTXt, zTXt, and unknown chunks that will be stored, with
|
||
|
|
|
||
|
|
png_set_chunk_cache_max(png_ptr, user_chunk_cache_max);
|
||
|
|
|
||
|
|
where 0x7fffffffL means unlimited. You can retrieve this limit with
|
||
|
|
|
||
|
|
chunk_cache_max = png_get_chunk_cache_max(png_ptr);
|
||
|
|
|
||
|
|
This limit also applies to the number of buffers that can be allocated
|
||
|
|
by png_decompress_chunk() while decompressing iTXt, zTXt, and iCCP chunks.
|
||
|
|
|
||
|
|
You can also set a limit on the amount of memory that a compressed chunk
|
||
|
|
other than IDAT can occupy, with
|
||
|
|
|
||
|
|
png_set_chunk_malloc_max(png_ptr, user_chunk_malloc_max);
|
||
|
|
|
||
|
|
and you can retrieve the limit with
|
||
|
|
|
||
|
|
chunk_malloc_max = png_get_chunk_malloc_max(png_ptr);
|
||
|
|
|
||
|
|
Any chunks that would cause either of these limits to be exceeded will
|
||
|
|
be ignored.
|
||
|
|
|
||
|
|
Information about your system
|
||
|
|
|
||
|
|
If you intend to display the PNG or to incorporate it in other image data you
|
||
|
|
need to tell libpng information about your display or drawing surface so that
|
||
|
|
libpng can convert the values in the image to match the display.
|
||
|
|
|
||
|
|
From libpng-1.5.4 this information can be set before reading the PNG file
|
||
|
|
header. In earlier versions png_set_gamma() existed but behaved incorrectly if
|
||
|
|
called before the PNG file header had been read and png_set_alpha_mode() did not
|
||
|
|
exist.
|
||
|
|
|
||
|
|
If you need to support versions prior to libpng-1.5.4 test the version number
|
||
|
|
as illustrated below using "PNG_LIBPNG_VER >= 10504" and follow the procedures
|
||
|
|
described in the appropriate manual page.
|
||
|
|
|
||
|
|
You give libpng the encoding expected by your system expressed as a 'gamma'
|
||
|
|
value. You can also specify a default encoding for the PNG file in
|
||
|
|
case the required information is missing from the file. By default libpng
|
||
|
|
assumes that the PNG data matches your system, to keep this default call:
|
||
|
|
|
||
|
|
png_set_gamma(png_ptr, screen_gamma, 1/screen_gamma/*file gamma*/);
|
||
|
|
|
||
|
|
or you can use the fixed point equivalent:
|
||
|
|
|
||
|
|
png_set_gamma_fixed(png_ptr, PNG_FP_1*screen_gamma, PNG_FP_1/screen_gamma);
|
||
|
|
|
||
|
|
If you don't know the gamma for your system it is probably 2.2 - a good
|
||
|
|
approximation to the IEC standard for display systems (sRGB). If images are
|
||
|
|
too contrasty or washed out you got the value wrong - check your system
|
||
|
|
documentation!
|
||
|
|
|
||
|
|
Many systems permit the system gamma to be changed via a lookup table in the
|
||
|
|
display driver, a few systems, including older Macs, change the response by
|
||
|
|
default. As of 1.5.4 three special values are available to handle common
|
||
|
|
situations:
|
||
|
|
|
||
|
|
PNG_DEFAULT_sRGB: Indicates that the system conforms to the IEC 61966-2-1
|
||
|
|
standard. This matches almost all systems.
|
||
|
|
PNG_GAMMA_MAC_18: Indicates that the system is an older (pre Mac OS 10.6)
|
||
|
|
Apple Macintosh system with the default settings.
|
||
|
|
PNG_GAMMA_LINEAR: Just the fixed point value for 1.0 - indicates that the
|
||
|
|
system expects data with no gamma encoding.
|
||
|
|
|
||
|
|
You would use the linear (unencoded) value if you need to process the pixel
|
||
|
|
values further because this avoids the need to decode and reencode each
|
||
|
|
component value whenever arithmetic is performed. A lot of graphics software
|
||
|
|
uses linear values for this reason, often with higher precision component values
|
||
|
|
to preserve overall accuracy.
|
||
|
|
|
||
|
|
The second thing you may need to tell libpng about is how your system handles
|
||
|
|
alpha channel information. Some, but not all, PNG files contain an alpha
|
||
|
|
channel. To display these files correctly you need to compose the data onto a
|
||
|
|
suitable background, as described in the PNG specification.
|
||
|
|
|
||
|
|
Libpng only supports composing onto a single color (using png_set_background;
|
||
|
|
see below). Otherwise you must do the composition yourself and, in this case,
|
||
|
|
you may need to call png_set_alpha_mode:
|
||
|
|
|
||
|
|
#if PNG_LIBPNG_VER >= 10504
|
||
|
|
png_set_alpha_mode(png_ptr, mode, screen_gamma);
|
||
|
|
#else
|
||
|
|
png_set_gamma(png_ptr, screen_gamma, 1.0/screen_gamma);
|
||
|
|
#endif
|
||
|
|
|
||
|
|
The screen_gamma value is the same as the argument to png_set_gamma; however,
|
||
|
|
how it affects the output depends on the mode. png_set_alpha_mode() sets the
|
||
|
|
file gamma default to 1/screen_gamma, so normally you don't need to call
|
||
|
|
png_set_gamma. If you need different defaults call png_set_gamma() before
|
||
|
|
png_set_alpha_mode() - if you call it after it will override the settings made
|
||
|
|
by png_set_alpha_mode().
|
||
|
|
|
||
|
|
The mode is as follows:
|
||
|
|
|
||
|
|
PNG_ALPHA_PNG: The data is encoded according to the PNG specification. Red,
|
||
|
|
green and blue, or gray, components are gamma encoded color
|
||
|
|
values and are not premultiplied by the alpha value. The
|
||
|
|
alpha value is a linear measure of the contribution of the
|
||
|
|
pixel to the corresponding final output pixel.
|
||
|
|
|
||
|
|
You should normally use this format if you intend to perform
|
||
|
|
color correction on the color values; most, maybe all, color
|
||
|
|
correction software has no handling for the alpha channel and,
|
||
|
|
anyway, the math to handle pre-multiplied component values is
|
||
|
|
unnecessarily complex.
|
||
|
|
|
||
|
|
Before you do any arithmetic on the component values you need
|
||
|
|
to remove the gamma encoding and multiply out the alpha
|
||
|
|
channel. See the PNG specification for more detail. It is
|
||
|
|
important to note that when an image with an alpha channel is
|
||
|
|
scaled, linear encoded, pre-multiplied component values must
|
||
|
|
be used!
|
||
|
|
|
||
|
|
The remaining modes assume you don't need to do any further color correction or
|
||
|
|
that if you do, your color correction software knows all about alpha (it
|
||
|
|
probably doesn't!)
|
||
|
|
|
||
|
|
PNG_ALPHA_STANDARD: The data libpng produces
|
||
|
|
is encoded in the standard way
|
||
|
|
assumed by most correctly written graphics software.
|
||
|
|
The gamma encoding will be removed by libpng and the
|
||
|
|
linear component values will be pre-multiplied by the
|
||
|
|
alpha channel.
|
||
|
|
|
||
|
|
With this format the final image must be re-encoded to
|
||
|
|
match the display gamma before the image is displayed.
|
||
|
|
If your system doesn't do that, yet still seems to
|
||
|
|
perform arithmetic on the pixels without decoding them,
|
||
|
|
it is broken - check out the modes below.
|
||
|
|
|
||
|
|
With PNG_ALPHA_STANDARD libpng always produces linear
|
||
|
|
component values, whatever screen_gamma you supply. The
|
||
|
|
screen_gamma value is, however, used as a default for
|
||
|
|
the file gamma if the PNG file has no gamma information.
|
||
|
|
|
||
|
|
If you call png_set_gamma() after png_set_alpha_mode() you
|
||
|
|
will override the linear encoding. Instead the
|
||
|
|
pre-multiplied pixel values will be gamma encoded but
|
||
|
|
the alpha channel will still be linear. This may
|
||
|
|
actually match the requirements of some broken software,
|
||
|
|
but it is unlikely.
|
||
|
|
|
||
|
|
While linear 8-bit data is often used it has
|
||
|
|
insufficient precision for any image with a reasonable
|
||
|
|
dynamic range. To avoid problems, and if your software
|
||
|
|
supports it, use png_set_expand_16() to force all
|
||
|
|
components to 16 bits.
|
||
|
|
|
||
|
|
PNG_ALPHA_OPTIMIZED: This mode is the same
|
||
|
|
as PNG_ALPHA_STANDARD except that
|
||
|
|
completely opaque pixels are gamma encoded according to
|
||
|
|
the screen_gamma value. Pixels with alpha less than 1.0
|
||
|
|
will still have linear components.
|
||
|
|
|
||
|
|
Use this format if you have control over your
|
||
|
|
compositing software and do don't do other arithmetic
|
||
|
|
(such as scaling) on the data you get from libpng. Your
|
||
|
|
compositing software can simply copy opaque pixels to
|
||
|
|
the output but still has linear values for the
|
||
|
|
non-opaque pixels.
|
||
|
|
|
||
|
|
In normal compositing, where the alpha channel encodes
|
||
|
|
partial pixel coverage (as opposed to broad area
|
||
|
|
translucency), the inaccuracies of the 8-bit
|
||
|
|
representation of non-opaque pixels are irrelevant.
|
||
|
|
|
||
|
|
You can also try this format if your software is broken;
|
||
|
|
it might look better.
|
||
|
|
|
||
|
|
PNG_ALPHA_BROKEN: This is PNG_ALPHA_STANDARD;
|
||
|
|
however, all component values,
|
||
|
|
including the alpha channel are gamma encoded. This is
|
||
|
|
an appropriate format to try if your software, or more
|
||
|
|
likely hardware, is totally broken, i.e., if it performs
|
||
|
|
linear arithmetic directly on gamma encoded values.
|
||
|
|
|
||
|
|
In most cases of broken software or hardware the bug in the final display
|
||
|
|
manifests as a subtle halo around composited parts of the image. You may not
|
||
|
|
even perceive this as a halo; the composited part of the image may simply appear
|
||
|
|
separate from the background, as though it had been cut out of paper and pasted
|
||
|
|
on afterward.
|
||
|
|
|
||
|
|
If you don't have to deal with bugs in software or hardware, or if you can fix
|
||
|
|
them, there are three recommended ways of using png_set_alpha_mode():
|
||
|
|
|
||
|
|
png_set_alpha_mode(png_ptr, PNG_ALPHA_PNG,
|
||
|
|
screen_gamma);
|
||
|
|
|
||
|
|
You can do color correction on the result (libpng does not currently
|
||
|
|
support color correction internally). When you handle the alpha channel
|
||
|
|
you need to undo the gamma encoding and multiply out the alpha.
|
||
|
|
|
||
|
|
png_set_alpha_mode(png_ptr, PNG_ALPHA_STANDARD,
|
||
|
|
screen_gamma);
|
||
|
|
png_set_expand_16(png_ptr);
|
||
|
|
|
||
|
|
If you are using the high level interface, don't call png_set_expand_16();
|
||
|
|
instead pass PNG_TRANSFORM_EXPAND_16 to the interface.
|
||
|
|
|
||
|
|
With this mode you can't do color correction, but you can do arithmetic,
|
||
|
|
including composition and scaling, on the data without further processing.
|
||
|
|
|
||
|
|
png_set_alpha_mode(png_ptr, PNG_ALPHA_OPTIMIZED,
|
||
|
|
screen_gamma);
|
||
|
|
|
||
|
|
You can avoid the expansion to 16-bit components with this mode, but you
|
||
|
|
lose the ability to scale the image or perform other linear arithmetic.
|
||
|
|
All you can do is compose the result onto a matching output. Since this
|
||
|
|
mode is libpng-specific you also need to write your own composition
|
||
|
|
software.
|
||
|
|
|
||
|
|
If you don't need, or can't handle, the alpha channel you can call
|
||
|
|
png_set_background() to remove it by compositing against a fixed color. Don't
|
||
|
|
call png_set_strip_alpha() to do this - it will leave spurious pixel values in
|
||
|
|
transparent parts of this image.
|
||
|
|
|
||
|
|
png_set_background(png_ptr, &background_color,
|
||
|
|
PNG_BACKGROUND_GAMMA_SCREEN, 0, 1);
|
||
|
|
|
||
|
|
The background_color is an RGB or grayscale value according to the data format
|
||
|
|
libpng will produce for you. Because you don't yet know the format of the PNG
|
||
|
|
file, if you call png_set_background at this point you must arrange for the
|
||
|
|
format produced by libpng to always have 8-bit or 16-bit components and then
|
||
|
|
store the color as an 8-bit or 16-bit color as appropriate. The color contains
|
||
|
|
separate gray and RGB component values, so you can let libpng produce gray or
|
||
|
|
RGB output according to the input format, but low bit depth grayscale images
|
||
|
|
must always be converted to at least 8-bit format. (Even though low bit depth
|
||
|
|
grayscale images can't have an alpha channel they can have a transparent
|
||
|
|
color!)
|
||
|
|
|
||
|
|
You set the transforms you need later, either as flags to the high level
|
||
|
|
interface or libpng API calls for the low level interface. For reference the
|
||
|
|
settings and API calls required are:
|
||
|
|
|
||
|
|
8-bit values:
|
||
|
|
PNG_TRANSFORM_SCALE_16 | PNG_EXPAND
|
||
|
|
png_set_expand(png_ptr); png_set_scale_16(png_ptr);
|
||
|
|
|
||
|
|
If you must get exactly the same inaccurate results
|
||
|
|
produced by default in versions prior to libpng-1.5.4,
|
||
|
|
use PNG_TRANSFORM_STRIP_16 and png_set_strip_16(png_ptr)
|
||
|
|
instead.
|
||
|
|
|
||
|
|
16-bit values:
|
||
|
|
PNG_TRANSFORM_EXPAND_16
|
||
|
|
png_set_expand_16(png_ptr);
|
||
|
|
|
||
|
|
In either case palette image data will be expanded to RGB. If you just want
|
||
|
|
color data you can add PNG_TRANSFORM_GRAY_TO_RGB or png_set_gray_to_rgb(png_ptr)
|
||
|
|
to the list.
|
||
|
|
|
||
|
|
Calling png_set_background before the PNG file header is read will not work
|
||
|
|
prior to libpng-1.5.4. Because the failure may result in unexpected warnings or
|
||
|
|
errors it is therefore much safer to call png_set_background after the head has
|
||
|
|
been read. Unfortunately this means that prior to libpng-1.5.4 it cannot be
|
||
|
|
used with the high level interface.
|
||
|
|
|
||
|
|
The high-level read interface
|
||
|
|
|
||
|
|
At this point there are two ways to proceed; through the high-level
|
||
|
|
read interface, or through a sequence of low-level read operations.
|
||
|
|
You can use the high-level interface if (a) you are willing to read
|
||
|
|
the entire image into memory, and (b) the input transformations
|
||
|
|
you want to do are limited to the following set:
|
||
|
|
|
||
|
|
PNG_TRANSFORM_IDENTITY No transformation
|
||
|
|
PNG_TRANSFORM_SCALE_16 Strip 16-bit samples to
|
||
|
|
8-bit accurately
|
||
|
|
PNG_TRANSFORM_STRIP_16 Chop 16-bit samples to
|
||
|
|
8-bit less accurately
|
||
|
|
PNG_TRANSFORM_STRIP_ALPHA Discard the alpha channel
|
||
|
|
PNG_TRANSFORM_PACKING Expand 1, 2 and 4-bit
|
||
|
|
samples to bytes
|
||
|
|
PNG_TRANSFORM_PACKSWAP Change order of packed
|
||
|
|
pixels to LSB first
|
||
|
|
PNG_TRANSFORM_EXPAND Perform set_expand()
|
||
|
|
PNG_TRANSFORM_INVERT_MONO Invert monochrome images
|
||
|
|
PNG_TRANSFORM_SHIFT Normalize pixels to the
|
||
|
|
sBIT depth
|
||
|
|
PNG_TRANSFORM_BGR Flip RGB to BGR, RGBA
|
||
|
|
to BGRA
|
||
|
|
PNG_TRANSFORM_SWAP_ALPHA Flip RGBA to ARGB or GA
|
||
|
|
to AG
|
||
|
|
PNG_TRANSFORM_INVERT_ALPHA Change alpha from opacity
|
||
|
|
to transparency
|
||
|
|
PNG_TRANSFORM_SWAP_ENDIAN Byte-swap 16-bit samples
|
||
|
|
PNG_TRANSFORM_GRAY_TO_RGB Expand grayscale samples
|
||
|
|
to RGB (or GA to RGBA)
|
||
|
|
PNG_TRANSFORM_EXPAND_16 Expand samples to 16 bits
|
||
|
|
|
||
|
|
(This excludes setting a background color, doing gamma transformation,
|
||
|
|
quantizing, and setting filler.) If this is the case, simply do this:
|
||
|
|
|
||
|
|
png_read_png(png_ptr, info_ptr, png_transforms, NULL)
|
||
|
|
|
||
|
|
where png_transforms is an integer containing the bitwise OR of some
|
||
|
|
set of transformation flags. This call is equivalent to png_read_info(),
|
||
|
|
followed the set of transformations indicated by the transform mask,
|
||
|
|
then png_read_image(), and finally png_read_end().
|
||
|
|
|
||
|
|
(The final parameter of this call is not yet used. Someday it might point
|
||
|
|
to transformation parameters required by some future input transform.)
|
||
|
|
|
||
|
|
You must use png_transforms and not call any png_set_transform() functions
|
||
|
|
when you use png_read_png().
|
||
|
|
|
||
|
|
After you have called png_read_png(), you can retrieve the image data
|
||
|
|
with
|
||
|
|
|
||
|
|
row_pointers = png_get_rows(png_ptr, info_ptr);
|
||
|
|
|
||
|
|
where row_pointers is an array of pointers to the pixel data for each row:
|
||
|
|
|
||
|
|
png_bytep row_pointers[height];
|
||
|
|
|
||
|
|
If you know your image size and pixel size ahead of time, you can allocate
|
||
|
|
row_pointers prior to calling png_read_png() with
|
||
|
|
|
||
|
|
if (height > PNG_UINT_32_MAX/png_sizeof(png_byte))
|
||
|
|
png_error (png_ptr,
|
||
|
|
"Image is too tall to process in memory");
|
||
|
|
|
||
|
|
if (width > PNG_UINT_32_MAX/pixel_size)
|
||
|
|
png_error (png_ptr,
|
||
|
|
"Image is too wide to process in memory");
|
||
|
|
|
||
|
|
row_pointers = png_malloc(png_ptr,
|
||
|
|
height*png_sizeof(png_bytep));
|
||
|
|
|
||
|
|
for (int i=0; i<height, i++)
|
||
|
|
row_pointers[i]=NULL; /* security precaution */
|
||
|
|
|
||
|
|
for (int i=0; i<height, i++)
|
||
|
|
row_pointers[i]=png_malloc(png_ptr,
|
||
|
|
width*pixel_size);
|
||
|
|
|
||
|
|
png_set_rows(png_ptr, info_ptr, &row_pointers);
|
||
|
|
|
||
|
|
Alternatively you could allocate your image in one big block and define
|
||
|
|
row_pointers[i] to point into the proper places in your block.
|
||
|
|
|
||
|
|
If you use png_set_rows(), the application is responsible for freeing
|
||
|
|
row_pointers (and row_pointers[i], if they were separately allocated).
|
||
|
|
|
||
|
|
If you don't allocate row_pointers ahead of time, png_read_png() will
|
||
|
|
do it, and it'll be free'ed by libpng when you call png_destroy_*().
|
||
|
|
|
||
|
|
The low-level read interface
|
||
|
|
|
||
|
|
If you are going the low-level route, you are now ready to read all
|
||
|
|
the file information up to the actual image data. You do this with a
|
||
|
|
call to png_read_info().
|
||
|
|
|
||
|
|
png_read_info(png_ptr, info_ptr);
|
||
|
|
|
||
|
|
This will process all chunks up to but not including the image data.
|
||
|
|
|
||
|
|
This also copies some of the data from the PNG file into the decode structure
|
||
|
|
for use in later transformations. Important information copied in is:
|
||
|
|
|
||
|
|
1) The PNG file gamma from the gAMA chunk. This overwrites the default value
|
||
|
|
provided by an earlier call to png_set_gamma or png_set_alpha_mode.
|
||
|
|
|
||
|
|
2) Prior to libpng-1.5.4 the background color from a bKGd chunk. This
|
||
|
|
damages the information provided by an earlier call to png_set_background
|
||
|
|
resulting in unexpected behavior. Libpng-1.5.4 no longer does this.
|
||
|
|
|
||
|
|
3) The number of significant bits in each component value. Libpng uses this to
|
||
|
|
optimize gamma handling by reducing the internal lookup table sizes.
|
||
|
|
|
||
|
|
4) The transparent color information from a tRNS chunk. This can be modified by
|
||
|
|
a later call to png_set_tRNS.
|
||
|
|
|
||
|
|
Querying the info structure
|
||
|
|
|
||
|
|
Functions are used to get the information from the info_ptr once it
|
||
|
|
has been read. Note that these fields may not be completely filled
|
||
|
|
in until png_read_end() has read the chunk data following the image.
|
||
|
|
|
||
|
|
png_get_IHDR(png_ptr, info_ptr, &width, &height,
|
||
|
|
&bit_depth, &color_type, &interlace_type,
|
||
|
|
&compression_type, &filter_method);
|
||
|
|
|
||
|
|
width - holds the width of the image
|
||
|
|
in pixels (up to 2^31).
|
||
|
|
|
||
|
|
height - holds the height of the image
|
||
|
|
in pixels (up to 2^31).
|
||
|
|
|
||
|
|
bit_depth - holds the bit depth of one of the
|
||
|
|
image channels. (valid values are
|
||
|
|
1, 2, 4, 8, 16 and depend also on
|
||
|
|
the color_type. See also
|
||
|
|
significant bits (sBIT) below).
|
||
|
|
|
||
|
|
color_type - describes which color/alpha channels
|
||
|
|
are present.
|
||
|
|
PNG_COLOR_TYPE_GRAY
|
||
|
|
(bit depths 1, 2, 4, 8, 16)
|
||
|
|
PNG_COLOR_TYPE_GRAY_ALPHA
|
||
|
|
(bit depths 8, 16)
|
||
|
|
PNG_COLOR_TYPE_PALETTE
|
||
|
|
(bit depths 1, 2, 4, 8)
|
||
|
|
PNG_COLOR_TYPE_RGB
|
||
|
|
(bit_depths 8, 16)
|
||
|
|
PNG_COLOR_TYPE_RGB_ALPHA
|
||
|
|
(bit_depths 8, 16)
|
||
|
|
|
||
|
|
PNG_COLOR_MASK_PALETTE
|
||
|
|
PNG_COLOR_MASK_COLOR
|
||
|
|
PNG_COLOR_MASK_ALPHA
|
||
|
|
|
||
|
|
interlace_type - (PNG_INTERLACE_NONE or
|
||
|
|
PNG_INTERLACE_ADAM7)
|
||
|
|
|
||
|
|
compression_type - (must be PNG_COMPRESSION_TYPE_BASE
|
||
|
|
for PNG 1.0)
|
||
|
|
|
||
|
|
filter_method - (must be PNG_FILTER_TYPE_BASE
|
||
|
|
for PNG 1.0, and can also be
|
||
|
|
PNG_INTRAPIXEL_DIFFERENCING if
|
||
|
|
the PNG datastream is embedded in
|
||
|
|
a MNG-1.0 datastream)
|
||
|
|
|
||
|
|
Any or all of interlace_type, compression_type, or
|
||
|
|
filter_method can be NULL if you are
|
||
|
|
not interested in their values.
|
||
|
|
|
||
|
|
Note that png_get_IHDR() returns 32-bit data into
|
||
|
|
the application's width and height variables.
|
||
|
|
This is an unsafe situation if these are 16-bit
|
||
|
|
variables. In such situations, the
|
||
|
|
png_get_image_width() and png_get_image_height()
|
||
|
|
functions described below are safer.
|
||
|
|
|
||
|
|
width = png_get_image_width(png_ptr,
|
||
|
|
info_ptr);
|
||
|
|
|
||
|
|
height = png_get_image_height(png_ptr,
|
||
|
|
info_ptr);
|
||
|
|
|
||
|
|
bit_depth = png_get_bit_depth(png_ptr,
|
||
|
|
info_ptr);
|
||
|
|
|
||
|
|
color_type = png_get_color_type(png_ptr,
|
||
|
|
info_ptr);
|
||
|
|
|
||
|
|
interlace_type = png_get_interlace_type(png_ptr,
|
||
|
|
info_ptr);
|
||
|
|
|
||
|
|
compression_type = png_get_compression_type(png_ptr,
|
||
|
|
info_ptr);
|
||
|
|
|
||
|
|
filter_method = png_get_filter_type(png_ptr,
|
||
|
|
info_ptr);
|
||
|
|
|
||
|
|
channels = png_get_channels(png_ptr, info_ptr);
|
||
|
|
|
||
|
|
channels - number of channels of info for the
|
||
|
|
color type (valid values are 1 (GRAY,
|
||
|
|
PALETTE), 2 (GRAY_ALPHA), 3 (RGB),
|
||
|
|
4 (RGB_ALPHA or RGB + filler byte))
|
||
|
|
|
||
|
|
rowbytes = png_get_rowbytes(png_ptr, info_ptr);
|
||
|
|
|
||
|
|
rowbytes - number of bytes needed to hold a row
|
||
|
|
|
||
|
|
signature = png_get_signature(png_ptr, info_ptr);
|
||
|
|
|
||
|
|
signature - holds the signature read from the
|
||
|
|
file (if any). The data is kept in
|
||
|
|
the same offset it would be if the
|
||
|
|
whole signature were read (i.e. if an
|
||
|
|
application had already read in 4
|
||
|
|
bytes of signature before starting
|
||
|
|
libpng, the remaining 4 bytes would
|
||
|
|
be in signature[4] through signature[7]
|
||
|
|
(see png_set_sig_bytes())).
|
||
|
|
|
||
|
|
These are also important, but their validity depends on whether the chunk
|
||
|
|
has been read. The png_get_valid(png_ptr, info_ptr, PNG_INFO_<chunk>) and
|
||
|
|
png_get_<chunk>(png_ptr, info_ptr, ...) functions return non-zero if the
|
||
|
|
data has been read, or zero if it is missing. The parameters to the
|
||
|
|
png_get_<chunk> are set directly if they are simple data types, or a
|
||
|
|
pointer into the info_ptr is returned for any complex types.
|
||
|
|
|
||
|
|
png_get_PLTE(png_ptr, info_ptr, &palette,
|
||
|
|
&num_palette);
|
||
|
|
|
||
|
|
palette - the palette for the file
|
||
|
|
(array of png_color)
|
||
|
|
|
||
|
|
num_palette - number of entries in the palette
|
||
|
|
|
||
|
|
png_get_gAMA(png_ptr, info_ptr, &file_gamma);
|
||
|
|
png_get_gAMA_fixed(png_ptr, info_ptr, &int_file_gamma);
|
||
|
|
|
||
|
|
file_gamma - the gamma at which the file is
|
||
|
|
written (PNG_INFO_gAMA)
|
||
|
|
|
||
|
|
int_file_gamma - 100,000 times the gamma at which the
|
||
|
|
file is written
|
||
|
|
|
||
|
|
png_get_cHRM(png_ptr, info_ptr, &white_x, &white_y, &red_x, &red_y,
|
||
|
|
&green_x, &green_y, &blue_x, &blue_y)
|
||
|
|
png_get_cHRM_XYZ(png_ptr, info_ptr, &red_X, &red_Y, &red_Z, &green_X,
|
||
|
|
&green_Y, &green_Z, &blue_X, &blue_Y, &blue_Z)
|
||
|
|
png_get_cHRM_fixed(png_ptr, info_ptr, &int_white_x, &int_white_y,
|
||
|
|
&int_red_x, &int_red_y, &int_green_x, &int_green_y,
|
||
|
|
&int_blue_x, &int_blue_y)
|
||
|
|
png_get_cHRM_XYZ_fixed(png_ptr, info_ptr, &int_red_X, &int_red_Y,
|
||
|
|
&int_red_Z, &int_green_X, &int_green_Y, &int_green_Z,
|
||
|
|
&int_blue_X, &int_blue_Y, &int_blue_Z)
|
||
|
|
|
||
|
|
{white,red,green,blue}_{x,y}
|
||
|
|
A color space encoding specified using the chromaticities
|
||
|
|
of the end points and the white point. (PNG_INFO_cHRM)
|
||
|
|
|
||
|
|
{red,green,blue}_{X,Y,Z}
|
||
|
|
A color space encoding specified using the encoding end
|
||
|
|
points - the CIE tristimulus specification of the intended
|
||
|
|
color of the red, green and blue channels in the PNG RGB
|
||
|
|
data. The white point is simply the sum of the three end
|
||
|
|
points. (PNG_INFO_cHRM)
|
||
|
|
|
||
|
|
png_get_sRGB(png_ptr, info_ptr, &srgb_intent);
|
||
|
|
|
||
|
|
file_srgb_intent - the rendering intent (PNG_INFO_sRGB)
|
||
|
|
The presence of the sRGB chunk
|
||
|
|
means that the pixel data is in the
|
||
|
|
sRGB color space. This chunk also
|
||
|
|
implies specific values of gAMA and
|
||
|
|
cHRM.
|
||
|
|
|
||
|
|
png_get_iCCP(png_ptr, info_ptr, &name,
|
||
|
|
&compression_type, &profile, &proflen);
|
||
|
|
|
||
|
|
name - The profile name.
|
||
|
|
|
||
|
|
compression_type - The compression type; always
|
||
|
|
PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
|
||
|
|
You may give NULL to this argument to
|
||
|
|
ignore it.
|
||
|
|
|
||
|
|
profile - International Color Consortium color
|
||
|
|
profile data. May contain NULs.
|
||
|
|
|
||
|
|
proflen - length of profile data in bytes.
|
||
|
|
|
||
|
|
png_get_sBIT(png_ptr, info_ptr, &sig_bit);
|
||
|
|
|
||
|
|
sig_bit - the number of significant bits for
|
||
|
|
(PNG_INFO_sBIT) each of the gray,
|
||
|
|
red, green, and blue channels,
|
||
|
|
whichever are appropriate for the
|
||
|
|
given color type (png_color_16)
|
||
|
|
|
||
|
|
png_get_tRNS(png_ptr, info_ptr, &trans_alpha,
|
||
|
|
&num_trans, &trans_color);
|
||
|
|
|
||
|
|
trans_alpha - array of alpha (transparency)
|
||
|
|
entries for palette (PNG_INFO_tRNS)
|
||
|
|
|
||
|
|
num_trans - number of transparent entries
|
||
|
|
(PNG_INFO_tRNS)
|
||
|
|
|
||
|
|
trans_color - graylevel or color sample values of
|
||
|
|
the single transparent color for
|
||
|
|
non-paletted images (PNG_INFO_tRNS)
|
||
|
|
|
||
|
|
png_get_hIST(png_ptr, info_ptr, &hist);
|
||
|
|
(PNG_INFO_hIST)
|
||
|
|
|
||
|
|
hist - histogram of palette (array of
|
||
|
|
png_uint_16)
|
||
|
|
|
||
|
|
png_get_tIME(png_ptr, info_ptr, &mod_time);
|
||
|
|
|
||
|
|
mod_time - time image was last modified
|
||
|
|
(PNG_VALID_tIME)
|
||
|
|
|
||
|
|
png_get_bKGD(png_ptr, info_ptr, &background);
|
||
|
|
|
||
|
|
background - background color (of type
|
||
|
|
png_color_16p) (PNG_VALID_bKGD)
|
||
|
|
valid 16-bit red, green and blue
|
||
|
|
values, regardless of color_type
|
||
|
|
|
||
|
|
num_comments = png_get_text(png_ptr, info_ptr,
|
||
|
|
&text_ptr, &num_text);
|
||
|
|
|
||
|
|
num_comments - number of comments
|
||
|
|
|
||
|
|
text_ptr - array of png_text holding image
|
||
|
|
comments
|
||
|
|
|
||
|
|
text_ptr[i].compression - type of compression used
|
||
|
|
on "text" PNG_TEXT_COMPRESSION_NONE
|
||
|
|
PNG_TEXT_COMPRESSION_zTXt
|
||
|
|
PNG_ITXT_COMPRESSION_NONE
|
||
|
|
PNG_ITXT_COMPRESSION_zTXt
|
||
|
|
|
||
|
|
text_ptr[i].key - keyword for comment. Must contain
|
||
|
|
1-79 characters.
|
||
|
|
|
||
|
|
text_ptr[i].text - text comments for current
|
||
|
|
keyword. Can be empty.
|
||
|
|
|
||
|
|
text_ptr[i].text_length - length of text string,
|
||
|
|
after decompression, 0 for iTXt
|
||
|
|
|
||
|
|
text_ptr[i].itxt_length - length of itxt string,
|
||
|
|
after decompression, 0 for tEXt/zTXt
|
||
|
|
|
||
|
|
text_ptr[i].lang - language of comment (empty
|
||
|
|
string for unknown).
|
||
|
|
|
||
|
|
text_ptr[i].lang_key - keyword in UTF-8
|
||
|
|
(empty string for unknown).
|
||
|
|
|
||
|
|
Note that the itxt_length, lang, and lang_key
|
||
|
|
members of the text_ptr structure only exist when the
|
||
|
|
library is built with iTXt chunk support. Prior to
|
||
|
|
libpng-1.4.0 the library was built by default without
|
||
|
|
iTXt support. Also note that when iTXt is supported,
|
||
|
|
they contain NULL pointers when the "compression"
|
||
|
|
field contains PNG_TEXT_COMPRESSION_NONE or
|
||
|
|
PNG_TEXT_COMPRESSION_zTXt.
|
||
|
|
|
||
|
|
num_text - number of comments (same as
|
||
|
|
num_comments; you can put NULL here
|
||
|
|
to avoid the duplication)
|
||
|
|
|
||
|
|
Note while png_set_text() will accept text, language,
|
||
|
|
and translated keywords that can be NULL pointers, the
|
||
|
|
structure returned by png_get_text will always contain
|
||
|
|
regular zero-terminated C strings. They might be
|
||
|
|
empty strings but they will never be NULL pointers.
|
||
|
|
|
||
|
|
num_spalettes = png_get_sPLT(png_ptr, info_ptr,
|
||
|
|
&palette_ptr);
|
||
|
|
|
||
|
|
num_spalettes - number of sPLT chunks read.
|
||
|
|
|
||
|
|
palette_ptr - array of palette structures holding
|
||
|
|
contents of one or more sPLT chunks
|
||
|
|
read.
|
||
|
|
|
||
|
|
png_get_oFFs(png_ptr, info_ptr, &offset_x, &offset_y,
|
||
|
|
&unit_type);
|
||
|
|
|
||
|
|
offset_x - positive offset from the left edge
|
||
|
|
of the screen (can be negative)
|
||
|
|
|
||
|
|
offset_y - positive offset from the top edge
|
||
|
|
of the screen (can be negative)
|
||
|
|
|
||
|
|
unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER
|
||
|
|
|
||
|
|
png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y,
|
||
|
|
&unit_type);
|
||
|
|
|
||
|
|
res_x - pixels/unit physical resolution in
|
||
|
|
x direction
|
||
|
|
|
||
|
|
res_y - pixels/unit physical resolution in
|
||
|
|
x direction
|
||
|
|
|
||
|
|
unit_type - PNG_RESOLUTION_UNKNOWN,
|
||
|
|
PNG_RESOLUTION_METER
|
||
|
|
|
||
|
|
png_get_sCAL(png_ptr, info_ptr, &unit, &width,
|
||
|
|
&height)
|
||
|
|
|
||
|
|
unit - physical scale units (an integer)
|
||
|
|
|
||
|
|
width - width of a pixel in physical scale units
|
||
|
|
|
||
|
|
height - height of a pixel in physical scale units
|
||
|
|
(width and height are doubles)
|
||
|
|
|
||
|
|
png_get_sCAL_s(png_ptr, info_ptr, &unit, &width,
|
||
|
|
&height)
|
||
|
|
|
||
|
|
unit - physical scale units (an integer)
|
||
|
|
|
||
|
|
width - width of a pixel in physical scale units
|
||
|
|
(expressed as a string)
|
||
|
|
|
||
|
|
height - height of a pixel in physical scale units
|
||
|
|
(width and height are strings like "2.54")
|
||
|
|
|
||
|
|
num_unknown_chunks = png_get_unknown_chunks(png_ptr,
|
||
|
|
info_ptr, &unknowns)
|
||
|
|
|
||
|
|
unknowns - array of png_unknown_chunk
|
||
|
|
structures holding unknown chunks
|
||
|
|
|
||
|
|
unknowns[i].name - name of unknown chunk
|
||
|
|
|
||
|
|
unknowns[i].data - data of unknown chunk
|
||
|
|
|
||
|
|
unknowns[i].size - size of unknown chunk's data
|
||
|
|
|
||
|
|
unknowns[i].location - position of chunk in file
|
||
|
|
|
||
|
|
The value of "i" corresponds to the order in which the
|
||
|
|
chunks were read from the PNG file or inserted with the
|
||
|
|
png_set_unknown_chunks() function.
|
||
|
|
|
||
|
|
The value of "location" is a bitwise "or" of
|
||
|
|
|
||
|
|
PNG_HAVE_IHDR (0x01)
|
||
|
|
PNG_HAVE_PLTE (0x02)
|
||
|
|
PNG_AFTER_IDAT (0x08)
|
||
|
|
|
||
|
|
The data from the pHYs chunk can be retrieved in several convenient
|
||
|
|
forms:
|
||
|
|
|
||
|
|
res_x = png_get_x_pixels_per_meter(png_ptr,
|
||
|
|
info_ptr)
|
||
|
|
|
||
|
|
res_y = png_get_y_pixels_per_meter(png_ptr,
|
||
|
|
info_ptr)
|
||
|
|
|
||
|
|
res_x_and_y = png_get_pixels_per_meter(png_ptr,
|
||
|
|
info_ptr)
|
||
|
|
|
||
|
|
res_x = png_get_x_pixels_per_inch(png_ptr,
|
||
|
|
info_ptr)
|
||
|
|
|
||
|
|
res_y = png_get_y_pixels_per_inch(png_ptr,
|
||
|
|
info_ptr)
|
||
|
|
|
||
|
|
res_x_and_y = png_get_pixels_per_inch(png_ptr,
|
||
|
|
info_ptr)
|
||
|
|
|
||
|
|
aspect_ratio = png_get_pixel_aspect_ratio(png_ptr,
|
||
|
|
info_ptr)
|
||
|
|
|
||
|
|
Each of these returns 0 [signifying "unknown"] if
|
||
|
|
the data is not present or if res_x is 0;
|
||
|
|
res_x_and_y is 0 if res_x != res_y
|
||
|
|
|
||
|
|
Note that because of the way the resolutions are
|
||
|
|
stored internally, the inch conversions won't
|
||
|
|
come out to exactly even number. For example,
|
||
|
|
72 dpi is stored as 0.28346 pixels/meter, and
|
||
|
|
when this is retrieved it is 71.9988 dpi, so
|
||
|
|
be sure to round the returned value appropriately
|
||
|
|
if you want to display a reasonable-looking result.
|
||
|
|
|
||
|
|
The data from the oFFs chunk can be retrieved in several convenient
|
||
|
|
forms:
|
||
|
|
|
||
|
|
x_offset = png_get_x_offset_microns(png_ptr, info_ptr);
|
||
|
|
|
||
|
|
y_offset = png_get_y_offset_microns(png_ptr, info_ptr);
|
||
|
|
|
||
|
|
x_offset = png_get_x_offset_inches(png_ptr, info_ptr);
|
||
|
|
|
||
|
|
y_offset = png_get_y_offset_inches(png_ptr, info_ptr);
|
||
|
|
|
||
|
|
Each of these returns 0 [signifying "unknown" if both
|
||
|
|
x and y are 0] if the data is not present or if the
|
||
|
|
chunk is present but the unit is the pixel. The
|
||
|
|
remark about inexact inch conversions applies here
|
||
|
|
as well, because a value in inches can't always be
|
||
|
|
converted to microns and back without some loss
|
||
|
|
of precision.
|
||
|
|
|
||
|
|
For more information, see the
|
||
|
|
PNG specification for chunk contents. Be careful with trusting
|
||
|
|
rowbytes, as some of the transformations could increase the space
|
||
|
|
needed to hold a row (expand, filler, gray_to_rgb, etc.).
|
||
|
|
See png_read_update_info(), below.
|
||
|
|
|
||
|
|
A quick word about text_ptr and num_text. PNG stores comments in
|
||
|
|
keyword/text pairs, one pair per chunk, with no limit on the number
|
||
|
|
of text chunks, and a 2^31 byte limit on their size. While there are
|
||
|
|
suggested keywords, there is no requirement to restrict the use to these
|
||
|
|
strings. It is strongly suggested that keywords and text be sensible
|
||
|
|
to humans (that's the point), so don't use abbreviations. Non-printing
|
||
|
|
symbols are not allowed. See the PNG specification for more details.
|
||
|
|
There is also no requirement to have text after the keyword.
|
||
|
|
|
||
|
|
Keywords should be limited to 79 Latin-1 characters without leading or
|
||
|
|
trailing spaces, but non-consecutive spaces are allowed within the
|
||
|
|
keyword. It is possible to have the same keyword any number of times.
|
||
|
|
The text_ptr is an array of png_text structures, each holding a
|
||
|
|
pointer to a language string, a pointer to a keyword and a pointer to
|
||
|
|
a text string. The text string, language code, and translated
|
||
|
|
keyword may be empty or NULL pointers. The keyword/text
|
||
|
|
pairs are put into the array in the order that they are received.
|
||
|
|
However, some or all of the text chunks may be after the image, so, to
|
||
|
|
make sure you have read all the text chunks, don't mess with these
|
||
|
|
until after you read the stuff after the image. This will be
|
||
|
|
mentioned again below in the discussion that goes with png_read_end().
|
||
|
|
|
||
|
|
Input transformations
|
||
|
|
|
||
|
|
After you've read the header information, you can set up the library
|
||
|
|
to handle any special transformations of the image data. The various
|
||
|
|
ways to transform the data will be described in the order that they
|
||
|
|
should occur. This is important, as some of these change the color
|
||
|
|
type and/or bit depth of the data, and some others only work on
|
||
|
|
certain color types and bit depths.
|
||
|
|
|
||
|
|
Transformations you request are ignored if they don't have any meaning for a
|
||
|
|
particular input data format. However some transformations can have an effect
|
||
|
|
as a result of a previous transformation. If you specify a contradictory set of
|
||
|
|
transformations, for example both adding and removing the alpha channel, you
|
||
|
|
cannot predict the final result.
|
||
|
|
|
||
|
|
The color used for the transparency values should be supplied in the same
|
||
|
|
format/depth as the current image data. It is stored in the same format/depth
|
||
|
|
as the image data in a tRNS chunk, so this is what libpng expects for this data.
|
||
|
|
|
||
|
|
The color used for the background value depends on the need_expand argument as
|
||
|
|
described below.
|
||
|
|
|
||
|
|
Data will be decoded into the supplied row buffers packed into bytes
|
||
|
|
unless the library has been told to transform it into another format.
|
||
|
|
For example, 4 bit/pixel paletted or grayscale data will be returned
|
||
|
|
2 pixels/byte with the leftmost pixel in the high-order bits of the
|
||
|
|
byte, unless png_set_packing() is called. 8-bit RGB data will be stored
|
||
|
|
in RGB RGB RGB format unless png_set_filler() or png_set_add_alpha()
|
||
|
|
is called to insert filler bytes, either before or after each RGB triplet.
|
||
|
|
16-bit RGB data will be returned RRGGBB RRGGBB, with the most significant
|
||
|
|
byte of the color value first, unless png_set_scale_16() is called to
|
||
|
|
transform it to regular RGB RGB triplets, or png_set_filler() or
|
||
|
|
png_set_add alpha() is called to insert filler bytes, either before or
|
||
|
|
after each RRGGBB triplet. Similarly, 8-bit or 16-bit grayscale data can
|
||
|
|
be modified with png_set_filler(), png_set_add_alpha(), png_set_strip_16(),
|
||
|
|
or png_set_scale_16().
|
||
|
|
|
||
|
|
The following code transforms grayscale images of less than 8 to 8 bits,
|
||
|
|
changes paletted images to RGB, and adds a full alpha channel if there is
|
||
|
|
transparency information in a tRNS chunk. This is most useful on
|
||
|
|
grayscale images with bit depths of 2 or 4 or if there is a multiple-image
|
||
|
|
viewing application that wishes to treat all images in the same way.
|
||
|
|
|
||
|
|
if (color_type == PNG_COLOR_TYPE_PALETTE)
|
||
|
|
png_set_palette_to_rgb(png_ptr);
|
||
|
|
|
||
|
|
if (png_get_valid(png_ptr, info_ptr,
|
||
|
|
PNG_INFO_tRNS)) png_set_tRNS_to_alpha(png_ptr);
|
||
|
|
|
||
|
|
if (color_type == PNG_COLOR_TYPE_GRAY &&
|
||
|
|
bit_depth < 8) png_set_expand_gray_1_2_4_to_8(png_ptr);
|
||
|
|
|
||
|
|
The first two functions are actually aliases for png_set_expand(), added
|
||
|
|
in libpng version 1.0.4, with the function names expanded to improve code
|
||
|
|
readability. In some future version they may actually do different
|
||
|
|
things.
|
||
|
|
|
||
|
|
As of libpng version 1.2.9, png_set_expand_gray_1_2_4_to_8() was
|
||
|
|
added. It expands the sample depth without changing tRNS to alpha.
|
||
|
|
|
||
|
|
As of libpng version 1.5.2, png_set_expand_16() was added. It behaves as
|
||
|
|
png_set_expand(); however, the resultant channels have 16 bits rather than 8.
|
||
|
|
Use this when the output color or gray channels are made linear to avoid fairly
|
||
|
|
severe accuracy loss.
|
||
|
|
|
||
|
|
if (bit_depth < 16)
|
||
|
|
png_set_expand_16(png_ptr);
|
||
|
|
|
||
|
|
PNG can have files with 16 bits per channel. If you only can handle
|
||
|
|
8 bits per channel, this will strip the pixels down to 8-bit.
|
||
|
|
|
||
|
|
if (bit_depth == 16)
|
||
|
|
#if PNG_LIBPNG_VER >= 10504
|
||
|
|
png_set_scale_16(png_ptr);
|
||
|
|
#else
|
||
|
|
png_set_strip_16(png_ptr);
|
||
|
|
#endif
|
||
|
|
|
||
|
|
(The more accurate "png_set_scale_16()" API became available in libpng version
|
||
|
|
1.5.4).
|
||
|
|
|
||
|
|
If you need to process the alpha channel on the image separately from the image
|
||
|
|
data (for example if you convert it to a bitmap mask) it is possible to have
|
||
|
|
libpng strip the channel leaving just RGB or gray data:
|
||
|
|
|
||
|
|
if (color_type & PNG_COLOR_MASK_ALPHA)
|
||
|
|
png_set_strip_alpha(png_ptr);
|
||
|
|
|
||
|
|
If you strip the alpha channel you need to find some other way of dealing with
|
||
|
|
the information. If, instead, you want to convert the image to an opaque
|
||
|
|
version with no alpha channel use png_set_background; see below.
|
||
|
|
|
||
|
|
As of libpng version 1.5.2, almost all useful expansions are supported, the
|
||
|
|
major ommissions are conversion of grayscale to indexed images (which can be
|
||
|
|
done trivially in the application) and conversion of indexed to grayscale (which
|
||
|
|
can be done by a trivial manipulation of the palette.)
|
||
|
|
|
||
|
|
In the following table, the 01 means grayscale with depth<8, 31 means
|
||
|
|
indexed with depth<8, other numerals represent the color type, "T" means
|
||
|
|
the tRNS chunk is present, A means an alpha channel is present, and O
|
||
|
|
means tRNS or alpha is present but all pixels in the image are opaque.
|
||
|
|
|
||
|
|
FROM 01 31 0 0T 0O 2 2T 2O 3 3T 3O 4A 4O 6A 6O
|
||
|
|
TO
|
||
|
|
01 - [G] - - - - - - - - - - - - -
|
||
|
|
31 [Q] Q [Q] [Q] [Q] Q Q Q Q Q Q [Q] [Q] Q Q
|
||
|
|
0 1 G + . . G G G G G G B B GB GB
|
||
|
|
0T lt Gt t + . Gt G G Gt G G Bt Bt GBt GBt
|
||
|
|
0O lt Gt t . + Gt Gt G Gt Gt G Bt Bt GBt GBt
|
||
|
|
2 C P C C C + . . C - - CB CB B B
|
||
|
|
2T Ct - Ct C C t + t - - - CBt CBt Bt Bt
|
||
|
|
2O Ct - Ct C C t t + - - - CBt CBt Bt Bt
|
||
|
|
3 [Q] p [Q] [Q] [Q] Q Q Q + . . [Q] [Q] Q Q
|
||
|
|
3T [Qt] p [Qt][Q] [Q] Qt Qt Qt t + t [Qt][Qt] Qt Qt
|
||
|
|
3O [Qt] p [Qt][Q] [Q] Qt Qt Qt t t + [Qt][Qt] Qt Qt
|
||
|
|
4A lA G A T T GA GT GT GA GT GT + BA G GBA
|
||
|
|
4O lA GBA A T T GA GT GT GA GT GT BA + GBA G
|
||
|
|
6A CA PA CA C C A T tT PA P P C CBA + BA
|
||
|
|
6O CA PBA CA C C A tT T PA P P CBA C BA +
|
||
|
|
|
||
|
|
Within the matrix,
|
||
|
|
"+" identifies entries where 'from' and 'to' are the same.
|
||
|
|
"-" means the transformation is not supported.
|
||
|
|
"." means nothing is necessary (a tRNS chunk can just be ignored).
|
||
|
|
"t" means the transformation is obtained by png_set_tRNS.
|
||
|
|
"A" means the transformation is obtained by png_set_add_alpha().
|
||
|
|
"X" means the transformation is obtained by png_set_expand().
|
||
|
|
"1" means the transformation is obtained by
|
||
|
|
png_set_expand_gray_1_2_4_to_8() (and by png_set_expand() if there
|
||
|
|
is no transparency in the original or the final format).
|
||
|
|
"C" means the transformation is obtained by png_set_gray_to_rgb().
|
||
|
|
"G" means the transformation is obtained by png_set_rgb_to_gray().
|
||
|
|
"P" means the transformation is obtained by
|
||
|
|
png_set_expand_palette_to_rgb().
|
||
|
|
"p" means the transformation is obtained by png_set_packing().
|
||
|
|
"Q" means the transformation is obtained by png_set_quantize().
|
||
|
|
"T" means the transformation is obtained by png_set_tRNS_to_alpha().
|
||
|
|
"B" means the transformation is obtained by png_set_background(), or
|
||
|
|
png_strip_alpha().
|
||
|
|
|
||
|
|
When an entry has multiple transforms listed all are required to cause the
|
||
|
|
right overall transformation. When two transforms are separated by a comma
|
||
|
|
either will do the job. When transforms are enclosed in [] the transform should
|
||
|
|
do the job but this is currently unimplemented - a different format will result
|
||
|
|
if the suggested transformations are used.
|
||
|
|
|
||
|
|
In PNG files, the alpha channel in an image
|
||
|
|
is the level of opacity. If you need the alpha channel in an image to
|
||
|
|
be the level of transparency instead of opacity, you can invert the
|
||
|
|
alpha channel (or the tRNS chunk data) after it's read, so that 0 is
|
||
|
|
fully opaque and 255 (in 8-bit or paletted images) or 65535 (in 16-bit
|
||
|
|
images) is fully transparent, with
|
||
|
|
|
||
|
|
png_set_invert_alpha(png_ptr);
|
||
|
|
|
||
|
|
PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
|
||
|
|
they can, resulting in, for example, 8 pixels per byte for 1 bit
|
||
|
|
files. This code expands to 1 pixel per byte without changing the
|
||
|
|
values of the pixels:
|
||
|
|
|
||
|
|
if (bit_depth < 8)
|
||
|
|
png_set_packing(png_ptr);
|
||
|
|
|
||
|
|
PNG files have possible bit depths of 1, 2, 4, 8, and 16. All pixels
|
||
|
|
stored in a PNG image have been "scaled" or "shifted" up to the next
|
||
|
|
higher possible bit depth (e.g. from 5 bits/sample in the range [0,31]
|
||
|
|
to 8 bits/sample in the range [0, 255]). However, it is also possible
|
||
|
|
to convert the PNG pixel data back to the original bit depth of the
|
||
|
|
image. This call reduces the pixels back down to the original bit depth:
|
||
|
|
|
||
|
|
png_color_8p sig_bit;
|
||
|
|
|
||
|
|
if (png_get_sBIT(png_ptr, info_ptr, &sig_bit))
|
||
|
|
png_set_shift(png_ptr, sig_bit);
|
||
|
|
|
||
|
|
PNG files store 3-color pixels in red, green, blue order. This code
|
||
|
|
changes the storage of the pixels to blue, green, red:
|
||
|
|
|
||
|
|
if (color_type == PNG_COLOR_TYPE_RGB ||
|
||
|
|
color_type == PNG_COLOR_TYPE_RGB_ALPHA)
|
||
|
|
png_set_bgr(png_ptr);
|
||
|
|
|
||
|
|
PNG files store RGB pixels packed into 3 or 6 bytes. This code expands them
|
||
|
|
into 4 or 8 bytes for windowing systems that need them in this format:
|
||
|
|
|
||
|
|
if (color_type == PNG_COLOR_TYPE_RGB)
|
||
|
|
png_set_filler(png_ptr, filler, PNG_FILLER_BEFORE);
|
||
|
|
|
||
|
|
where "filler" is the 8 or 16-bit number to fill with, and the location is
|
||
|
|
either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether
|
||
|
|
you want the filler before the RGB or after. This transformation
|
||
|
|
does not affect images that already have full alpha channels. To add an
|
||
|
|
opaque alpha channel, use filler=0xff or 0xffff and PNG_FILLER_AFTER which
|
||
|
|
will generate RGBA pixels.
|
||
|
|
|
||
|
|
Note that png_set_filler() does not change the color type. If you want
|
||
|
|
to do that, you can add a true alpha channel with
|
||
|
|
|
||
|
|
if (color_type == PNG_COLOR_TYPE_RGB ||
|
||
|
|
color_type == PNG_COLOR_TYPE_GRAY)
|
||
|
|
png_set_add_alpha(png_ptr, filler, PNG_FILLER_AFTER);
|
||
|
|
|
||
|
|
where "filler" contains the alpha value to assign to each pixel.
|
||
|
|
This function was added in libpng-1.2.7.
|
||
|
|
|
||
|
|
If you are reading an image with an alpha channel, and you need the
|
||
|
|
data as ARGB instead of the normal PNG format RGBA:
|
||
|
|
|
||
|
|
if (color_type == PNG_COLOR_TYPE_RGB_ALPHA)
|
||
|
|
png_set_swap_alpha(png_ptr);
|
||
|
|
|
||
|
|
For some uses, you may want a grayscale image to be represented as
|
||
|
|
RGB. This code will do that conversion:
|
||
|
|
|
||
|
|
if (color_type == PNG_COLOR_TYPE_GRAY ||
|
||
|
|
color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
|
||
|
|
png_set_gray_to_rgb(png_ptr);
|
||
|
|
|
||
|
|
Conversely, you can convert an RGB or RGBA image to grayscale or grayscale
|
||
|
|
with alpha.
|
||
|
|
|
||
|
|
if (color_type == PNG_COLOR_TYPE_RGB ||
|
||
|
|
color_type == PNG_COLOR_TYPE_RGB_ALPHA)
|
||
|
|
png_set_rgb_to_gray(png_ptr, error_action, double red_weight,
|
||
|
|
double green_weight);
|
||
|
|
|
||
|
|
error_action = 1: silently do the conversion
|
||
|
|
|
||
|
|
error_action = 2: issue a warning if the original
|
||
|
|
image has any pixel where
|
||
|
|
red != green or red != blue
|
||
|
|
|
||
|
|
error_action = 3: issue an error and abort the
|
||
|
|
conversion if the original
|
||
|
|
image has any pixel where
|
||
|
|
red != green or red != blue
|
||
|
|
|
||
|
|
red_weight: weight of red component
|
||
|
|
|
||
|
|
green_weight: weight of green component
|
||
|
|
If either weight is negative, default
|
||
|
|
weights are used.
|
||
|
|
|
||
|
|
In the corresponding fixed point API the red_weight and green_weight values are
|
||
|
|
simply scaled by 100,000:
|
||
|
|
|
||
|
|
png_set_rgb_to_gray(png_ptr, error_action, png_fixed_point red_weight,
|
||
|
|
png_fixed_point green_weight);
|
||
|
|
|
||
|
|
If you have set error_action = 1 or 2, you can
|
||
|
|
later check whether the image really was gray, after processing
|
||
|
|
the image rows, with the png_get_rgb_to_gray_status(png_ptr) function.
|
||
|
|
It will return a png_byte that is zero if the image was gray or
|
||
|
|
1 if there were any non-gray pixels. Background and sBIT data
|
||
|
|
will be silently converted to grayscale, using the green channel
|
||
|
|
data for sBIT, regardless of the error_action setting.
|
||
|
|
|
||
|
|
The default values come from the PNG file cHRM chunk if present; otherwise, the
|
||
|
|
defaults correspond to the ITU-R recommendation 709, and also the sRGB color
|
||
|
|
space, as recommended in the Charles Poynton's Colour FAQ,
|
||
|
|
<http://www.poynton.com/>, in section 9:
|
||
|
|
|
||
|
|
<http://www.poynton.com/notes/colour_and_gamma/ColorFAQ.html#RTFToC9>
|
||
|
|
|
||
|
|
Y = 0.2126 * R + 0.7152 * G + 0.0722 * B
|
||
|
|
|
||
|
|
Previous versions of this document, 1998 through 2002, recommended a slightly
|
||
|
|
different formula:
|
||
|
|
|
||
|
|
Y = 0.212671 * R + 0.715160 * G + 0.072169 * B
|
||
|
|
|
||
|
|
Libpng uses an integer approximation:
|
||
|
|
|
||
|
|
Y = (6968 * R + 23434 * G + 2366 * B)/32768
|
||
|
|
|
||
|
|
The calculation is done in a linear colorspace, if the image gamma
|
||
|
|
can be determined.
|
||
|
|
|
||
|
|
The png_set_background() function has been described already; it tells libpng to
|
||
|
|
composite images with alpha or simple transparency against the supplied
|
||
|
|
background color. For compatibility with versions of libpng earlier than
|
||
|
|
libpng-1.5.4 it is recommended that you call the function after reading the file
|
||
|
|
header, even if you don't want to use the color in a bKGD chunk, if one exists.
|
||
|
|
|
||
|
|
If the PNG file contains a bKGD chunk (PNG_INFO_bKGD valid),
|
||
|
|
you may use this color, or supply another color more suitable for
|
||
|
|
the current display (e.g., the background color from a web page). You
|
||
|
|
need to tell libpng how the color is represented, both the format of the
|
||
|
|
component values in the color (the number of bits) and the gamma encoding of the
|
||
|
|
color. The function takes two arguments, background_gamma_mode and need_expand
|
||
|
|
to convey this information, however only two combinations are likely to be
|
||
|
|
useful:
|
||
|
|
|
||
|
|
png_color_16 my_background;
|
||
|
|
png_color_16p image_background;
|
||
|
|
|
||
|
|
if (png_get_bKGD(png_ptr, info_ptr, &image_background))
|
||
|
|
png_set_background(png_ptr, image_background,
|
||
|
|
PNG_BACKGROUND_GAMMA_FILE, 1/*needs to be expanded*/, 1);
|
||
|
|
else
|
||
|
|
png_set_background(png_ptr, &my_background,
|
||
|
|
PNG_BACKGROUND_GAMMA_SCREEN, 0/*do not expand*/, 1);
|
||
|
|
|
||
|
|
The second call was described above - my_background is in the format of the
|
||
|
|
final, display, output produced by libpng. Because you now know the format of
|
||
|
|
the PNG it is possible to avoid the need to choose either 8-bit or 16-bit
|
||
|
|
output and to retain palette images (the palette colors will be modified
|
||
|
|
appropriately and the tRNS chunk removed.) However, if you are doing this,
|
||
|
|
take great care not to ask for transformations without checking first that
|
||
|
|
they apply!
|
||
|
|
|
||
|
|
In the first call the background color has the original bit depth and color type
|
||
|
|
of the PNG file. So, for palette images the color is supplied as a palette
|
||
|
|
index and for low bit greyscale images the color is a reduced bit value in
|
||
|
|
image_background->gray.
|
||
|
|
|
||
|
|
If you didn't call png_set_gamma() before reading the file header, for example
|
||
|
|
if you need your code to remain compatible with older versions of libpng prior
|
||
|
|
to libpng-1.5.4, this is the place to call it.
|
||
|
|
|
||
|
|
Do not call it if you called png_set_alpha_mode(); doing so will damage the
|
||
|
|
settings put in place by png_set_alpha_mode(). (If png_set_alpha_mode() is
|
||
|
|
supported then you can certainly do png_set_gamma() before reading the PNG
|
||
|
|
header.)
|
||
|
|
|
||
|
|
This API unconditionally sets the screen and file gamma values, so it will
|
||
|
|
override the value in the PNG file unless it is called before the PNG file
|
||
|
|
reading starts. For this reason you must always call it with the PNG file
|
||
|
|
value when you call it in this position:
|
||
|
|
|
||
|
|
if (png_get_gAMA(png_ptr, info_ptr, &file_gamma))
|
||
|
|
png_set_gamma(png_ptr, screen_gamma, file_gamma);
|
||
|
|
|
||
|
|
else
|
||
|
|
png_set_gamma(png_ptr, screen_gamma, 0.45455);
|
||
|
|
|
||
|
|
If you need to reduce an RGB file to a paletted file, or if a paletted
|
||
|
|
file has more entries then will fit on your screen, png_set_quantize()
|
||
|
|
will do that. Note that this is a simple match quantization that merely
|
||
|
|
finds the closest color available. This should work fairly well with
|
||
|
|
optimized palettes, but fairly badly with linear color cubes. If you
|
||
|
|
pass a palette that is larger than maximum_colors, the file will
|
||
|
|
reduce the number of colors in the palette so it will fit into
|
||
|
|
maximum_colors. If there is a histogram, libpng will use it to make
|
||
|
|
more intelligent choices when reducing the palette. If there is no
|
||
|
|
histogram, it may not do as good a job.
|
||
|
|
|
||
|
|
if (color_type & PNG_COLOR_MASK_COLOR)
|
||
|
|
{
|
||
|
|
if (png_get_valid(png_ptr, info_ptr,
|
||
|
|
PNG_INFO_PLTE))
|
||
|
|
{
|
||
|
|
png_uint_16p histogram = NULL;
|
||
|
|
|
||
|
|
png_get_hIST(png_ptr, info_ptr,
|
||
|
|
&histogram);
|
||
|
|
png_set_quantize(png_ptr, palette, num_palette,
|
||
|
|
max_screen_colors, histogram, 1);
|
||
|
|
}
|
||
|
|
|
||
|
|
else
|
||
|
|
{
|
||
|
|
png_color std_color_cube[MAX_SCREEN_COLORS] =
|
||
|
|
{ ... colors ... };
|
||
|
|
|
||
|
|
png_set_quantize(png_ptr, std_color_cube,
|
||
|
|
MAX_SCREEN_COLORS, MAX_SCREEN_COLORS,
|
||
|
|
NULL,0);
|
||
|
|
}
|
||
|
|
}
|
||
|
|
|
||
|
|
PNG files describe monochrome as black being zero and white being one.
|
||
|
|
The following code will reverse this (make black be one and white be
|
||
|
|
zero):
|
||
|
|
|
||
|
|
if (bit_depth == 1 && color_type == PNG_COLOR_TYPE_GRAY)
|
||
|
|
png_set_invert_mono(png_ptr);
|
||
|
|
|
||
|
|
This function can also be used to invert grayscale and gray-alpha images:
|
||
|
|
|
||
|
|
if (color_type == PNG_COLOR_TYPE_GRAY ||
|
||
|
|
color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
|
||
|
|
png_set_invert_mono(png_ptr);
|
||
|
|
|
||
|
|
PNG files store 16-bit pixels in network byte order (big-endian,
|
||
|
|
ie. most significant bits first). This code changes the storage to the
|
||
|
|
other way (little-endian, i.e. least significant bits first, the
|
||
|
|
way PCs store them):
|
||
|
|
|
||
|
|
if (bit_depth == 16)
|
||
|
|
png_set_swap(png_ptr);
|
||
|
|
|
||
|
|
If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you
|
||
|
|
need to change the order the pixels are packed into bytes, you can use:
|
||
|
|
|
||
|
|
if (bit_depth < 8)
|
||
|
|
png_set_packswap(png_ptr);
|
||
|
|
|
||
|
|
Finally, you can write your own transformation function if none of
|
||
|
|
the existing ones meets your needs. This is done by setting a callback
|
||
|
|
with
|
||
|
|
|
||
|
|
png_set_read_user_transform_fn(png_ptr,
|
||
|
|
read_transform_fn);
|
||
|
|
|
||
|
|
You must supply the function
|
||
|
|
|
||
|
|
void read_transform_fn(png_structp png_ptr, png_row_infop
|
||
|
|
row_info, png_bytep data)
|
||
|
|
|
||
|
|
See pngtest.c for a working example. Your function will be called
|
||
|
|
after all of the other transformations have been processed. Take care with
|
||
|
|
interlaced images if you do the interlace yourself - the width of the row is the
|
||
|
|
width in 'row_info', not the overall image width.
|
||
|
|
|
||
|
|
If supported, libpng provides two information routines that you can use to find
|
||
|
|
where you are in processing the image:
|
||
|
|
|
||
|
|
png_get_current_pass_number(png_structp png_ptr);
|
||
|
|
png_get_current_row_number(png_structp png_ptr);
|
||
|
|
|
||
|
|
Don't try using these outside a transform callback - firstly they are only
|
||
|
|
supported if user transforms are supported, secondly they may well return
|
||
|
|
unexpected results unless the row is actually being processed at the moment they
|
||
|
|
are called.
|
||
|
|
|
||
|
|
With interlaced
|
||
|
|
images the value returned is the row in the input sub-image image. Use
|
||
|
|
PNG_ROW_FROM_PASS_ROW(row, pass) and PNG_COL_FROM_PASS_COL(col, pass) to
|
||
|
|
find the output pixel (x,y) given an interlaced sub-image pixel (row,col,pass).
|
||
|
|
|
||
|
|
The discussion of interlace handling above contains more information on how to
|
||
|
|
use these values.
|
||
|
|
|
||
|
|
You can also set up a pointer to a user structure for use by your
|
||
|
|
callback function, and you can inform libpng that your transform
|
||
|
|
function will change the number of channels or bit depth with the
|
||
|
|
function
|
||
|
|
|
||
|
|
png_set_user_transform_info(png_ptr, user_ptr,
|
||
|
|
user_depth, user_channels);
|
||
|
|
|
||
|
|
The user's application, not libpng, is responsible for allocating and
|
||
|
|
freeing any memory required for the user structure.
|
||
|
|
|
||
|
|
You can retrieve the pointer via the function
|
||
|
|
png_get_user_transform_ptr(). For example:
|
||
|
|
|
||
|
|
voidp read_user_transform_ptr =
|
||
|
|
png_get_user_transform_ptr(png_ptr);
|
||
|
|
|
||
|
|
The last thing to handle is interlacing; this is covered in detail below,
|
||
|
|
but you must call the function here if you want libpng to handle expansion
|
||
|
|
of the interlaced image.
|
||
|
|
|
||
|
|
number_of_passes = png_set_interlace_handling(png_ptr);
|
||
|
|
|
||
|
|
After setting the transformations, libpng can update your png_info
|
||
|
|
structure to reflect any transformations you've requested with this
|
||
|
|
call.
|
||
|
|
|
||
|
|
png_read_update_info(png_ptr, info_ptr);
|
||
|
|
|
||
|
|
This is most useful to update the info structure's rowbytes
|
||
|
|
field so you can use it to allocate your image memory. This function
|
||
|
|
will also update your palette with the correct screen_gamma and
|
||
|
|
background if these have been given with the calls above. You may
|
||
|
|
only call png_read_update_info() once with a particular info_ptr.
|
||
|
|
|
||
|
|
After you call png_read_update_info(), you can allocate any
|
||
|
|
memory you need to hold the image. The row data is simply
|
||
|
|
raw byte data for all forms of images. As the actual allocation
|
||
|
|
varies among applications, no example will be given. If you
|
||
|
|
are allocating one large chunk, you will need to build an
|
||
|
|
array of pointers to each row, as it will be needed for some
|
||
|
|
of the functions below.
|
||
|
|
|
||
|
|
Remember: Before you call png_read_update_info(), the png_get_*()
|
||
|
|
functions return the values corresponding to the original PNG image.
|
||
|
|
After you call png_read_update_info the values refer to the image
|
||
|
|
that libpng will output. Consequently you must call all the png_set_
|
||
|
|
functions before you call png_read_update_info(). This is particularly
|
||
|
|
important for png_set_interlace_handling() - if you are going to call
|
||
|
|
png_read_update_info() you must call png_set_interlace_handling() before
|
||
|
|
it unless you want to receive interlaced output.
|
||
|
|
|
||
|
|
Reading image data
|
||
|
|
|
||
|
|
After you've allocated memory, you can read the image data.
|
||
|
|
The simplest way to do this is in one function call. If you are
|
||
|
|
allocating enough memory to hold the whole image, you can just
|
||
|
|
call png_read_image() and libpng will read in all the image data
|
||
|
|
and put it in the memory area supplied. You will need to pass in
|
||
|
|
an array of pointers to each row.
|
||
|
|
|
||
|
|
This function automatically handles interlacing, so you don't
|
||
|
|
need to call png_set_interlace_handling() (unless you call
|
||
|
|
png_read_update_info()) or call this function multiple times, or any
|
||
|
|
of that other stuff necessary with png_read_rows().
|
||
|
|
|
||
|
|
png_read_image(png_ptr, row_pointers);
|
||
|
|
|
||
|
|
where row_pointers is:
|
||
|
|
|
||
|
|
png_bytep row_pointers[height];
|
||
|
|
|
||
|
|
You can point to void or char or whatever you use for pixels.
|
||
|
|
|
||
|
|
If you don't want to read in the whole image at once, you can
|
||
|
|
use png_read_rows() instead. If there is no interlacing (check
|
||
|
|
interlace_type == PNG_INTERLACE_NONE), this is simple:
|
||
|
|
|
||
|
|
png_read_rows(png_ptr, row_pointers, NULL,
|
||
|
|
number_of_rows);
|
||
|
|
|
||
|
|
where row_pointers is the same as in the png_read_image() call.
|
||
|
|
|
||
|
|
If you are doing this just one row at a time, you can do this with
|
||
|
|
a single row_pointer instead of an array of row_pointers:
|
||
|
|
|
||
|
|
png_bytep row_pointer = row;
|
||
|
|
png_read_row(png_ptr, row_pointer, NULL);
|
||
|
|
|
||
|
|
If the file is interlaced (interlace_type != 0 in the IHDR chunk), things
|
||
|
|
get somewhat harder. The only current (PNG Specification version 1.2)
|
||
|
|
interlacing type for PNG is (interlace_type == PNG_INTERLACE_ADAM7);
|
||
|
|
a somewhat complicated 2D interlace scheme, known as Adam7, that
|
||
|
|
breaks down an image into seven smaller images of varying size, based
|
||
|
|
on an 8x8 grid. This number is defined (from libpng 1.5) as
|
||
|
|
PNG_INTERLACE_ADAM7_PASSES in png.h
|
||
|
|
|
||
|
|
libpng can fill out those images or it can give them to you "as is".
|
||
|
|
It is almost always better to have libpng handle the interlacing for you.
|
||
|
|
If you want the images filled out, there are two ways to do that. The one
|
||
|
|
mentioned in the PNG specification is to expand each pixel to cover
|
||
|
|
those pixels that have not been read yet (the "rectangle" method).
|
||
|
|
This results in a blocky image for the first pass, which gradually
|
||
|
|
smooths out as more pixels are read. The other method is the "sparkle"
|
||
|
|
method, where pixels are drawn only in their final locations, with the
|
||
|
|
rest of the image remaining whatever colors they were initialized to
|
||
|
|
before the start of the read. The first method usually looks better,
|
||
|
|
but tends to be slower, as there are more pixels to put in the rows.
|
||
|
|
|
||
|
|
If, as is likely, you want libpng to expand the images, call this before
|
||
|
|
calling png_start_read_image() or png_read_update_info():
|
||
|
|
|
||
|
|
if (interlace_type == PNG_INTERLACE_ADAM7)
|
||
|
|
number_of_passes
|
||
|
|
= png_set_interlace_handling(png_ptr);
|
||
|
|
|
||
|
|
This will return the number of passes needed. Currently, this is seven,
|
||
|
|
but may change if another interlace type is added. This function can be
|
||
|
|
called even if the file is not interlaced, where it will return one pass.
|
||
|
|
You then need to read the whole image 'number_of_passes' times. Each time
|
||
|
|
will distribute the pixels from the current pass to the correct place in
|
||
|
|
the output image, so you need to supply the same rows to png_read_rows in
|
||
|
|
each pass.
|
||
|
|
|
||
|
|
If you are not going to display the image after each pass, but are
|
||
|
|
going to wait until the entire image is read in, use the sparkle
|
||
|
|
effect. This effect is faster and the end result of either method
|
||
|
|
is exactly the same. If you are planning on displaying the image
|
||
|
|
after each pass, the "rectangle" effect is generally considered the
|
||
|
|
better looking one.
|
||
|
|
|
||
|
|
If you only want the "sparkle" effect, just call png_read_rows() as
|
||
|
|
normal, with the third parameter NULL. Make sure you make pass over
|
||
|
|
the image number_of_passes times, and you don't change the data in the
|
||
|
|
rows between calls. You can change the locations of the data, just
|
||
|
|
not the data. Each pass only writes the pixels appropriate for that
|
||
|
|
pass, and assumes the data from previous passes is still valid.
|
||
|
|
|
||
|
|
png_read_rows(png_ptr, row_pointers, NULL,
|
||
|
|
number_of_rows);
|
||
|
|
|
||
|
|
If you only want the first effect (the rectangles), do the same as
|
||
|
|
before except pass the row buffer in the third parameter, and leave
|
||
|
|
the second parameter NULL.
|
||
|
|
|
||
|
|
png_read_rows(png_ptr, NULL, row_pointers,
|
||
|
|
number_of_rows);
|
||
|
|
|
||
|
|
If you don't want libpng to handle the interlacing details, just call
|
||
|
|
png_read_rows() PNG_INTERLACE_ADAM7_PASSES times to read in all the images.
|
||
|
|
Each of the images is a valid image by itself, however you will almost
|
||
|
|
certainly need to distribute the pixels from each sub-image to the
|
||
|
|
correct place. This is where everything gets very tricky.
|
||
|
|
|
||
|
|
If you want to retrieve the separate images you must pass the correct
|
||
|
|
number of rows to each successive call of png_read_rows(). The calculation
|
||
|
|
gets pretty complicated for small images, where some sub-images may
|
||
|
|
not even exist because either their width or height ends up zero.
|
||
|
|
libpng provides two macros to help you in 1.5 and later versions:
|
||
|
|
|
||
|
|
png_uint_32 width = PNG_PASS_COLS(image_width, pass_number);
|
||
|
|
png_uint_32 height = PNG_PASS_ROWS(image_height, pass_number);
|
||
|
|
|
||
|
|
Respectively these tell you the width and height of the sub-image
|
||
|
|
corresponding to the numbered pass. 'pass' is in in the range 0 to 6 -
|
||
|
|
this can be confusing because the specification refers to the same passes
|
||
|
|
as 1 to 7! Be careful, you must check both the width and height before
|
||
|
|
calling png_read_rows() and not call it for that pass if either is zero.
|
||
|
|
|
||
|
|
You can, of course, read each sub-image row by row. If you want to
|
||
|
|
produce optimal code to make a pixel-by-pixel transformation of an
|
||
|
|
interlaced image this is the best approach; read each row of each pass,
|
||
|
|
transform it, and write it out to a new interlaced image.
|
||
|
|
|
||
|
|
If you want to de-interlace the image yourself libpng provides further
|
||
|
|
macros to help that tell you where to place the pixels in the output image.
|
||
|
|
Because the interlacing scheme is rectangular - sub-image pixels are always
|
||
|
|
arranged on a rectangular grid - all you need to know for each pass is the
|
||
|
|
starting column and row in the output image of the first pixel plus the
|
||
|
|
spacing between each pixel. As of libpng 1.5 there are four macros to
|
||
|
|
retrieve this information:
|
||
|
|
|
||
|
|
png_uint_32 x = PNG_PASS_START_COL(pass);
|
||
|
|
png_uint_32 y = PNG_PASS_START_ROW(pass);
|
||
|
|
png_uint_32 xStep = 1U << PNG_PASS_COL_SHIFT(pass);
|
||
|
|
png_uint_32 yStep = 1U << PNG_PASS_ROW_SHIFT(pass);
|
||
|
|
|
||
|
|
These allow you to write the obvious loop:
|
||
|
|
|
||
|
|
png_uint_32 input_y = 0;
|
||
|
|
png_uint_32 output_y = PNG_PASS_START_ROW(pass);
|
||
|
|
|
||
|
|
while (output_y < output_image_height)
|
||
|
|
{
|
||
|
|
png_uint_32 input_x = 0;
|
||
|
|
png_uint_32 output_x = PNG_PASS_START_COL(pass);
|
||
|
|
|
||
|
|
while (output_x < output_image_width)
|
||
|
|
{
|
||
|
|
image[output_y][output_x] =
|
||
|
|
subimage[pass][input_y][input_x++];
|
||
|
|
|
||
|
|
output_x += xStep;
|
||
|
|
}
|
||
|
|
|
||
|
|
++input_y;
|
||
|
|
output_y += yStep;
|
||
|
|
}
|
||
|
|
|
||
|
|
Notice that the steps between successive output rows and columns are
|
||
|
|
returned as shifts. This is possible because the pixels in the subimages
|
||
|
|
are always a power of 2 apart - 1, 2, 4 or 8 pixels - in the original
|
||
|
|
image. In practice you may need to directly calculate the output coordinate
|
||
|
|
given an input coordinate. libpng provides two further macros for this
|
||
|
|
purpose:
|
||
|
|
|
||
|
|
png_uint_32 output_x = PNG_COL_FROM_PASS_COL(input_x, pass);
|
||
|
|
png_uint_32 output_y = PNG_ROW_FROM_PASS_ROW(input_y, pass);
|
||
|
|
|
||
|
|
Finally a pair of macros are provided to tell you if a particular image
|
||
|
|
row or column appears in a given pass:
|
||
|
|
|
||
|
|
int col_in_pass = PNG_COL_IN_INTERLACE_PASS(output_x, pass);
|
||
|
|
int row_in_pass = PNG_ROW_IN_INTERLACE_PASS(output_y, pass);
|
||
|
|
|
||
|
|
Bear in mind that you will probably also need to check the width and height
|
||
|
|
of the pass in addition to the above to be sure the pass even exists!
|
||
|
|
|
||
|
|
With any luck you are convinced by now that you don't want to do your own
|
||
|
|
interlace handling. In reality normally the only good reason for doing this
|
||
|
|
is if you are processing PNG files on a pixel-by-pixel basis and don't want
|
||
|
|
to load the whole file into memory when it is interlaced.
|
||
|
|
|
||
|
|
libpng includes a test program, pngvalid, that illustrates reading and
|
||
|
|
writing of interlaced images. If you can't get interlacing to work in your
|
||
|
|
code and don't want to leave it to libpng (the recommended approach), see
|
||
|
|
how pngvalid.c does it.
|
||
|
|
|
||
|
|
Finishing a sequential read
|
||
|
|
|
||
|
|
After you are finished reading the image through the
|
||
|
|
low-level interface, you can finish reading the file. If you are
|
||
|
|
interested in comments or time, which may be stored either before or
|
||
|
|
after the image data, you should pass the separate png_info struct if
|
||
|
|
you want to keep the comments from before and after the image
|
||
|
|
separate.
|
||
|
|
|
||
|
|
png_infop end_info = png_create_info_struct(png_ptr);
|
||
|
|
|
||
|
|
if (!end_info)
|
||
|
|
{
|
||
|
|
png_destroy_read_struct(&png_ptr, &info_ptr,
|
||
|
|
(png_infopp)NULL);
|
||
|
|
return (ERROR);
|
||
|
|
}
|
||
|
|
|
||
|
|
png_read_end(png_ptr, end_info);
|
||
|
|
|
||
|
|
If you are not interested, you should still call png_read_end()
|
||
|
|
but you can pass NULL, avoiding the need to create an end_info structure.
|
||
|
|
|
||
|
|
png_read_end(png_ptr, (png_infop)NULL);
|
||
|
|
|
||
|
|
If you don't call png_read_end(), then your file pointer will be
|
||
|
|
left pointing to the first chunk after the last IDAT, which is probably
|
||
|
|
not what you want if you expect to read something beyond the end of
|
||
|
|
the PNG datastream.
|
||
|
|
|
||
|
|
When you are done, you can free all memory allocated by libpng like this:
|
||
|
|
|
||
|
|
png_destroy_read_struct(&png_ptr, &info_ptr,
|
||
|
|
&end_info);
|
||
|
|
|
||
|
|
or, if you didn't create an end_info structure,
|
||
|
|
|
||
|
|
png_destroy_read_struct(&png_ptr, &info_ptr,
|
||
|
|
(png_infopp)NULL);
|
||
|
|
|
||
|
|
It is also possible to individually free the info_ptr members that
|
||
|
|
point to libpng-allocated storage with the following function:
|
||
|
|
|
||
|
|
png_free_data(png_ptr, info_ptr, mask, seq)
|
||
|
|
|
||
|
|
mask - identifies data to be freed, a mask
|
||
|
|
containing the bitwise OR of one or
|
||
|
|
more of
|
||
|
|
PNG_FREE_PLTE, PNG_FREE_TRNS,
|
||
|
|
PNG_FREE_HIST, PNG_FREE_ICCP,
|
||
|
|
PNG_FREE_PCAL, PNG_FREE_ROWS,
|
||
|
|
PNG_FREE_SCAL, PNG_FREE_SPLT,
|
||
|
|
PNG_FREE_TEXT, PNG_FREE_UNKN,
|
||
|
|
or simply PNG_FREE_ALL
|
||
|
|
|
||
|
|
seq - sequence number of item to be freed
|
||
|
|
(-1 for all items)
|
||
|
|
|
||
|
|
This function may be safely called when the relevant storage has
|
||
|
|
already been freed, or has not yet been allocated, or was allocated
|
||
|
|
by the user and not by libpng, and will in those cases do nothing.
|
||
|
|
The "seq" parameter is ignored if only one item of the selected data
|
||
|
|
type, such as PLTE, is allowed. If "seq" is not -1, and multiple items
|
||
|
|
are allowed for the data type identified in the mask, such as text or
|
||
|
|
sPLT, only the n'th item in the structure is freed, where n is "seq".
|
||
|
|
|
||
|
|
The default behavior is only to free data that was allocated internally
|
||
|
|
by libpng. This can be changed, so that libpng will not free the data,
|
||
|
|
or so that it will free data that was allocated by the user with png_malloc()
|
||
|
|
or png_zalloc() and passed in via a png_set_*() function, with
|
||
|
|
|
||
|
|
png_data_freer(png_ptr, info_ptr, freer, mask)
|
||
|
|
|
||
|
|
freer - one of
|
||
|
|
PNG_DESTROY_WILL_FREE_DATA
|
||
|
|
PNG_SET_WILL_FREE_DATA
|
||
|
|
PNG_USER_WILL_FREE_DATA
|
||
|
|
|
||
|
|
mask - which data elements are affected
|
||
|
|
same choices as in png_free_data()
|
||
|
|
|
||
|
|
This function only affects data that has already been allocated.
|
||
|
|
You can call this function after reading the PNG data but before calling
|
||
|
|
any png_set_*() functions, to control whether the user or the png_set_*()
|
||
|
|
function is responsible for freeing any existing data that might be present,
|
||
|
|
and again after the png_set_*() functions to control whether the user
|
||
|
|
or png_destroy_*() is supposed to free the data. When the user assumes
|
||
|
|
responsibility for libpng-allocated data, the application must use
|
||
|
|
png_free() to free it, and when the user transfers responsibility to libpng
|
||
|
|
for data that the user has allocated, the user must have used png_malloc()
|
||
|
|
or png_zalloc() to allocate it.
|
||
|
|
|
||
|
|
If you allocated your row_pointers in a single block, as suggested above in
|
||
|
|
the description of the high level read interface, you must not transfer
|
||
|
|
responsibility for freeing it to the png_set_rows or png_read_destroy function,
|
||
|
|
because they would also try to free the individual row_pointers[i].
|
||
|
|
|
||
|
|
If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword
|
||
|
|
separately, do not transfer responsibility for freeing text_ptr to libpng,
|
||
|
|
because when libpng fills a png_text structure it combines these members with
|
||
|
|
the key member, and png_free_data() will free only text_ptr.key. Similarly,
|
||
|
|
if you transfer responsibility for free'ing text_ptr from libpng to your
|
||
|
|
application, your application must not separately free those members.
|
||
|
|
|
||
|
|
The png_free_data() function will turn off the "valid" flag for anything
|
||
|
|
it frees. If you need to turn the flag off for a chunk that was freed by
|
||
|
|
your application instead of by libpng, you can use
|
||
|
|
|
||
|
|
png_set_invalid(png_ptr, info_ptr, mask);
|
||
|
|
|
||
|
|
mask - identifies the chunks to be made invalid,
|
||
|
|
containing the bitwise OR of one or
|
||
|
|
more of
|
||
|
|
PNG_INFO_gAMA, PNG_INFO_sBIT,
|
||
|
|
PNG_INFO_cHRM, PNG_INFO_PLTE,
|
||
|
|
PNG_INFO_tRNS, PNG_INFO_bKGD,
|
||
|
|
PNG_INFO_hIST, PNG_INFO_pHYs,
|
||
|
|
PNG_INFO_oFFs, PNG_INFO_tIME,
|
||
|
|
PNG_INFO_pCAL, PNG_INFO_sRGB,
|
||
|
|
PNG_INFO_iCCP, PNG_INFO_sPLT,
|
||
|
|
PNG_INFO_sCAL, PNG_INFO_IDAT
|
||
|
|
|
||
|
|
For a more compact example of reading a PNG image, see the file example.c.
|
||
|
|
|
||
|
|
Reading PNG files progressively
|
||
|
|
|
||
|
|
The progressive reader is slightly different then the non-progressive
|
||
|
|
reader. Instead of calling png_read_info(), png_read_rows(), and
|
||
|
|
png_read_end(), you make one call to png_process_data(), which calls
|
||
|
|
callbacks when it has the info, a row, or the end of the image. You
|
||
|
|
set up these callbacks with png_set_progressive_read_fn(). You don't
|
||
|
|
have to worry about the input/output functions of libpng, as you are
|
||
|
|
giving the library the data directly in png_process_data(). I will
|
||
|
|
assume that you have read the section on reading PNG files above,
|
||
|
|
so I will only highlight the differences (although I will show
|
||
|
|
all of the code).
|
||
|
|
|
||
|
|
png_structp png_ptr;
|
||
|
|
png_infop info_ptr;
|
||
|
|
|
||
|
|
/* An example code fragment of how you would
|
||
|
|
initialize the progressive reader in your
|
||
|
|
application. */
|
||
|
|
int
|
||
|
|
initialize_png_reader()
|
||
|
|
{
|
||
|
|
png_ptr = png_create_read_struct
|
||
|
|
(PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
|
||
|
|
user_error_fn, user_warning_fn);
|
||
|
|
|
||
|
|
if (!png_ptr)
|
||
|
|
return (ERROR);
|
||
|
|
|
||
|
|
info_ptr = png_create_info_struct(png_ptr);
|
||
|
|
|
||
|
|
if (!info_ptr)
|
||
|
|
{
|
||
|
|
png_destroy_read_struct(&png_ptr,
|
||
|
|
(png_infopp)NULL, (png_infopp)NULL);
|
||
|
|
return (ERROR);
|
||
|
|
}
|
||
|
|
|
||
|
|
if (setjmp(png_jmpbuf(png_ptr)))
|
||
|
|
{
|
||
|
|
png_destroy_read_struct(&png_ptr, &info_ptr,
|
||
|
|
(png_infopp)NULL);
|
||
|
|
return (ERROR);
|
||
|
|
}
|
||
|
|
|
||
|
|
/* This one's new. You can provide functions
|
||
|
|
to be called when the header info is valid,
|
||
|
|
when each row is completed, and when the image
|
||
|
|
is finished. If you aren't using all functions,
|
||
|
|
you can specify NULL parameters. Even when all
|
||
|
|
three functions are NULL, you need to call
|
||
|
|
png_set_progressive_read_fn(). You can use
|
||
|
|
any struct as the user_ptr (cast to a void pointer
|
||
|
|
for the function call), and retrieve the pointer
|
||
|
|
from inside the callbacks using the function
|
||
|
|
|
||
|
|
png_get_progressive_ptr(png_ptr);
|
||
|
|
|
||
|
|
which will return a void pointer, which you have
|
||
|
|
to cast appropriately.
|
||
|
|
*/
|
||
|
|
png_set_progressive_read_fn(png_ptr, (void *)user_ptr,
|
||
|
|
info_callback, row_callback, end_callback);
|
||
|
|
|
||
|
|
return 0;
|
||
|
|
}
|
||
|
|
|
||
|
|
/* A code fragment that you call as you receive blocks
|
||
|
|
of data */
|
||
|
|
int
|
||
|
|
process_data(png_bytep buffer, png_uint_32 length)
|
||
|
|
{
|
||
|
|
if (setjmp(png_jmpbuf(png_ptr)))
|
||
|
|
{
|
||
|
|
png_destroy_read_struct(&png_ptr, &info_ptr,
|
||
|
|
(png_infopp)NULL);
|
||
|
|
return (ERROR);
|
||
|
|
}
|
||
|
|
|
||
|
|
/* This one's new also. Simply give it a chunk
|
||
|
|
of data from the file stream (in order, of
|
||
|
|
course). On machines with segmented memory
|
||
|
|
models machines, don't give it any more than
|
||
|
|
64K. The library seems to run fine with sizes
|
||
|
|
of 4K. Although you can give it much less if
|
||
|
|
necessary (I assume you can give it chunks of
|
||
|
|
1 byte, I haven't tried less then 256 bytes
|
||
|
|
yet). When this function returns, you may
|
||
|
|
want to display any rows that were generated
|
||
|
|
in the row callback if you don't already do
|
||
|
|
so there.
|
||
|
|
*/
|
||
|
|
png_process_data(png_ptr, info_ptr, buffer, length);
|
||
|
|
|
||
|
|
/* At this point you can call png_process_data_skip if
|
||
|
|
you want to handle data the library will skip yourself;
|
||
|
|
it simply returns the number of bytes to skip (and stops
|
||
|
|
libpng skipping that number of bytes on the next
|
||
|
|
png_process_data call).
|
||
|
|
return 0;
|
||
|
|
}
|
||
|
|
|
||
|
|
/* This function is called (as set by
|
||
|
|
png_set_progressive_read_fn() above) when enough data
|
||
|
|
has been supplied so all of the header has been
|
||
|
|
read.
|
||
|
|
*/
|
||
|
|
void
|
||
|
|
info_callback(png_structp png_ptr, png_infop info)
|
||
|
|
{
|
||
|
|
/* Do any setup here, including setting any of
|
||
|
|
the transformations mentioned in the Reading
|
||
|
|
PNG files section. For now, you _must_ call
|
||
|
|
either png_start_read_image() or
|
||
|
|
png_read_update_info() after all the
|
||
|
|
transformations are set (even if you don't set
|
||
|
|
any). You may start getting rows before
|
||
|
|
png_process_data() returns, so this is your
|
||
|
|
last chance to prepare for that.
|
||
|
|
|
||
|
|
This is where you turn on interlace handling,
|
||
|
|
assuming you don't want to do it yourself.
|
||
|
|
|
||
|
|
If you need to you can stop the processing of
|
||
|
|
your original input data at this point by calling
|
||
|
|
png_process_data_pause. This returns the number
|
||
|
|
of unprocessed bytes from the last png_process_data
|
||
|
|
call - it is up to you to ensure that the next call
|
||
|
|
sees these bytes again. If you don't want to bother
|
||
|
|
with this you can get libpng to cache the unread
|
||
|
|
bytes by setting the 'save' parameter (see png.h) but
|
||
|
|
then libpng will have to copy the data internally.
|
||
|
|
*/
|
||
|
|
}
|
||
|
|
|
||
|
|
/* This function is called when each row of image
|
||
|
|
data is complete */
|
||
|
|
void
|
||
|
|
row_callback(png_structp png_ptr, png_bytep new_row,
|
||
|
|
png_uint_32 row_num, int pass)
|
||
|
|
{
|
||
|
|
/* If the image is interlaced, and you turned
|
||
|
|
on the interlace handler, this function will
|
||
|
|
be called for every row in every pass. Some
|
||
|
|
of these rows will not be changed from the
|
||
|
|
previous pass. When the row is not changed,
|
||
|
|
the new_row variable will be NULL. The rows
|
||
|
|
and passes are called in order, so you don't
|
||
|
|
really need the row_num and pass, but I'm
|
||
|
|
supplying them because it may make your life
|
||
|
|
easier.
|
||
|
|
|
||
|
|
If you did not turn on interlace handling then
|
||
|
|
the callback is called for each row of each
|
||
|
|
sub-image when the image is interlaced. In this
|
||
|
|
case 'row_num' is the row in the sub-image, not
|
||
|
|
the row in the output image as it is in all other
|
||
|
|
cases.
|
||
|
|
|
||
|
|
For the non-NULL rows of interlaced images when
|
||
|
|
you have switched on libpng interlace handling,
|
||
|
|
you must call png_progressive_combine_row()
|
||
|
|
passing in the row and the old row. You can
|
||
|
|
call this function for NULL rows (it will just
|
||
|
|
return) and for non-interlaced images (it just
|
||
|
|
does the memcpy for you) if it will make the
|
||
|
|
code easier. Thus, you can just do this for
|
||
|
|
all cases if you switch on interlace handling;
|
||
|
|
*/
|
||
|
|
|
||
|
|
png_progressive_combine_row(png_ptr, old_row,
|
||
|
|
new_row);
|
||
|
|
|
||
|
|
/* where old_row is what was displayed for
|
||
|
|
previously for the row. Note that the first
|
||
|
|
pass (pass == 0, really) will completely cover
|
||
|
|
the old row, so the rows do not have to be
|
||
|
|
initialized. After the first pass (and only
|
||
|
|
for interlaced images), you will have to pass
|
||
|
|
the current row, and the function will combine
|
||
|
|
the old row and the new row.
|
||
|
|
|
||
|
|
You can also call png_process_data_pause in this
|
||
|
|
callback - see above.
|
||
|
|
*/
|
||
|
|
}
|
||
|
|
|
||
|
|
void
|
||
|
|
end_callback(png_structp png_ptr, png_infop info)
|
||
|
|
{
|
||
|
|
/* This function is called after the whole image
|
||
|
|
has been read, including any chunks after the
|
||
|
|
image (up to and including the IEND). You
|
||
|
|
will usually have the same info chunk as you
|
||
|
|
had in the header, although some data may have
|
||
|
|
been added to the comments and time fields.
|
||
|
|
|
||
|
|
Most people won't do much here, perhaps setting
|
||
|
|
a flag that marks the image as finished.
|
||
|
|
*/
|
||
|
|
}
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
IV. Writing
|
||
|
|
|
||
|
|
Much of this is very similar to reading. However, everything of
|
||
|
|
importance is repeated here, so you won't have to constantly look
|
||
|
|
back up in the reading section to understand writing.
|
||
|
|
|
||
|
|
Setup
|
||
|
|
|
||
|
|
You will want to do the I/O initialization before you get into libpng,
|
||
|
|
so if it doesn't work, you don't have anything to undo. If you are not
|
||
|
|
using the standard I/O functions, you will need to replace them with
|
||
|
|
custom writing functions. See the discussion under Customizing libpng.
|
||
|
|
|
||
|
|
FILE *fp = fopen(file_name, "wb");
|
||
|
|
|
||
|
|
if (!fp)
|
||
|
|
return (ERROR);
|
||
|
|
|
||
|
|
Next, png_struct and png_info need to be allocated and initialized.
|
||
|
|
As these can be both relatively large, you may not want to store these
|
||
|
|
on the stack, unless you have stack space to spare. Of course, you
|
||
|
|
will want to check if they return NULL. If you are also reading,
|
||
|
|
you won't want to name your read structure and your write structure
|
||
|
|
both "png_ptr"; you can call them anything you like, such as
|
||
|
|
"read_ptr" and "write_ptr". Look at pngtest.c, for example.
|
||
|
|
|
||
|
|
png_structp png_ptr = png_create_write_struct
|
||
|
|
(PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
|
||
|
|
user_error_fn, user_warning_fn);
|
||
|
|
|
||
|
|
if (!png_ptr)
|
||
|
|
return (ERROR);
|
||
|
|
|
||
|
|
png_infop info_ptr = png_create_info_struct(png_ptr);
|
||
|
|
if (!info_ptr)
|
||
|
|
{
|
||
|
|
png_destroy_write_struct(&png_ptr,
|
||
|
|
(png_infopp)NULL);
|
||
|
|
return (ERROR);
|
||
|
|
}
|
||
|
|
|
||
|
|
If you want to use your own memory allocation routines,
|
||
|
|
define PNG_USER_MEM_SUPPORTED and use
|
||
|
|
png_create_write_struct_2() instead of png_create_write_struct():
|
||
|
|
|
||
|
|
png_structp png_ptr = png_create_write_struct_2
|
||
|
|
(PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
|
||
|
|
user_error_fn, user_warning_fn, (png_voidp)
|
||
|
|
user_mem_ptr, user_malloc_fn, user_free_fn);
|
||
|
|
|
||
|
|
After you have these structures, you will need to set up the
|
||
|
|
error handling. When libpng encounters an error, it expects to
|
||
|
|
longjmp() back to your routine. Therefore, you will need to call
|
||
|
|
setjmp() and pass the png_jmpbuf(png_ptr). If you
|
||
|
|
write the file from different routines, you will need to update
|
||
|
|
the png_jmpbuf(png_ptr) every time you enter a new routine that will
|
||
|
|
call a png_*() function. See your documentation of setjmp/longjmp
|
||
|
|
for your compiler for more information on setjmp/longjmp. See
|
||
|
|
the discussion on libpng error handling in the Customizing Libpng
|
||
|
|
section below for more information on the libpng error handling.
|
||
|
|
|
||
|
|
if (setjmp(png_jmpbuf(png_ptr)))
|
||
|
|
{
|
||
|
|
png_destroy_write_struct(&png_ptr, &info_ptr);
|
||
|
|
fclose(fp);
|
||
|
|
return (ERROR);
|
||
|
|
}
|
||
|
|
...
|
||
|
|
return;
|
||
|
|
|
||
|
|
If you would rather avoid the complexity of setjmp/longjmp issues,
|
||
|
|
you can compile libpng with PNG_NO_SETJMP, in which case
|
||
|
|
errors will result in a call to PNG_ABORT() which defaults to abort().
|
||
|
|
|
||
|
|
You can #define PNG_ABORT() to a function that does something
|
||
|
|
more useful than abort(), as long as your function does not
|
||
|
|
return.
|
||
|
|
|
||
|
|
Now you need to set up the output code. The default for libpng is to
|
||
|
|
use the C function fwrite(). If you use this, you will need to pass a
|
||
|
|
valid FILE * in the function png_init_io(). Be sure that the file is
|
||
|
|
opened in binary mode. Again, if you wish to handle writing data in
|
||
|
|
another way, see the discussion on libpng I/O handling in the Customizing
|
||
|
|
Libpng section below.
|
||
|
|
|
||
|
|
png_init_io(png_ptr, fp);
|
||
|
|
|
||
|
|
If you are embedding your PNG into a datastream such as MNG, and don't
|
||
|
|
want libpng to write the 8-byte signature, or if you have already
|
||
|
|
written the signature in your application, use
|
||
|
|
|
||
|
|
png_set_sig_bytes(png_ptr, 8);
|
||
|
|
|
||
|
|
to inform libpng that it should not write a signature.
|
||
|
|
|
||
|
|
Write callbacks
|
||
|
|
|
||
|
|
At this point, you can set up a callback function that will be
|
||
|
|
called after each row has been written, which you can use to control
|
||
|
|
a progress meter or the like. It's demonstrated in pngtest.c.
|
||
|
|
You must supply a function
|
||
|
|
|
||
|
|
void write_row_callback(png_structp png_ptr, png_uint_32 row,
|
||
|
|
int pass);
|
||
|
|
{
|
||
|
|
/* put your code here */
|
||
|
|
}
|
||
|
|
|
||
|
|
(You can give it another name that you like instead of "write_row_callback")
|
||
|
|
|
||
|
|
To inform libpng about your function, use
|
||
|
|
|
||
|
|
png_set_write_status_fn(png_ptr, write_row_callback);
|
||
|
|
|
||
|
|
When this function is called the row has already been completely processed and
|
||
|
|
it has also been written out. The 'row' and 'pass' refer to the next row to be
|
||
|
|
handled. For the
|
||
|
|
non-interlaced case the row that was just handled is simply one less than the
|
||
|
|
passed in row number, and pass will always be 0. For the interlaced case the
|
||
|
|
same applies unless the row value is 0, in which case the row just handled was
|
||
|
|
the last one from one of the preceding passes. Because interlacing may skip a
|
||
|
|
pass you cannot be sure that the preceding pass is just 'pass-1', if you really
|
||
|
|
need to know what the last pass is record (row,pass) from the callback and use
|
||
|
|
the last recorded value each time.
|
||
|
|
|
||
|
|
As with the user transform you can find the output row using the
|
||
|
|
PNG_ROW_FROM_PASS_ROW macro.
|
||
|
|
|
||
|
|
You now have the option of modifying how the compression library will
|
||
|
|
run. The following functions are mainly for testing, but may be useful
|
||
|
|
in some cases, like if you need to write PNG files extremely fast and
|
||
|
|
are willing to give up some compression, or if you want to get the
|
||
|
|
maximum possible compression at the expense of slower writing. If you
|
||
|
|
have no special needs in this area, let the library do what it wants by
|
||
|
|
not calling this function at all, as it has been tuned to deliver a good
|
||
|
|
speed/compression ratio. The second parameter to png_set_filter() is
|
||
|
|
the filter method, for which the only valid values are 0 (as of the
|
||
|
|
July 1999 PNG specification, version 1.2) or 64 (if you are writing
|
||
|
|
a PNG datastream that is to be embedded in a MNG datastream). The third
|
||
|
|
parameter is a flag that indicates which filter type(s) are to be tested
|
||
|
|
for each scanline. See the PNG specification for details on the specific
|
||
|
|
filter types.
|
||
|
|
|
||
|
|
|
||
|
|
/* turn on or off filtering, and/or choose
|
||
|
|
specific filters. You can use either a single
|
||
|
|
PNG_FILTER_VALUE_NAME or the bitwise OR of one
|
||
|
|
or more PNG_FILTER_NAME masks.
|
||
|
|
*/
|
||
|
|
png_set_filter(png_ptr, 0,
|
||
|
|
PNG_FILTER_NONE | PNG_FILTER_VALUE_NONE |
|
||
|
|
PNG_FILTER_SUB | PNG_FILTER_VALUE_SUB |
|
||
|
|
PNG_FILTER_UP | PNG_FILTER_VALUE_UP |
|
||
|
|
PNG_FILTER_AVG | PNG_FILTER_VALUE_AVG |
|
||
|
|
PNG_FILTER_PAETH | PNG_FILTER_VALUE_PAETH|
|
||
|
|
PNG_ALL_FILTERS);
|
||
|
|
|
||
|
|
If an application wants to start and stop using particular filters during
|
||
|
|
compression, it should start out with all of the filters (to ensure that
|
||
|
|
the previous row of pixels will be stored in case it's needed later),
|
||
|
|
and then add and remove them after the start of compression.
|
||
|
|
|
||
|
|
If you are writing a PNG datastream that is to be embedded in a MNG
|
||
|
|
datastream, the second parameter can be either 0 or 64.
|
||
|
|
|
||
|
|
The png_set_compression_*() functions interface to the zlib compression
|
||
|
|
library, and should mostly be ignored unless you really know what you are
|
||
|
|
doing. The only generally useful call is png_set_compression_level()
|
||
|
|
which changes how much time zlib spends on trying to compress the image
|
||
|
|
data. See the Compression Library (zlib.h and algorithm.txt, distributed
|
||
|
|
with zlib) for details on the compression levels.
|
||
|
|
|
||
|
|
#include zlib.h
|
||
|
|
|
||
|
|
/* Set the zlib compression level */
|
||
|
|
png_set_compression_level(png_ptr,
|
||
|
|
Z_BEST_COMPRESSION);
|
||
|
|
|
||
|
|
/* Set other zlib parameters for compressing IDAT */
|
||
|
|
png_set_compression_mem_level(png_ptr, 8);
|
||
|
|
png_set_compression_strategy(png_ptr,
|
||
|
|
Z_DEFAULT_STRATEGY);
|
||
|
|
png_set_compression_window_bits(png_ptr, 15);
|
||
|
|
png_set_compression_method(png_ptr, 8);
|
||
|
|
png_set_compression_buffer_size(png_ptr, 8192)
|
||
|
|
|
||
|
|
/* Set zlib parameters for text compression
|
||
|
|
* If you don't call these, the parameters
|
||
|
|
* fall back on those defined for IDAT chunks
|
||
|
|
*/
|
||
|
|
png_set_text_compression_mem_level(png_ptr, 8);
|
||
|
|
png_set_text_compression_strategy(png_ptr,
|
||
|
|
Z_DEFAULT_STRATEGY);
|
||
|
|
png_set_text_compression_window_bits(png_ptr, 15);
|
||
|
|
png_set_text_compression_method(png_ptr, 8);
|
||
|
|
|
||
|
|
Setting the contents of info for output
|
||
|
|
|
||
|
|
You now need to fill in the png_info structure with all the data you
|
||
|
|
wish to write before the actual image. Note that the only thing you
|
||
|
|
are allowed to write after the image is the text chunks and the time
|
||
|
|
chunk (as of PNG Specification 1.2, anyway). See png_write_end() and
|
||
|
|
the latest PNG specification for more information on that. If you
|
||
|
|
wish to write them before the image, fill them in now, and flag that
|
||
|
|
data as being valid. If you want to wait until after the data, don't
|
||
|
|
fill them until png_write_end(). For all the fields in png_info and
|
||
|
|
their data types, see png.h. For explanations of what the fields
|
||
|
|
contain, see the PNG specification.
|
||
|
|
|
||
|
|
Some of the more important parts of the png_info are:
|
||
|
|
|
||
|
|
png_set_IHDR(png_ptr, info_ptr, width, height,
|
||
|
|
bit_depth, color_type, interlace_type,
|
||
|
|
compression_type, filter_method)
|
||
|
|
|
||
|
|
width - holds the width of the image
|
||
|
|
in pixels (up to 2^31).
|
||
|
|
|
||
|
|
height - holds the height of the image
|
||
|
|
in pixels (up to 2^31).
|
||
|
|
|
||
|
|
bit_depth - holds the bit depth of one of the
|
||
|
|
image channels.
|
||
|
|
(valid values are 1, 2, 4, 8, 16
|
||
|
|
and depend also on the
|
||
|
|
color_type. See also significant
|
||
|
|
bits (sBIT) below).
|
||
|
|
|
||
|
|
color_type - describes which color/alpha
|
||
|
|
channels are present.
|
||
|
|
PNG_COLOR_TYPE_GRAY
|
||
|
|
(bit depths 1, 2, 4, 8, 16)
|
||
|
|
PNG_COLOR_TYPE_GRAY_ALPHA
|
||
|
|
(bit depths 8, 16)
|
||
|
|
PNG_COLOR_TYPE_PALETTE
|
||
|
|
(bit depths 1, 2, 4, 8)
|
||
|
|
PNG_COLOR_TYPE_RGB
|
||
|
|
(bit_depths 8, 16)
|
||
|
|
PNG_COLOR_TYPE_RGB_ALPHA
|
||
|
|
(bit_depths 8, 16)
|
||
|
|
|
||
|
|
PNG_COLOR_MASK_PALETTE
|
||
|
|
PNG_COLOR_MASK_COLOR
|
||
|
|
PNG_COLOR_MASK_ALPHA
|
||
|
|
|
||
|
|
interlace_type - PNG_INTERLACE_NONE or
|
||
|
|
PNG_INTERLACE_ADAM7
|
||
|
|
|
||
|
|
compression_type - (must be
|
||
|
|
PNG_COMPRESSION_TYPE_DEFAULT)
|
||
|
|
|
||
|
|
filter_method - (must be PNG_FILTER_TYPE_DEFAULT
|
||
|
|
or, if you are writing a PNG to
|
||
|
|
be embedded in a MNG datastream,
|
||
|
|
can also be
|
||
|
|
PNG_INTRAPIXEL_DIFFERENCING)
|
||
|
|
|
||
|
|
If you call png_set_IHDR(), the call must appear before any of the
|
||
|
|
other png_set_*() functions, because they might require access to some of
|
||
|
|
the IHDR settings. The remaining png_set_*() functions can be called
|
||
|
|
in any order.
|
||
|
|
|
||
|
|
If you wish, you can reset the compression_type, interlace_type, or
|
||
|
|
filter_method later by calling png_set_IHDR() again; if you do this, the
|
||
|
|
width, height, bit_depth, and color_type must be the same in each call.
|
||
|
|
|
||
|
|
png_set_PLTE(png_ptr, info_ptr, palette,
|
||
|
|
num_palette);
|
||
|
|
|
||
|
|
palette - the palette for the file
|
||
|
|
(array of png_color)
|
||
|
|
num_palette - number of entries in the palette
|
||
|
|
|
||
|
|
png_set_gAMA(png_ptr, info_ptr, file_gamma);
|
||
|
|
png_set_gAMA_fixed(png_ptr, info_ptr, int_file_gamma);
|
||
|
|
|
||
|
|
file_gamma - the gamma at which the image was
|
||
|
|
created (PNG_INFO_gAMA)
|
||
|
|
|
||
|
|
int_file_gamma - 100,000 times the gamma at which
|
||
|
|
the image was created
|
||
|
|
|
||
|
|
png_set_cHRM(png_ptr, info_ptr, white_x, white_y, red_x, red_y,
|
||
|
|
green_x, green_y, blue_x, blue_y)
|
||
|
|
png_set_cHRM_XYZ(png_ptr, info_ptr, red_X, red_Y, red_Z, green_X,
|
||
|
|
green_Y, green_Z, blue_X, blue_Y, blue_Z)
|
||
|
|
png_set_cHRM_fixed(png_ptr, info_ptr, int_white_x, int_white_y,
|
||
|
|
int_red_x, int_red_y, int_green_x, int_green_y,
|
||
|
|
int_blue_x, int_blue_y)
|
||
|
|
png_set_cHRM_XYZ_fixed(png_ptr, info_ptr, int_red_X, int_red_Y,
|
||
|
|
int_red_Z, int_green_X, int_green_Y, int_green_Z,
|
||
|
|
int_blue_X, int_blue_Y, int_blue_Z)
|
||
|
|
|
||
|
|
{white,red,green,blue}_{x,y}
|
||
|
|
A color space encoding specified using the chromaticities
|
||
|
|
of the end points and the white point.
|
||
|
|
|
||
|
|
{red,green,blue}_{X,Y,Z}
|
||
|
|
A color space encoding specified using the encoding end
|
||
|
|
points - the CIE tristimulus specification of the intended
|
||
|
|
color of the red, green and blue channels in the PNG RGB
|
||
|
|
data. The white point is simply the sum of the three end
|
||
|
|
points.
|
||
|
|
|
||
|
|
png_set_sRGB(png_ptr, info_ptr, srgb_intent);
|
||
|
|
|
||
|
|
srgb_intent - the rendering intent
|
||
|
|
(PNG_INFO_sRGB) The presence of
|
||
|
|
the sRGB chunk means that the pixel
|
||
|
|
data is in the sRGB color space.
|
||
|
|
This chunk also implies specific
|
||
|
|
values of gAMA and cHRM. Rendering
|
||
|
|
intent is the CSS-1 property that
|
||
|
|
has been defined by the International
|
||
|
|
Color Consortium
|
||
|
|
(http://www.color.org).
|
||
|
|
It can be one of
|
||
|
|
PNG_sRGB_INTENT_SATURATION,
|
||
|
|
PNG_sRGB_INTENT_PERCEPTUAL,
|
||
|
|
PNG_sRGB_INTENT_ABSOLUTE, or
|
||
|
|
PNG_sRGB_INTENT_RELATIVE.
|
||
|
|
|
||
|
|
|
||
|
|
png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr,
|
||
|
|
srgb_intent);
|
||
|
|
|
||
|
|
srgb_intent - the rendering intent
|
||
|
|
(PNG_INFO_sRGB) The presence of the
|
||
|
|
sRGB chunk means that the pixel
|
||
|
|
data is in the sRGB color space.
|
||
|
|
This function also causes gAMA and
|
||
|
|
cHRM chunks with the specific values
|
||
|
|
that are consistent with sRGB to be
|
||
|
|
written.
|
||
|
|
|
||
|
|
png_set_iCCP(png_ptr, info_ptr, name, compression_type,
|
||
|
|
profile, proflen);
|
||
|
|
|
||
|
|
name - The profile name.
|
||
|
|
|
||
|
|
compression_type - The compression type; always
|
||
|
|
PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
|
||
|
|
You may give NULL to this argument to
|
||
|
|
ignore it.
|
||
|
|
|
||
|
|
profile - International Color Consortium color
|
||
|
|
profile data. May contain NULs.
|
||
|
|
|
||
|
|
proflen - length of profile data in bytes.
|
||
|
|
|
||
|
|
png_set_sBIT(png_ptr, info_ptr, sig_bit);
|
||
|
|
|
||
|
|
sig_bit - the number of significant bits for
|
||
|
|
(PNG_INFO_sBIT) each of the gray, red,
|
||
|
|
green, and blue channels, whichever are
|
||
|
|
appropriate for the given color type
|
||
|
|
(png_color_16)
|
||
|
|
|
||
|
|
png_set_tRNS(png_ptr, info_ptr, trans_alpha,
|
||
|
|
num_trans, trans_color);
|
||
|
|
|
||
|
|
trans_alpha - array of alpha (transparency)
|
||
|
|
entries for palette (PNG_INFO_tRNS)
|
||
|
|
|
||
|
|
num_trans - number of transparent entries
|
||
|
|
(PNG_INFO_tRNS)
|
||
|
|
|
||
|
|
trans_color - graylevel or color sample values
|
||
|
|
(in order red, green, blue) of the
|
||
|
|
single transparent color for
|
||
|
|
non-paletted images (PNG_INFO_tRNS)
|
||
|
|
|
||
|
|
png_set_hIST(png_ptr, info_ptr, hist);
|
||
|
|
|
||
|
|
hist - histogram of palette (array of
|
||
|
|
png_uint_16) (PNG_INFO_hIST)
|
||
|
|
|
||
|
|
png_set_tIME(png_ptr, info_ptr, mod_time);
|
||
|
|
|
||
|
|
mod_time - time image was last modified
|
||
|
|
(PNG_VALID_tIME)
|
||
|
|
|
||
|
|
png_set_bKGD(png_ptr, info_ptr, background);
|
||
|
|
|
||
|
|
background - background color (of type
|
||
|
|
png_color_16p) (PNG_VALID_bKGD)
|
||
|
|
|
||
|
|
png_set_text(png_ptr, info_ptr, text_ptr, num_text);
|
||
|
|
|
||
|
|
text_ptr - array of png_text holding image
|
||
|
|
comments
|
||
|
|
|
||
|
|
text_ptr[i].compression - type of compression used
|
||
|
|
on "text" PNG_TEXT_COMPRESSION_NONE
|
||
|
|
PNG_TEXT_COMPRESSION_zTXt
|
||
|
|
PNG_ITXT_COMPRESSION_NONE
|
||
|
|
PNG_ITXT_COMPRESSION_zTXt
|
||
|
|
text_ptr[i].key - keyword for comment. Must contain
|
||
|
|
1-79 characters.
|
||
|
|
text_ptr[i].text - text comments for current
|
||
|
|
keyword. Can be NULL or empty.
|
||
|
|
text_ptr[i].text_length - length of text string,
|
||
|
|
after decompression, 0 for iTXt
|
||
|
|
text_ptr[i].itxt_length - length of itxt string,
|
||
|
|
after decompression, 0 for tEXt/zTXt
|
||
|
|
text_ptr[i].lang - language of comment (NULL or
|
||
|
|
empty for unknown).
|
||
|
|
text_ptr[i].translated_keyword - keyword in UTF-8 (NULL
|
||
|
|
or empty for unknown).
|
||
|
|
|
||
|
|
Note that the itxt_length, lang, and lang_key
|
||
|
|
members of the text_ptr structure only exist when the
|
||
|
|
library is built with iTXt chunk support. Prior to
|
||
|
|
libpng-1.4.0 the library was built by default without
|
||
|
|
iTXt support. Also note that when iTXt is supported,
|
||
|
|
they contain NULL pointers when the "compression"
|
||
|
|
field contains PNG_TEXT_COMPRESSION_NONE or
|
||
|
|
PNG_TEXT_COMPRESSION_zTXt.
|
||
|
|
|
||
|
|
num_text - number of comments
|
||
|
|
|
||
|
|
png_set_sPLT(png_ptr, info_ptr, &palette_ptr,
|
||
|
|
num_spalettes);
|
||
|
|
|
||
|
|
palette_ptr - array of png_sPLT_struct structures
|
||
|
|
to be added to the list of palettes
|
||
|
|
in the info structure.
|
||
|
|
num_spalettes - number of palette structures to be
|
||
|
|
added.
|
||
|
|
|
||
|
|
png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y,
|
||
|
|
unit_type);
|
||
|
|
|
||
|
|
offset_x - positive offset from the left
|
||
|
|
edge of the screen
|
||
|
|
|
||
|
|
offset_y - positive offset from the top
|
||
|
|
edge of the screen
|
||
|
|
|
||
|
|
unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER
|
||
|
|
|
||
|
|
png_set_pHYs(png_ptr, info_ptr, res_x, res_y,
|
||
|
|
unit_type);
|
||
|
|
|
||
|
|
res_x - pixels/unit physical resolution
|
||
|
|
in x direction
|
||
|
|
|
||
|
|
res_y - pixels/unit physical resolution
|
||
|
|
in y direction
|
||
|
|
|
||
|
|
unit_type - PNG_RESOLUTION_UNKNOWN,
|
||
|
|
PNG_RESOLUTION_METER
|
||
|
|
|
||
|
|
png_set_sCAL(png_ptr, info_ptr, unit, width, height)
|
||
|
|
|
||
|
|
unit - physical scale units (an integer)
|
||
|
|
|
||
|
|
width - width of a pixel in physical scale units
|
||
|
|
|
||
|
|
height - height of a pixel in physical scale units
|
||
|
|
(width and height are doubles)
|
||
|
|
|
||
|
|
png_set_sCAL_s(png_ptr, info_ptr, unit, width, height)
|
||
|
|
|
||
|
|
unit - physical scale units (an integer)
|
||
|
|
|
||
|
|
width - width of a pixel in physical scale units
|
||
|
|
expressed as a string
|
||
|
|
|
||
|
|
height - height of a pixel in physical scale units
|
||
|
|
(width and height are strings like "2.54")
|
||
|
|
|
||
|
|
png_set_unknown_chunks(png_ptr, info_ptr, &unknowns,
|
||
|
|
num_unknowns)
|
||
|
|
|
||
|
|
unknowns - array of png_unknown_chunk
|
||
|
|
structures holding unknown chunks
|
||
|
|
unknowns[i].name - name of unknown chunk
|
||
|
|
unknowns[i].data - data of unknown chunk
|
||
|
|
unknowns[i].size - size of unknown chunk's data
|
||
|
|
unknowns[i].location - position to write chunk in file
|
||
|
|
0: do not write chunk
|
||
|
|
PNG_HAVE_IHDR: before PLTE
|
||
|
|
PNG_HAVE_PLTE: before IDAT
|
||
|
|
PNG_AFTER_IDAT: after IDAT
|
||
|
|
|
||
|
|
The "location" member is set automatically according to
|
||
|
|
what part of the output file has already been written.
|
||
|
|
You can change its value after calling png_set_unknown_chunks()
|
||
|
|
as demonstrated in pngtest.c. Within each of the "locations",
|
||
|
|
the chunks are sequenced according to their position in the
|
||
|
|
structure (that is, the value of "i", which is the order in which
|
||
|
|
the chunk was either read from the input file or defined with
|
||
|
|
png_set_unknown_chunks).
|
||
|
|
|
||
|
|
A quick word about text and num_text. text is an array of png_text
|
||
|
|
structures. num_text is the number of valid structures in the array.
|
||
|
|
Each png_text structure holds a language code, a keyword, a text value,
|
||
|
|
and a compression type.
|
||
|
|
|
||
|
|
The compression types have the same valid numbers as the compression
|
||
|
|
types of the image data. Currently, the only valid number is zero.
|
||
|
|
However, you can store text either compressed or uncompressed, unlike
|
||
|
|
images, which always have to be compressed. So if you don't want the
|
||
|
|
text compressed, set the compression type to PNG_TEXT_COMPRESSION_NONE.
|
||
|
|
Because tEXt and zTXt chunks don't have a language field, if you
|
||
|
|
specify PNG_TEXT_COMPRESSION_NONE or PNG_TEXT_COMPRESSION_zTXt
|
||
|
|
any language code or translated keyword will not be written out.
|
||
|
|
|
||
|
|
Until text gets around a few hundred bytes, it is not worth compressing it.
|
||
|
|
After the text has been written out to the file, the compression type
|
||
|
|
is set to PNG_TEXT_COMPRESSION_NONE_WR or PNG_TEXT_COMPRESSION_zTXt_WR,
|
||
|
|
so that it isn't written out again at the end (in case you are calling
|
||
|
|
png_write_end() with the same struct).
|
||
|
|
|
||
|
|
The keywords that are given in the PNG Specification are:
|
||
|
|
|
||
|
|
Title Short (one line) title or
|
||
|
|
caption for image
|
||
|
|
|
||
|
|
Author Name of image's creator
|
||
|
|
|
||
|
|
Description Description of image (possibly long)
|
||
|
|
|
||
|
|
Copyright Copyright notice
|
||
|
|
|
||
|
|
Creation Time Time of original image creation
|
||
|
|
(usually RFC 1123 format, see below)
|
||
|
|
|
||
|
|
Software Software used to create the image
|
||
|
|
|
||
|
|
Disclaimer Legal disclaimer
|
||
|
|
|
||
|
|
Warning Warning of nature of content
|
||
|
|
|
||
|
|
Source Device used to create the image
|
||
|
|
|
||
|
|
Comment Miscellaneous comment; conversion
|
||
|
|
from other image format
|
||
|
|
|
||
|
|
The keyword-text pairs work like this. Keywords should be short
|
||
|
|
simple descriptions of what the comment is about. Some typical
|
||
|
|
keywords are found in the PNG specification, as is some recommendations
|
||
|
|
on keywords. You can repeat keywords in a file. You can even write
|
||
|
|
some text before the image and some after. For example, you may want
|
||
|
|
to put a description of the image before the image, but leave the
|
||
|
|
disclaimer until after, so viewers working over modem connections
|
||
|
|
don't have to wait for the disclaimer to go over the modem before
|
||
|
|
they start seeing the image. Finally, keywords should be full
|
||
|
|
words, not abbreviations. Keywords and text are in the ISO 8859-1
|
||
|
|
(Latin-1) character set (a superset of regular ASCII) and can not
|
||
|
|
contain NUL characters, and should not contain control or other
|
||
|
|
unprintable characters. To make the comments widely readable, stick
|
||
|
|
with basic ASCII, and avoid machine specific character set extensions
|
||
|
|
like the IBM-PC character set. The keyword must be present, but
|
||
|
|
you can leave off the text string on non-compressed pairs.
|
||
|
|
Compressed pairs must have a text string, as only the text string
|
||
|
|
is compressed anyway, so the compression would be meaningless.
|
||
|
|
|
||
|
|
PNG supports modification time via the png_time structure. Two
|
||
|
|
conversion routines are provided, png_convert_from_time_t() for
|
||
|
|
time_t and png_convert_from_struct_tm() for struct tm. The
|
||
|
|
time_t routine uses gmtime(). You don't have to use either of
|
||
|
|
these, but if you wish to fill in the png_time structure directly,
|
||
|
|
you should provide the time in universal time (GMT) if possible
|
||
|
|
instead of your local time. Note that the year number is the full
|
||
|
|
year (e.g. 1998, rather than 98 - PNG is year 2000 compliant!), and
|
||
|
|
that months start with 1.
|
||
|
|
|
||
|
|
If you want to store the time of the original image creation, you should
|
||
|
|
use a plain tEXt chunk with the "Creation Time" keyword. This is
|
||
|
|
necessary because the "creation time" of a PNG image is somewhat vague,
|
||
|
|
depending on whether you mean the PNG file, the time the image was
|
||
|
|
created in a non-PNG format, a still photo from which the image was
|
||
|
|
scanned, or possibly the subject matter itself. In order to facilitate
|
||
|
|
machine-readable dates, it is recommended that the "Creation Time"
|
||
|
|
tEXt chunk use RFC 1123 format dates (e.g. "22 May 1997 18:07:10 GMT"),
|
||
|
|
although this isn't a requirement. Unlike the tIME chunk, the
|
||
|
|
"Creation Time" tEXt chunk is not expected to be automatically changed
|
||
|
|
by the software. To facilitate the use of RFC 1123 dates, a function
|
||
|
|
png_convert_to_rfc1123(png_timep) is provided to convert from PNG
|
||
|
|
time to an RFC 1123 format string.
|
||
|
|
|
||
|
|
Writing unknown chunks
|
||
|
|
|
||
|
|
You can use the png_set_unknown_chunks function to queue up chunks
|
||
|
|
for writing. You give it a chunk name, raw data, and a size; that's
|
||
|
|
all there is to it. The chunks will be written by the next following
|
||
|
|
png_write_info_before_PLTE, png_write_info, or png_write_end function.
|
||
|
|
Any chunks previously read into the info structure's unknown-chunk
|
||
|
|
list will also be written out in a sequence that satisfies the PNG
|
||
|
|
specification's ordering rules.
|
||
|
|
|
||
|
|
The high-level write interface
|
||
|
|
|
||
|
|
At this point there are two ways to proceed; through the high-level
|
||
|
|
write interface, or through a sequence of low-level write operations.
|
||
|
|
You can use the high-level interface if your image data is present
|
||
|
|
in the info structure. All defined output
|
||
|
|
transformations are permitted, enabled by the following masks.
|
||
|
|
|
||
|
|
PNG_TRANSFORM_IDENTITY No transformation
|
||
|
|
PNG_TRANSFORM_PACKING Pack 1, 2 and 4-bit samples
|
||
|
|
PNG_TRANSFORM_PACKSWAP Change order of packed
|
||
|
|
pixels to LSB first
|
||
|
|
PNG_TRANSFORM_INVERT_MONO Invert monochrome images
|
||
|
|
PNG_TRANSFORM_SHIFT Normalize pixels to the
|
||
|
|
sBIT depth
|
||
|
|
PNG_TRANSFORM_BGR Flip RGB to BGR, RGBA
|
||
|
|
to BGRA
|
||
|
|
PNG_TRANSFORM_SWAP_ALPHA Flip RGBA to ARGB or GA
|
||
|
|
to AG
|
||
|
|
PNG_TRANSFORM_INVERT_ALPHA Change alpha from opacity
|
||
|
|
to transparency
|
||
|
|
PNG_TRANSFORM_SWAP_ENDIAN Byte-swap 16-bit samples
|
||
|
|
PNG_TRANSFORM_STRIP_FILLER Strip out filler
|
||
|
|
bytes (deprecated).
|
||
|
|
PNG_TRANSFORM_STRIP_FILLER_BEFORE Strip out leading
|
||
|
|
filler bytes
|
||
|
|
PNG_TRANSFORM_STRIP_FILLER_AFTER Strip out trailing
|
||
|
|
filler bytes
|
||
|
|
|
||
|
|
If you have valid image data in the info structure (you can use
|
||
|
|
png_set_rows() to put image data in the info structure), simply do this:
|
||
|
|
|
||
|
|
png_write_png(png_ptr, info_ptr, png_transforms, NULL)
|
||
|
|
|
||
|
|
where png_transforms is an integer containing the bitwise OR of some set of
|
||
|
|
transformation flags. This call is equivalent to png_write_info(),
|
||
|
|
followed the set of transformations indicated by the transform mask,
|
||
|
|
then png_write_image(), and finally png_write_end().
|
||
|
|
|
||
|
|
(The final parameter of this call is not yet used. Someday it might point
|
||
|
|
to transformation parameters required by some future output transform.)
|
||
|
|
|
||
|
|
You must use png_transforms and not call any png_set_transform() functions
|
||
|
|
when you use png_write_png().
|
||
|
|
|
||
|
|
The low-level write interface
|
||
|
|
|
||
|
|
If you are going the low-level route instead, you are now ready to
|
||
|
|
write all the file information up to the actual image data. You do
|
||
|
|
this with a call to png_write_info().
|
||
|
|
|
||
|
|
png_write_info(png_ptr, info_ptr);
|
||
|
|
|
||
|
|
Note that there is one transformation you may need to do before
|
||
|
|
png_write_info(). In PNG files, the alpha channel in an image is the
|
||
|
|
level of opacity. If your data is supplied as a level of transparency,
|
||
|
|
you can invert the alpha channel before you write it, so that 0 is
|
||
|
|
fully transparent and 255 (in 8-bit or paletted images) or 65535
|
||
|
|
(in 16-bit images) is fully opaque, with
|
||
|
|
|
||
|
|
png_set_invert_alpha(png_ptr);
|
||
|
|
|
||
|
|
This must appear before png_write_info() instead of later with the
|
||
|
|
other transformations because in the case of paletted images the tRNS
|
||
|
|
chunk data has to be inverted before the tRNS chunk is written. If
|
||
|
|
your image is not a paletted image, the tRNS data (which in such cases
|
||
|
|
represents a single color to be rendered as transparent) won't need to
|
||
|
|
be changed, and you can safely do this transformation after your
|
||
|
|
png_write_info() call.
|
||
|
|
|
||
|
|
If you need to write a private chunk that you want to appear before
|
||
|
|
the PLTE chunk when PLTE is present, you can write the PNG info in
|
||
|
|
two steps, and insert code to write your own chunk between them:
|
||
|
|
|
||
|
|
png_write_info_before_PLTE(png_ptr, info_ptr);
|
||
|
|
png_set_unknown_chunks(png_ptr, info_ptr, ...);
|
||
|
|
png_write_info(png_ptr, info_ptr);
|
||
|
|
|
||
|
|
After you've written the file information, you can set up the library
|
||
|
|
to handle any special transformations of the image data. The various
|
||
|
|
ways to transform the data will be described in the order that they
|
||
|
|
should occur. This is important, as some of these change the color
|
||
|
|
type and/or bit depth of the data, and some others only work on
|
||
|
|
certain color types and bit depths. Even though each transformation
|
||
|
|
checks to see if it has data that it can do something with, you should
|
||
|
|
make sure to only enable a transformation if it will be valid for the
|
||
|
|
data. For example, don't swap red and blue on grayscale data.
|
||
|
|
|
||
|
|
PNG files store RGB pixels packed into 3 or 6 bytes. This code tells
|
||
|
|
the library to strip input data that has 4 or 8 bytes per pixel down
|
||
|
|
to 3 or 6 bytes (or strip 2 or 4-byte grayscale+filler data to 1 or 2
|
||
|
|
bytes per pixel).
|
||
|
|
|
||
|
|
png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);
|
||
|
|
|
||
|
|
where the 0 is unused, and the location is either PNG_FILLER_BEFORE or
|
||
|
|
PNG_FILLER_AFTER, depending upon whether the filler byte in the pixel
|
||
|
|
is stored XRGB or RGBX.
|
||
|
|
|
||
|
|
PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
|
||
|
|
they can, resulting in, for example, 8 pixels per byte for 1 bit files.
|
||
|
|
If the data is supplied at 1 pixel per byte, use this code, which will
|
||
|
|
correctly pack the pixels into a single byte:
|
||
|
|
|
||
|
|
png_set_packing(png_ptr);
|
||
|
|
|
||
|
|
PNG files reduce possible bit depths to 1, 2, 4, 8, and 16. If your
|
||
|
|
data is of another bit depth, you can write an sBIT chunk into the
|
||
|
|
file so that decoders can recover the original data if desired.
|
||
|
|
|
||
|
|
/* Set the true bit depth of the image data */
|
||
|
|
if (color_type & PNG_COLOR_MASK_COLOR)
|
||
|
|
{
|
||
|
|
sig_bit.red = true_bit_depth;
|
||
|
|
sig_bit.green = true_bit_depth;
|
||
|
|
sig_bit.blue = true_bit_depth;
|
||
|
|
}
|
||
|
|
|
||
|
|
else
|
||
|
|
{
|
||
|
|
sig_bit.gray = true_bit_depth;
|
||
|
|
}
|
||
|
|
|
||
|
|
if (color_type & PNG_COLOR_MASK_ALPHA)
|
||
|
|
{
|
||
|
|
sig_bit.alpha = true_bit_depth;
|
||
|
|
}
|
||
|
|
|
||
|
|
png_set_sBIT(png_ptr, info_ptr, &sig_bit);
|
||
|
|
|
||
|
|
If the data is stored in the row buffer in a bit depth other than
|
||
|
|
one supported by PNG (e.g. 3 bit data in the range 0-7 for a 4-bit PNG),
|
||
|
|
this will scale the values to appear to be the correct bit depth as
|
||
|
|
is required by PNG.
|
||
|
|
|
||
|
|
png_set_shift(png_ptr, &sig_bit);
|
||
|
|
|
||
|
|
PNG files store 16-bit pixels in network byte order (big-endian,
|
||
|
|
ie. most significant bits first). This code would be used if they are
|
||
|
|
supplied the other way (little-endian, i.e. least significant bits
|
||
|
|
first, the way PCs store them):
|
||
|
|
|
||
|
|
if (bit_depth > 8)
|
||
|
|
png_set_swap(png_ptr);
|
||
|
|
|
||
|
|
If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you
|
||
|
|
need to change the order the pixels are packed into bytes, you can use:
|
||
|
|
|
||
|
|
if (bit_depth < 8)
|
||
|
|
png_set_packswap(png_ptr);
|
||
|
|
|
||
|
|
PNG files store 3 color pixels in red, green, blue order. This code
|
||
|
|
would be used if they are supplied as blue, green, red:
|
||
|
|
|
||
|
|
png_set_bgr(png_ptr);
|
||
|
|
|
||
|
|
PNG files describe monochrome as black being zero and white being
|
||
|
|
one. This code would be used if the pixels are supplied with this reversed
|
||
|
|
(black being one and white being zero):
|
||
|
|
|
||
|
|
png_set_invert_mono(png_ptr);
|
||
|
|
|
||
|
|
Finally, you can write your own transformation function if none of
|
||
|
|
the existing ones meets your needs. This is done by setting a callback
|
||
|
|
with
|
||
|
|
|
||
|
|
png_set_write_user_transform_fn(png_ptr,
|
||
|
|
write_transform_fn);
|
||
|
|
|
||
|
|
You must supply the function
|
||
|
|
|
||
|
|
void write_transform_fn(png_structp png_ptr, png_row_infop
|
||
|
|
row_info, png_bytep data)
|
||
|
|
|
||
|
|
See pngtest.c for a working example. Your function will be called
|
||
|
|
before any of the other transformations are processed. If supported
|
||
|
|
libpng also supplies an information routine that may be called from
|
||
|
|
your callback:
|
||
|
|
|
||
|
|
png_get_current_row_number(png_ptr);
|
||
|
|
png_get_current_pass_number(png_ptr);
|
||
|
|
|
||
|
|
This returns the current row passed to the transform. With interlaced
|
||
|
|
images the value returned is the row in the input sub-image image. Use
|
||
|
|
PNG_ROW_FROM_PASS_ROW(row, pass) and PNG_COL_FROM_PASS_COL(col, pass) to
|
||
|
|
find the output pixel (x,y) given an interlaced sub-image pixel (row,col,pass).
|
||
|
|
|
||
|
|
The discussion of interlace handling above contains more information on how to
|
||
|
|
use these values.
|
||
|
|
|
||
|
|
You can also set up a pointer to a user structure for use by your
|
||
|
|
callback function.
|
||
|
|
|
||
|
|
png_set_user_transform_info(png_ptr, user_ptr, 0, 0);
|
||
|
|
|
||
|
|
The user_channels and user_depth parameters of this function are ignored
|
||
|
|
when writing; you can set them to zero as shown.
|
||
|
|
|
||
|
|
You can retrieve the pointer via the function png_get_user_transform_ptr().
|
||
|
|
For example:
|
||
|
|
|
||
|
|
voidp write_user_transform_ptr =
|
||
|
|
png_get_user_transform_ptr(png_ptr);
|
||
|
|
|
||
|
|
It is possible to have libpng flush any pending output, either manually,
|
||
|
|
or automatically after a certain number of lines have been written. To
|
||
|
|
flush the output stream a single time call:
|
||
|
|
|
||
|
|
png_write_flush(png_ptr);
|
||
|
|
|
||
|
|
and to have libpng flush the output stream periodically after a certain
|
||
|
|
number of scanlines have been written, call:
|
||
|
|
|
||
|
|
png_set_flush(png_ptr, nrows);
|
||
|
|
|
||
|
|
Note that the distance between rows is from the last time png_write_flush()
|
||
|
|
was called, or the first row of the image if it has never been called.
|
||
|
|
So if you write 50 lines, and then png_set_flush 25, it will flush the
|
||
|
|
output on the next scanline, and every 25 lines thereafter, unless
|
||
|
|
png_write_flush() is called before 25 more lines have been written.
|
||
|
|
If nrows is too small (less than about 10 lines for a 640 pixel wide
|
||
|
|
RGB image) the image compression may decrease noticeably (although this
|
||
|
|
may be acceptable for real-time applications). Infrequent flushing will
|
||
|
|
only degrade the compression performance by a few percent over images
|
||
|
|
that do not use flushing.
|
||
|
|
|
||
|
|
Writing the image data
|
||
|
|
|
||
|
|
That's it for the transformations. Now you can write the image data.
|
||
|
|
The simplest way to do this is in one function call. If you have the
|
||
|
|
whole image in memory, you can just call png_write_image() and libpng
|
||
|
|
will write the image. You will need to pass in an array of pointers to
|
||
|
|
each row. This function automatically handles interlacing, so you don't
|
||
|
|
need to call png_set_interlace_handling() or call this function multiple
|
||
|
|
times, or any of that other stuff necessary with png_write_rows().
|
||
|
|
|
||
|
|
png_write_image(png_ptr, row_pointers);
|
||
|
|
|
||
|
|
where row_pointers is:
|
||
|
|
|
||
|
|
png_byte *row_pointers[height];
|
||
|
|
|
||
|
|
You can point to void or char or whatever you use for pixels.
|
||
|
|
|
||
|
|
If you don't want to write the whole image at once, you can
|
||
|
|
use png_write_rows() instead. If the file is not interlaced,
|
||
|
|
this is simple:
|
||
|
|
|
||
|
|
png_write_rows(png_ptr, row_pointers,
|
||
|
|
number_of_rows);
|
||
|
|
|
||
|
|
row_pointers is the same as in the png_write_image() call.
|
||
|
|
|
||
|
|
If you are just writing one row at a time, you can do this with
|
||
|
|
a single row_pointer instead of an array of row_pointers:
|
||
|
|
|
||
|
|
png_bytep row_pointer = row;
|
||
|
|
|
||
|
|
png_write_row(png_ptr, row_pointer);
|
||
|
|
|
||
|
|
When the file is interlaced, things can get a good deal more complicated.
|
||
|
|
The only currently (as of the PNG Specification version 1.2, dated July
|
||
|
|
1999) defined interlacing scheme for PNG files is the "Adam7" interlace
|
||
|
|
scheme, that breaks down an image into seven smaller images of varying
|
||
|
|
size. libpng will build these images for you, or you can do them
|
||
|
|
yourself. If you want to build them yourself, see the PNG specification
|
||
|
|
for details of which pixels to write when.
|
||
|
|
|
||
|
|
If you don't want libpng to handle the interlacing details, just
|
||
|
|
use png_set_interlace_handling() and call png_write_rows() the
|
||
|
|
correct number of times to write all the sub-images
|
||
|
|
(png_set_interlace_handling() returns the number of sub-images.)
|
||
|
|
|
||
|
|
If you want libpng to build the sub-images, call this before you start
|
||
|
|
writing any rows:
|
||
|
|
|
||
|
|
number_of_passes = png_set_interlace_handling(png_ptr);
|
||
|
|
|
||
|
|
This will return the number of passes needed. Currently, this is seven,
|
||
|
|
but may change if another interlace type is added.
|
||
|
|
|
||
|
|
Then write the complete image number_of_passes times.
|
||
|
|
|
||
|
|
png_write_rows(png_ptr, row_pointers, number_of_rows);
|
||
|
|
|
||
|
|
Think carefully before you write an interlaced image. Typically code that
|
||
|
|
reads such images reads all the image data into memory, uncompressed, before
|
||
|
|
doing any processing. Only code that can display an image on the fly can
|
||
|
|
take advantage of the interlacing and even then the image has to be exactly
|
||
|
|
the correct size for the output device, because scaling an image requires
|
||
|
|
adjacent pixels and these are not available until all the passes have been
|
||
|
|
read.
|
||
|
|
|
||
|
|
If you do write an interlaced image you will hardly ever need to handle
|
||
|
|
the interlacing yourself. Call png_set_interlace_handling() and use the
|
||
|
|
approach described above.
|
||
|
|
|
||
|
|
The only time it is conceivable that you will really need to write an
|
||
|
|
interlaced image pass-by-pass is when you have read one pass by pass and
|
||
|
|
made some pixel-by-pixel transformation to it, as described in the read
|
||
|
|
code above. In this case use the PNG_PASS_ROWS and PNG_PASS_COLS macros
|
||
|
|
to determine the size of each sub-image in turn and simply write the rows
|
||
|
|
you obtained from the read code.
|
||
|
|
|
||
|
|
Finishing a sequential write
|
||
|
|
|
||
|
|
After you are finished writing the image, you should finish writing
|
||
|
|
the file. If you are interested in writing comments or time, you should
|
||
|
|
pass an appropriately filled png_info pointer. If you are not interested,
|
||
|
|
you can pass NULL.
|
||
|
|
|
||
|
|
png_write_end(png_ptr, info_ptr);
|
||
|
|
|
||
|
|
When you are done, you can free all memory used by libpng like this:
|
||
|
|
|
||
|
|
png_destroy_write_struct(&png_ptr, &info_ptr);
|
||
|
|
|
||
|
|
It is also possible to individually free the info_ptr members that
|
||
|
|
point to libpng-allocated storage with the following function:
|
||
|
|
|
||
|
|
png_free_data(png_ptr, info_ptr, mask, seq)
|
||
|
|
|
||
|
|
mask - identifies data to be freed, a mask
|
||
|
|
containing the bitwise OR of one or
|
||
|
|
more of
|
||
|
|
PNG_FREE_PLTE, PNG_FREE_TRNS,
|
||
|
|
PNG_FREE_HIST, PNG_FREE_ICCP,
|
||
|
|
PNG_FREE_PCAL, PNG_FREE_ROWS,
|
||
|
|
PNG_FREE_SCAL, PNG_FREE_SPLT,
|
||
|
|
PNG_FREE_TEXT, PNG_FREE_UNKN,
|
||
|
|
or simply PNG_FREE_ALL
|
||
|
|
|
||
|
|
seq - sequence number of item to be freed
|
||
|
|
(-1 for all items)
|
||
|
|
|
||
|
|
This function may be safely called when the relevant storage has
|
||
|
|
already been freed, or has not yet been allocated, or was allocated
|
||
|
|
by the user and not by libpng, and will in those cases do nothing.
|
||
|
|
The "seq" parameter is ignored if only one item of the selected data
|
||
|
|
type, such as PLTE, is allowed. If "seq" is not -1, and multiple items
|
||
|
|
are allowed for the data type identified in the mask, such as text or
|
||
|
|
sPLT, only the n'th item in the structure is freed, where n is "seq".
|
||
|
|
|
||
|
|
If you allocated data such as a palette that you passed in to libpng
|
||
|
|
with png_set_*, you must not free it until just before the call to
|
||
|
|
png_destroy_write_struct().
|
||
|
|
|
||
|
|
The default behavior is only to free data that was allocated internally
|
||
|
|
by libpng. This can be changed, so that libpng will not free the data,
|
||
|
|
or so that it will free data that was allocated by the user with png_malloc()
|
||
|
|
or png_zalloc() and passed in via a png_set_*() function, with
|
||
|
|
|
||
|
|
png_data_freer(png_ptr, info_ptr, freer, mask)
|
||
|
|
|
||
|
|
freer - one of
|
||
|
|
PNG_DESTROY_WILL_FREE_DATA
|
||
|
|
PNG_SET_WILL_FREE_DATA
|
||
|
|
PNG_USER_WILL_FREE_DATA
|
||
|
|
|
||
|
|
mask - which data elements are affected
|
||
|
|
same choices as in png_free_data()
|
||
|
|
|
||
|
|
For example, to transfer responsibility for some data from a read structure
|
||
|
|
to a write structure, you could use
|
||
|
|
|
||
|
|
png_data_freer(read_ptr, read_info_ptr,
|
||
|
|
PNG_USER_WILL_FREE_DATA,
|
||
|
|
PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)
|
||
|
|
|
||
|
|
png_data_freer(write_ptr, write_info_ptr,
|
||
|
|
PNG_DESTROY_WILL_FREE_DATA,
|
||
|
|
PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)
|
||
|
|
|
||
|
|
thereby briefly reassigning responsibility for freeing to the user but
|
||
|
|
immediately afterwards reassigning it once more to the write_destroy
|
||
|
|
function. Having done this, it would then be safe to destroy the read
|
||
|
|
structure and continue to use the PLTE, tRNS, and hIST data in the write
|
||
|
|
structure.
|
||
|
|
|
||
|
|
This function only affects data that has already been allocated.
|
||
|
|
You can call this function before calling after the png_set_*() functions
|
||
|
|
to control whether the user or png_destroy_*() is supposed to free the data.
|
||
|
|
When the user assumes responsibility for libpng-allocated data, the
|
||
|
|
application must use
|
||
|
|
png_free() to free it, and when the user transfers responsibility to libpng
|
||
|
|
for data that the user has allocated, the user must have used png_malloc()
|
||
|
|
or png_zalloc() to allocate it.
|
||
|
|
|
||
|
|
If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword
|
||
|
|
separately, do not transfer responsibility for freeing text_ptr to libpng,
|
||
|
|
because when libpng fills a png_text structure it combines these members with
|
||
|
|
the key member, and png_free_data() will free only text_ptr.key. Similarly,
|
||
|
|
if you transfer responsibility for free'ing text_ptr from libpng to your
|
||
|
|
application, your application must not separately free those members.
|
||
|
|
For a more compact example of writing a PNG image, see the file example.c.
|
||
|
|
|
||
|
|
V. Modifying/Customizing libpng:
|
||
|
|
|
||
|
|
There are two issues here. The first is changing how libpng does
|
||
|
|
standard things like memory allocation, input/output, and error handling.
|
||
|
|
The second deals with more complicated things like adding new chunks,
|
||
|
|
adding new transformations, and generally changing how libpng works.
|
||
|
|
Both of those are compile-time issues; that is, they are generally
|
||
|
|
determined at the time the code is written, and there is rarely a need
|
||
|
|
to provide the user with a means of changing them.
|
||
|
|
|
||
|
|
Memory allocation, input/output, and error handling
|
||
|
|
|
||
|
|
All of the memory allocation, input/output, and error handling in libpng
|
||
|
|
goes through callbacks that are user-settable. The default routines are
|
||
|
|
in pngmem.c, pngrio.c, pngwio.c, and pngerror.c, respectively. To change
|
||
|
|
these functions, call the appropriate png_set_*_fn() function.
|
||
|
|
|
||
|
|
Memory allocation is done through the functions png_malloc(), png_calloc(),
|
||
|
|
and png_free(). These currently just call the standard C functions.
|
||
|
|
png_calloc() calls png_malloc() and then clears the newly
|
||
|
|
allocated memory to zero. There is limited support for certain systems
|
||
|
|
with segmented memory architectures and the types of pointers declared by
|
||
|
|
png.h match this; you will have to use appropriate pointers in your
|
||
|
|
application. Since it is
|
||
|
|
unlikely that the method of handling memory allocation on a platform
|
||
|
|
will change between applications, these functions must be modified in
|
||
|
|
the library at compile time. If you prefer to use a different method
|
||
|
|
of allocating and freeing data, you can use png_create_read_struct_2() or
|
||
|
|
png_create_write_struct_2() to register your own functions as described
|
||
|
|
above. These functions also provide a void pointer that can be retrieved
|
||
|
|
via
|
||
|
|
|
||
|
|
mem_ptr=png_get_mem_ptr(png_ptr);
|
||
|
|
|
||
|
|
Your replacement memory functions must have prototypes as follows:
|
||
|
|
|
||
|
|
png_voidp malloc_fn(png_structp png_ptr,
|
||
|
|
png_alloc_size_t size);
|
||
|
|
|
||
|
|
void free_fn(png_structp png_ptr, png_voidp ptr);
|
||
|
|
|
||
|
|
Your malloc_fn() must return NULL in case of failure. The png_malloc()
|
||
|
|
function will normally call png_error() if it receives a NULL from the
|
||
|
|
system memory allocator or from your replacement malloc_fn().
|
||
|
|
|
||
|
|
Your free_fn() will never be called with a NULL ptr, since libpng's
|
||
|
|
png_free() checks for NULL before calling free_fn().
|
||
|
|
|
||
|
|
Input/Output in libpng is done through png_read() and png_write(),
|
||
|
|
which currently just call fread() and fwrite(). The FILE * is stored in
|
||
|
|
png_struct and is initialized via png_init_io(). If you wish to change
|
||
|
|
the method of I/O, the library supplies callbacks that you can set
|
||
|
|
through the function png_set_read_fn() and png_set_write_fn() at run
|
||
|
|
time, instead of calling the png_init_io() function. These functions
|
||
|
|
also provide a void pointer that can be retrieved via the function
|
||
|
|
png_get_io_ptr(). For example:
|
||
|
|
|
||
|
|
png_set_read_fn(png_structp read_ptr,
|
||
|
|
voidp read_io_ptr, png_rw_ptr read_data_fn)
|
||
|
|
|
||
|
|
png_set_write_fn(png_structp write_ptr,
|
||
|
|
voidp write_io_ptr, png_rw_ptr write_data_fn,
|
||
|
|
png_flush_ptr output_flush_fn);
|
||
|
|
|
||
|
|
voidp read_io_ptr = png_get_io_ptr(read_ptr);
|
||
|
|
voidp write_io_ptr = png_get_io_ptr(write_ptr);
|
||
|
|
|
||
|
|
The replacement I/O functions must have prototypes as follows:
|
||
|
|
|
||
|
|
void user_read_data(png_structp png_ptr,
|
||
|
|
png_bytep data, png_size_t length);
|
||
|
|
|
||
|
|
void user_write_data(png_structp png_ptr,
|
||
|
|
png_bytep data, png_size_t length);
|
||
|
|
|
||
|
|
void user_flush_data(png_structp png_ptr);
|
||
|
|
|
||
|
|
The user_read_data() function is responsible for detecting and
|
||
|
|
handling end-of-data errors.
|
||
|
|
|
||
|
|
Supplying NULL for the read, write, or flush functions sets them back
|
||
|
|
to using the default C stream functions, which expect the io_ptr to
|
||
|
|
point to a standard *FILE structure. It is probably a mistake
|
||
|
|
to use NULL for one of write_data_fn and output_flush_fn but not both
|
||
|
|
of them, unless you have built libpng with PNG_NO_WRITE_FLUSH defined.
|
||
|
|
It is an error to read from a write stream, and vice versa.
|
||
|
|
|
||
|
|
Error handling in libpng is done through png_error() and png_warning().
|
||
|
|
Errors handled through png_error() are fatal, meaning that png_error()
|
||
|
|
should never return to its caller. Currently, this is handled via
|
||
|
|
setjmp() and longjmp() (unless you have compiled libpng with
|
||
|
|
PNG_NO_SETJMP, in which case it is handled via PNG_ABORT()),
|
||
|
|
but you could change this to do things like exit() if you should wish,
|
||
|
|
as long as your function does not return.
|
||
|
|
|
||
|
|
On non-fatal errors, png_warning() is called
|
||
|
|
to print a warning message, and then control returns to the calling code.
|
||
|
|
By default png_error() and png_warning() print a message on stderr via
|
||
|
|
fprintf() unless the library is compiled with PNG_NO_CONSOLE_IO defined
|
||
|
|
(because you don't want the messages) or PNG_NO_STDIO defined (because
|
||
|
|
fprintf() isn't available). If you wish to change the behavior of the error
|
||
|
|
functions, you will need to set up your own message callbacks. These
|
||
|
|
functions are normally supplied at the time that the png_struct is created.
|
||
|
|
It is also possible to redirect errors and warnings to your own replacement
|
||
|
|
functions after png_create_*_struct() has been called by calling:
|
||
|
|
|
||
|
|
png_set_error_fn(png_structp png_ptr,
|
||
|
|
png_voidp error_ptr, png_error_ptr error_fn,
|
||
|
|
png_error_ptr warning_fn);
|
||
|
|
|
||
|
|
png_voidp error_ptr = png_get_error_ptr(png_ptr);
|
||
|
|
|
||
|
|
If NULL is supplied for either error_fn or warning_fn, then the libpng
|
||
|
|
default function will be used, calling fprintf() and/or longjmp() if a
|
||
|
|
problem is encountered. The replacement error functions should have
|
||
|
|
parameters as follows:
|
||
|
|
|
||
|
|
void user_error_fn(png_structp png_ptr,
|
||
|
|
png_const_charp error_msg);
|
||
|
|
|
||
|
|
void user_warning_fn(png_structp png_ptr,
|
||
|
|
png_const_charp warning_msg);
|
||
|
|
|
||
|
|
The motivation behind using setjmp() and longjmp() is the C++ throw and
|
||
|
|
catch exception handling methods. This makes the code much easier to write,
|
||
|
|
as there is no need to check every return code of every function call.
|
||
|
|
However, there are some uncertainties about the status of local variables
|
||
|
|
after a longjmp, so the user may want to be careful about doing anything
|
||
|
|
after setjmp returns non-zero besides returning itself. Consult your
|
||
|
|
compiler documentation for more details. For an alternative approach, you
|
||
|
|
may wish to use the "cexcept" facility (see http://cexcept.sourceforge.net),
|
||
|
|
which is illustrated in pngvalid.c and in contrib/visupng.
|
||
|
|
|
||
|
|
Custom chunks
|
||
|
|
|
||
|
|
If you need to read or write custom chunks, you may need to get deeper
|
||
|
|
into the libpng code. The library now has mechanisms for storing
|
||
|
|
and writing chunks of unknown type; you can even declare callbacks
|
||
|
|
for custom chunks. However, this may not be good enough if the
|
||
|
|
library code itself needs to know about interactions between your
|
||
|
|
chunk and existing `intrinsic' chunks.
|
||
|
|
|
||
|
|
If you need to write a new intrinsic chunk, first read the PNG
|
||
|
|
specification. Acquire a first level of understanding of how it works.
|
||
|
|
Pay particular attention to the sections that describe chunk names,
|
||
|
|
and look at how other chunks were designed, so you can do things
|
||
|
|
similarly. Second, check out the sections of libpng that read and
|
||
|
|
write chunks. Try to find a chunk that is similar to yours and use
|
||
|
|
it as a template. More details can be found in the comments inside
|
||
|
|
the code. It is best to handle private or unknown chunks in a generic method,
|
||
|
|
via callback functions, instead of by modifying libpng functions. This
|
||
|
|
is illustrated in pngtest.c, which uses a callback function to handle a
|
||
|
|
private "vpAg" chunk and the new "sTER" chunk, which are both unknown to
|
||
|
|
libpng.
|
||
|
|
|
||
|
|
If you wish to write your own transformation for the data, look through
|
||
|
|
the part of the code that does the transformations, and check out some of
|
||
|
|
the simpler ones to get an idea of how they work. Try to find a similar
|
||
|
|
transformation to the one you want to add and copy off of it. More details
|
||
|
|
can be found in the comments inside the code itself.
|
||
|
|
|
||
|
|
Configuring for 16-bit platforms
|
||
|
|
|
||
|
|
You will want to look into zconf.h to tell zlib (and thus libpng) that
|
||
|
|
it cannot allocate more then 64K at a time. Even if you can, the memory
|
||
|
|
won't be accessible. So limit zlib and libpng to 64K by defining MAXSEG_64K.
|
||
|
|
|
||
|
|
Configuring for DOS
|
||
|
|
|
||
|
|
For DOS users who only have access to the lower 640K, you will
|
||
|
|
have to limit zlib's memory usage via a png_set_compression_mem_level()
|
||
|
|
call. See zlib.h or zconf.h in the zlib library for more information.
|
||
|
|
|
||
|
|
Configuring for Medium Model
|
||
|
|
|
||
|
|
Libpng's support for medium model has been tested on most of the popular
|
||
|
|
compilers. Make sure MAXSEG_64K gets defined, USE_FAR_KEYWORD gets
|
||
|
|
defined, and FAR gets defined to far in pngconf.h, and you should be
|
||
|
|
all set. Everything in the library (except for zlib's structure) is
|
||
|
|
expecting far data. You must use the typedefs with the p or pp on
|
||
|
|
the end for pointers (or at least look at them and be careful). Make
|
||
|
|
note that the rows of data are defined as png_bytepp, which is
|
||
|
|
an "unsigned char far * far *".
|
||
|
|
|
||
|
|
Configuring for gui/windowing platforms:
|
||
|
|
|
||
|
|
You will need to write new error and warning functions that use the GUI
|
||
|
|
interface, as described previously, and set them to be the error and
|
||
|
|
warning functions at the time that png_create_*_struct() is called,
|
||
|
|
in order to have them available during the structure initialization.
|
||
|
|
They can be changed later via png_set_error_fn(). On some compilers,
|
||
|
|
you may also have to change the memory allocators (png_malloc, etc.).
|
||
|
|
|
||
|
|
Configuring for compiler xxx:
|
||
|
|
|
||
|
|
All includes for libpng are in pngconf.h. If you need to add, change
|
||
|
|
or delete an include, this is the place to do it.
|
||
|
|
The includes that are not needed outside libpng are placed in pngpriv.h,
|
||
|
|
which is only used by the routines inside libpng itself.
|
||
|
|
The files in libpng proper only include pngpriv.h and png.h, which
|
||
|
|
in turn includes pngconf.h and, as of libpng-1.5.0, pnglibconf.h.
|
||
|
|
As of libpng-1.5.0, pngpriv.h also includes three other private header
|
||
|
|
files, pngstruct.h, pnginfo.h, and pngdebug.h, which contain material
|
||
|
|
that previously appeared in the public headers.
|
||
|
|
|
||
|
|
Configuring zlib:
|
||
|
|
|
||
|
|
There are special functions to configure the compression. Perhaps the
|
||
|
|
most useful one changes the compression level, which currently uses
|
||
|
|
input compression values in the range 0 - 9. The library normally
|
||
|
|
uses the default compression level (Z_DEFAULT_COMPRESSION = 6). Tests
|
||
|
|
have shown that for a large majority of images, compression values in
|
||
|
|
the range 3-6 compress nearly as well as higher levels, and do so much
|
||
|
|
faster. For online applications it may be desirable to have maximum speed
|
||
|
|
(Z_BEST_SPEED = 1). With versions of zlib after v0.99, you can also
|
||
|
|
specify no compression (Z_NO_COMPRESSION = 0), but this would create
|
||
|
|
files larger than just storing the raw bitmap. You can specify the
|
||
|
|
compression level by calling:
|
||
|
|
|
||
|
|
#include zlib.h
|
||
|
|
png_set_compression_level(png_ptr, level);
|
||
|
|
|
||
|
|
Another useful one is to reduce the memory level used by the library.
|
||
|
|
The memory level defaults to 8, but it can be lowered if you are
|
||
|
|
short on memory (running DOS, for example, where you only have 640K).
|
||
|
|
Note that the memory level does have an effect on compression; among
|
||
|
|
other things, lower levels will result in sections of incompressible
|
||
|
|
data being emitted in smaller stored blocks, with a correspondingly
|
||
|
|
larger relative overhead of up to 15% in the worst case.
|
||
|
|
|
||
|
|
#include zlib.h
|
||
|
|
png_set_compression_mem_level(png_ptr, level);
|
||
|
|
|
||
|
|
The other functions are for configuring zlib. They are not recommended
|
||
|
|
for normal use and may result in writing an invalid PNG file. See
|
||
|
|
zlib.h for more information on what these mean.
|
||
|
|
|
||
|
|
#include zlib.h
|
||
|
|
png_set_compression_strategy(png_ptr,
|
||
|
|
strategy);
|
||
|
|
|
||
|
|
png_set_compression_window_bits(png_ptr,
|
||
|
|
window_bits);
|
||
|
|
|
||
|
|
png_set_compression_method(png_ptr, method);
|
||
|
|
|
||
|
|
png_set_compression_buffer_size(png_ptr, size);
|
||
|
|
|
||
|
|
As of libpng version 1.5.4, additional APIs became
|
||
|
|
available to set these separately for non-IDAT
|
||
|
|
compressed chunks such as zTXt, iTXt, and iCCP:
|
||
|
|
|
||
|
|
#include zlib.h
|
||
|
|
#if PNG_LIBPNG_VER <= 10504
|
||
|
|
png_set_text_compression_level(png_ptr, level);
|
||
|
|
|
||
|
|
png_set_text_compression_mem_level(png_ptr, level);
|
||
|
|
|
||
|
|
png_set_text_compression_strategy(png_ptr,
|
||
|
|
strategy);
|
||
|
|
|
||
|
|
png_set_text_compression_window_bits(png_ptr,
|
||
|
|
window_bits);
|
||
|
|
|
||
|
|
png_set_text_compression_method(png_ptr, method);
|
||
|
|
#endif
|
||
|
|
|
||
|
|
Controlling row filtering
|
||
|
|
|
||
|
|
If you want to control whether libpng uses filtering or not, which
|
||
|
|
filters are used, and how it goes about picking row filters, you
|
||
|
|
can call one of these functions. The selection and configuration
|
||
|
|
of row filters can have a significant impact on the size and
|
||
|
|
encoding speed and a somewhat lesser impact on the decoding speed
|
||
|
|
of an image. Filtering is enabled by default for RGB and grayscale
|
||
|
|
images (with and without alpha), but not for paletted images nor
|
||
|
|
for any images with bit depths less than 8 bits/pixel.
|
||
|
|
|
||
|
|
The 'method' parameter sets the main filtering method, which is
|
||
|
|
currently only '0' in the PNG 1.2 specification. The 'filters'
|
||
|
|
parameter sets which filter(s), if any, should be used for each
|
||
|
|
scanline. Possible values are PNG_ALL_FILTERS and PNG_NO_FILTERS
|
||
|
|
to turn filtering on and off, respectively.
|
||
|
|
|
||
|
|
Individual filter types are PNG_FILTER_NONE, PNG_FILTER_SUB,
|
||
|
|
PNG_FILTER_UP, PNG_FILTER_AVG, PNG_FILTER_PAETH, which can be bitwise
|
||
|
|
ORed together with '|' to specify one or more filters to use.
|
||
|
|
These filters are described in more detail in the PNG specification.
|
||
|
|
If you intend to change the filter type during the course of writing
|
||
|
|
the image, you should start with flags set for all of the filters
|
||
|
|
you intend to use so that libpng can initialize its internal
|
||
|
|
structures appropriately for all of the filter types. (Note that this
|
||
|
|
means the first row must always be adaptively filtered, because libpng
|
||
|
|
currently does not allocate the filter buffers until png_write_row()
|
||
|
|
is called for the first time.)
|
||
|
|
|
||
|
|
filters = PNG_FILTER_NONE | PNG_FILTER_SUB
|
||
|
|
PNG_FILTER_UP | PNG_FILTER_AVG |
|
||
|
|
PNG_FILTER_PAETH | PNG_ALL_FILTERS;
|
||
|
|
|
||
|
|
png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE,
|
||
|
|
filters);
|
||
|
|
The second parameter can also be
|
||
|
|
PNG_INTRAPIXEL_DIFFERENCING if you are
|
||
|
|
writing a PNG to be embedded in a MNG
|
||
|
|
datastream. This parameter must be the
|
||
|
|
same as the value of filter_method used
|
||
|
|
in png_set_IHDR().
|
||
|
|
|
||
|
|
It is also possible to influence how libpng chooses from among the
|
||
|
|
available filters. This is done in one or both of two ways - by
|
||
|
|
telling it how important it is to keep the same filter for successive
|
||
|
|
rows, and by telling it the relative computational costs of the filters.
|
||
|
|
|
||
|
|
double weights[3] = {1.5, 1.3, 1.1},
|
||
|
|
costs[PNG_FILTER_VALUE_LAST] =
|
||
|
|
{1.0, 1.3, 1.3, 1.5, 1.7};
|
||
|
|
|
||
|
|
png_set_filter_heuristics(png_ptr,
|
||
|
|
PNG_FILTER_HEURISTIC_WEIGHTED, 3,
|
||
|
|
weights, costs);
|
||
|
|
|
||
|
|
The weights are multiplying factors that indicate to libpng that the
|
||
|
|
row filter should be the same for successive rows unless another row filter
|
||
|
|
is that many times better than the previous filter. In the above example,
|
||
|
|
if the previous 3 filters were SUB, SUB, NONE, the SUB filter could have a
|
||
|
|
"sum of absolute differences" 1.5 x 1.3 times higher than other filters
|
||
|
|
and still be chosen, while the NONE filter could have a sum 1.1 times
|
||
|
|
higher than other filters and still be chosen. Unspecified weights are
|
||
|
|
taken to be 1.0, and the specified weights should probably be declining
|
||
|
|
like those above in order to emphasize recent filters over older filters.
|
||
|
|
|
||
|
|
The filter costs specify for each filter type a relative decoding cost
|
||
|
|
to be considered when selecting row filters. This means that filters
|
||
|
|
with higher costs are less likely to be chosen over filters with lower
|
||
|
|
costs, unless their "sum of absolute differences" is that much smaller.
|
||
|
|
The costs do not necessarily reflect the exact computational speeds of
|
||
|
|
the various filters, since this would unduly influence the final image
|
||
|
|
size.
|
||
|
|
|
||
|
|
Note that the numbers above were invented purely for this example and
|
||
|
|
are given only to help explain the function usage. Little testing has
|
||
|
|
been done to find optimum values for either the costs or the weights.
|
||
|
|
|
||
|
|
Removing unwanted object code
|
||
|
|
|
||
|
|
There are a bunch of #define's in pngconf.h that control what parts of
|
||
|
|
libpng are compiled. All the defines end in _SUPPORTED. If you are
|
||
|
|
never going to use a capability, you can change the #define to #undef
|
||
|
|
before recompiling libpng and save yourself code and data space, or
|
||
|
|
you can turn off individual capabilities with defines that begin with
|
||
|
|
PNG_NO_.
|
||
|
|
|
||
|
|
In libpng-1.5.0 and later, the #define's are in pnglibconf.h instead.
|
||
|
|
|
||
|
|
You can also turn all of the transforms and ancillary chunk capabilities
|
||
|
|
off en masse with compiler directives that define
|
||
|
|
PNG_NO_READ[or WRITE]_TRANSFORMS, or PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS,
|
||
|
|
or all four,
|
||
|
|
along with directives to turn on any of the capabilities that you do
|
||
|
|
want. The PNG_NO_READ[or WRITE]_TRANSFORMS directives disable the extra
|
||
|
|
transformations but still leave the library fully capable of reading
|
||
|
|
and writing PNG files with all known public chunks. Use of the
|
||
|
|
PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS directive produces a library
|
||
|
|
that is incapable of reading or writing ancillary chunks. If you are
|
||
|
|
not using the progressive reading capability, you can turn that off
|
||
|
|
with PNG_NO_PROGRESSIVE_READ (don't confuse this with the INTERLACING
|
||
|
|
capability, which you'll still have).
|
||
|
|
|
||
|
|
All the reading and writing specific code are in separate files, so the
|
||
|
|
linker should only grab the files it needs. However, if you want to
|
||
|
|
make sure, or if you are building a stand alone library, all the
|
||
|
|
reading files start with "pngr" and all the writing files start with "pngw".
|
||
|
|
The files that don't match either (like png.c, pngtrans.c, etc.)
|
||
|
|
are used for both reading and writing, and always need to be included.
|
||
|
|
The progressive reader is in pngpread.c
|
||
|
|
|
||
|
|
If you are creating or distributing a dynamically linked library (a .so
|
||
|
|
or DLL file), you should not remove or disable any parts of the library,
|
||
|
|
as this will cause applications linked with different versions of the
|
||
|
|
library to fail if they call functions not available in your library.
|
||
|
|
The size of the library itself should not be an issue, because only
|
||
|
|
those sections that are actually used will be loaded into memory.
|
||
|
|
|
||
|
|
Requesting debug printout
|
||
|
|
|
||
|
|
The macro definition PNG_DEBUG can be used to request debugging
|
||
|
|
printout. Set it to an integer value in the range 0 to 3. Higher
|
||
|
|
numbers result in increasing amounts of debugging information. The
|
||
|
|
information is printed to the "stderr" file, unless another file
|
||
|
|
name is specified in the PNG_DEBUG_FILE macro definition.
|
||
|
|
|
||
|
|
When PNG_DEBUG > 0, the following functions (macros) become available:
|
||
|
|
|
||
|
|
png_debug(level, message)
|
||
|
|
png_debug1(level, message, p1)
|
||
|
|
png_debug2(level, message, p1, p2)
|
||
|
|
|
||
|
|
in which "level" is compared to PNG_DEBUG to decide whether to print
|
||
|
|
the message, "message" is the formatted string to be printed,
|
||
|
|
and p1 and p2 are parameters that are to be embedded in the string
|
||
|
|
according to printf-style formatting directives. For example,
|
||
|
|
|
||
|
|
png_debug1(2, "foo=%d\n", foo);
|
||
|
|
|
||
|
|
is expanded to
|
||
|
|
|
||
|
|
if (PNG_DEBUG > 2)
|
||
|
|
fprintf(PNG_DEBUG_FILE, "foo=%d\n", foo);
|
||
|
|
|
||
|
|
When PNG_DEBUG is defined but is zero, the macros aren't defined, but you
|
||
|
|
can still use PNG_DEBUG to control your own debugging:
|
||
|
|
|
||
|
|
#ifdef PNG_DEBUG
|
||
|
|
fprintf(stderr, ...
|
||
|
|
#endif
|
||
|
|
|
||
|
|
When PNG_DEBUG = 1, the macros are defined, but only png_debug statements
|
||
|
|
having level = 0 will be printed. There aren't any such statements in
|
||
|
|
this version of libpng, but if you insert some they will be printed.
|
||
|
|
|
||
|
|
VI. MNG support
|
||
|
|
|
||
|
|
The MNG specification (available at http://www.libpng.org/pub/mng) allows
|
||
|
|
certain extensions to PNG for PNG images that are embedded in MNG datastreams.
|
||
|
|
Libpng can support some of these extensions. To enable them, use the
|
||
|
|
png_permit_mng_features() function:
|
||
|
|
|
||
|
|
feature_set = png_permit_mng_features(png_ptr, mask)
|
||
|
|
|
||
|
|
mask is a png_uint_32 containing the bitwise OR of the
|
||
|
|
features you want to enable. These include
|
||
|
|
PNG_FLAG_MNG_EMPTY_PLTE
|
||
|
|
PNG_FLAG_MNG_FILTER_64
|
||
|
|
PNG_ALL_MNG_FEATURES
|
||
|
|
|
||
|
|
feature_set is a png_uint_32 that is the bitwise AND of
|
||
|
|
your mask with the set of MNG features that is
|
||
|
|
supported by the version of libpng that you are using.
|
||
|
|
|
||
|
|
It is an error to use this function when reading or writing a standalone
|
||
|
|
PNG file with the PNG 8-byte signature. The PNG datastream must be wrapped
|
||
|
|
in a MNG datastream. As a minimum, it must have the MNG 8-byte signature
|
||
|
|
and the MHDR and MEND chunks. Libpng does not provide support for these
|
||
|
|
or any other MNG chunks; your application must provide its own support for
|
||
|
|
them. You may wish to consider using libmng (available at
|
||
|
|
http://www.libmng.com) instead.
|
||
|
|
|
||
|
|
VII. Changes to Libpng from version 0.88
|
||
|
|
|
||
|
|
It should be noted that versions of libpng later than 0.96 are not
|
||
|
|
distributed by the original libpng author, Guy Schalnat, nor by
|
||
|
|
Andreas Dilger, who had taken over from Guy during 1996 and 1997, and
|
||
|
|
distributed versions 0.89 through 0.96, but rather by another member
|
||
|
|
of the original PNG Group, Glenn Randers-Pehrson. Guy and Andreas are
|
||
|
|
still alive and well, but they have moved on to other things.
|
||
|
|
|
||
|
|
The old libpng functions png_read_init(), png_write_init(),
|
||
|
|
png_info_init(), png_read_destroy(), and png_write_destroy() have been
|
||
|
|
moved to PNG_INTERNAL in version 0.95 to discourage their use. These
|
||
|
|
functions will be removed from libpng version 1.4.0.
|
||
|
|
|
||
|
|
The preferred method of creating and initializing the libpng structures is
|
||
|
|
via the png_create_read_struct(), png_create_write_struct(), and
|
||
|
|
png_create_info_struct() because they isolate the size of the structures
|
||
|
|
from the application, allow version error checking, and also allow the
|
||
|
|
use of custom error handling routines during the initialization, which
|
||
|
|
the old functions do not. The functions png_read_destroy() and
|
||
|
|
png_write_destroy() do not actually free the memory that libpng
|
||
|
|
allocated for these structs, but just reset the data structures, so they
|
||
|
|
can be used instead of png_destroy_read_struct() and
|
||
|
|
png_destroy_write_struct() if you feel there is too much system overhead
|
||
|
|
allocating and freeing the png_struct for each image read.
|
||
|
|
|
||
|
|
Setting the error callbacks via png_set_message_fn() before
|
||
|
|
png_read_init() as was suggested in libpng-0.88 is no longer supported
|
||
|
|
because this caused applications that do not use custom error functions
|
||
|
|
to fail if the png_ptr was not initialized to zero. It is still possible
|
||
|
|
to set the error callbacks AFTER png_read_init(), or to change them with
|
||
|
|
png_set_error_fn(), which is essentially the same function, but with a new
|
||
|
|
name to force compilation errors with applications that try to use the old
|
||
|
|
method.
|
||
|
|
|
||
|
|
Starting with version 1.0.7, you can find out which version of the library
|
||
|
|
you are using at run-time:
|
||
|
|
|
||
|
|
png_uint_32 libpng_vn = png_access_version_number();
|
||
|
|
|
||
|
|
The number libpng_vn is constructed from the major version, minor
|
||
|
|
version with leading zero, and release number with leading zero,
|
||
|
|
(e.g., libpng_vn for version 1.0.7 is 10007).
|
||
|
|
|
||
|
|
Note that this function does not take a png_ptr, so you can call it
|
||
|
|
before you've created one.
|
||
|
|
|
||
|
|
You can also check which version of png.h you used when compiling your
|
||
|
|
application:
|
||
|
|
|
||
|
|
png_uint_32 application_vn = PNG_LIBPNG_VER;
|
||
|
|
|
||
|
|
VIII. Changes to Libpng from version 1.0.x to 1.2.x
|
||
|
|
|
||
|
|
Support for user memory management was enabled by default. To
|
||
|
|
accomplish this, the functions png_create_read_struct_2(),
|
||
|
|
png_create_write_struct_2(), png_set_mem_fn(), png_get_mem_ptr(),
|
||
|
|
png_malloc_default(), and png_free_default() were added.
|
||
|
|
|
||
|
|
Support for the iTXt chunk has been enabled by default as of
|
||
|
|
version 1.2.41.
|
||
|
|
|
||
|
|
Support for certain MNG features was enabled.
|
||
|
|
|
||
|
|
Support for numbered error messages was added. However, we never got
|
||
|
|
around to actually numbering the error messages. The function
|
||
|
|
png_set_strip_error_numbers() was added (Note: the prototype for this
|
||
|
|
function was inadvertently removed from png.h in PNG_NO_ASSEMBLER_CODE
|
||
|
|
builds of libpng-1.2.15. It was restored in libpng-1.2.36).
|
||
|
|
|
||
|
|
The png_malloc_warn() function was added at libpng-1.2.3. This issues
|
||
|
|
a png_warning and returns NULL instead of aborting when it fails to
|
||
|
|
acquire the requested memory allocation.
|
||
|
|
|
||
|
|
Support for setting user limits on image width and height was enabled
|
||
|
|
by default. The functions png_set_user_limits(), png_get_user_width_max(),
|
||
|
|
and png_get_user_height_max() were added at libpng-1.2.6.
|
||
|
|
|
||
|
|
The png_set_add_alpha() function was added at libpng-1.2.7.
|
||
|
|
|
||
|
|
The function png_set_expand_gray_1_2_4_to_8() was added at libpng-1.2.9.
|
||
|
|
Unlike png_set_gray_1_2_4_to_8(), the new function does not expand the
|
||
|
|
tRNS chunk to alpha. The png_set_gray_1_2_4_to_8() function is
|
||
|
|
deprecated.
|
||
|
|
|
||
|
|
A number of macro definitions in support of runtime selection of
|
||
|
|
assembler code features (especially Intel MMX code support) were
|
||
|
|
added at libpng-1.2.0:
|
||
|
|
|
||
|
|
PNG_ASM_FLAG_MMX_SUPPORT_COMPILED
|
||
|
|
PNG_ASM_FLAG_MMX_SUPPORT_IN_CPU
|
||
|
|
PNG_ASM_FLAG_MMX_READ_COMBINE_ROW
|
||
|
|
PNG_ASM_FLAG_MMX_READ_INTERLACE
|
||
|
|
PNG_ASM_FLAG_MMX_READ_FILTER_SUB
|
||
|
|
PNG_ASM_FLAG_MMX_READ_FILTER_UP
|
||
|
|
PNG_ASM_FLAG_MMX_READ_FILTER_AVG
|
||
|
|
PNG_ASM_FLAG_MMX_READ_FILTER_PAETH
|
||
|
|
PNG_ASM_FLAGS_INITIALIZED
|
||
|
|
PNG_MMX_READ_FLAGS
|
||
|
|
PNG_MMX_FLAGS
|
||
|
|
PNG_MMX_WRITE_FLAGS
|
||
|
|
PNG_MMX_FLAGS
|
||
|
|
|
||
|
|
We added the following functions in support of runtime
|
||
|
|
selection of assembler code features:
|
||
|
|
|
||
|
|
png_get_mmx_flagmask()
|
||
|
|
png_set_mmx_thresholds()
|
||
|
|
png_get_asm_flags()
|
||
|
|
png_get_mmx_bitdepth_threshold()
|
||
|
|
png_get_mmx_rowbytes_threshold()
|
||
|
|
png_set_asm_flags()
|
||
|
|
|
||
|
|
We replaced all of these functions with simple stubs in libpng-1.2.20,
|
||
|
|
when the Intel assembler code was removed due to a licensing issue.
|
||
|
|
|
||
|
|
These macros are deprecated:
|
||
|
|
|
||
|
|
PNG_READ_TRANSFORMS_NOT_SUPPORTED
|
||
|
|
PNG_PROGRESSIVE_READ_NOT_SUPPORTED
|
||
|
|
PNG_NO_SEQUENTIAL_READ_SUPPORTED
|
||
|
|
PNG_WRITE_TRANSFORMS_NOT_SUPPORTED
|
||
|
|
PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED
|
||
|
|
PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED
|
||
|
|
|
||
|
|
They have been replaced, respectively, by:
|
||
|
|
|
||
|
|
PNG_NO_READ_TRANSFORMS
|
||
|
|
PNG_NO_PROGRESSIVE_READ
|
||
|
|
PNG_NO_SEQUENTIAL_READ
|
||
|
|
PNG_NO_WRITE_TRANSFORMS
|
||
|
|
PNG_NO_READ_ANCILLARY_CHUNKS
|
||
|
|
PNG_NO_WRITE_ANCILLARY_CHUNKS
|
||
|
|
|
||
|
|
PNG_MAX_UINT was replaced with PNG_UINT_31_MAX. It has been
|
||
|
|
deprecated since libpng-1.0.16 and libpng-1.2.6.
|
||
|
|
|
||
|
|
The function
|
||
|
|
png_check_sig(sig, num)
|
||
|
|
was replaced with
|
||
|
|
!png_sig_cmp(sig, 0, num)
|
||
|
|
It has been deprecated since libpng-0.90.
|
||
|
|
|
||
|
|
The function
|
||
|
|
png_set_gray_1_2_4_to_8()
|
||
|
|
which also expands tRNS to alpha was replaced with
|
||
|
|
png_set_expand_gray_1_2_4_to_8()
|
||
|
|
which does not. It has been deprecated since libpng-1.0.18 and 1.2.9.
|
||
|
|
|
||
|
|
IX. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x
|
||
|
|
|
||
|
|
Private libpng prototypes and macro definitions were moved from
|
||
|
|
png.h and pngconf.h into a new pngpriv.h header file.
|
||
|
|
|
||
|
|
Functions png_set_benign_errors(), png_benign_error(), and
|
||
|
|
png_chunk_benign_error() were added.
|
||
|
|
|
||
|
|
Support for setting the maximum amount of memory that the application
|
||
|
|
will allocate for reading chunks was added, as a security measure.
|
||
|
|
The functions png_set_chunk_cache_max() and png_get_chunk_cache_max()
|
||
|
|
were added to the library.
|
||
|
|
|
||
|
|
We implemented support for I/O states by adding png_ptr member io_state
|
||
|
|
and functions png_get_io_chunk_name() and png_get_io_state() in pngget.c
|
||
|
|
|
||
|
|
We added PNG_TRANSFORM_GRAY_TO_RGB to the available high-level
|
||
|
|
input transforms.
|
||
|
|
|
||
|
|
Checking for and reporting of errors in the IHDR chunk is more thorough.
|
||
|
|
|
||
|
|
Support for global arrays was removed, to improve thread safety.
|
||
|
|
|
||
|
|
Some obsolete/deprecated macros and functions have been removed.
|
||
|
|
|
||
|
|
Typecasted NULL definitions such as
|
||
|
|
#define png_voidp_NULL (png_voidp)NULL
|
||
|
|
were eliminated. If you used these in your application, just use
|
||
|
|
NULL instead.
|
||
|
|
|
||
|
|
The png_struct and info_struct members "trans" and "trans_values" were
|
||
|
|
changed to "trans_alpha" and "trans_color", respectively.
|
||
|
|
|
||
|
|
The obsolete, unused pnggccrd.c and pngvcrd.c files and related makefiles
|
||
|
|
were removed.
|
||
|
|
|
||
|
|
The PNG_1_0_X and PNG_1_2_X macros were eliminated.
|
||
|
|
|
||
|
|
The PNG_LEGACY_SUPPORTED macro was eliminated.
|
||
|
|
|
||
|
|
Many WIN32_WCE #ifdefs were removed.
|
||
|
|
|
||
|
|
The functions png_read_init(info_ptr), png_write_init(info_ptr),
|
||
|
|
png_info_init(info_ptr), png_read_destroy(), and png_write_destroy()
|
||
|
|
have been removed. They have been deprecated since libpng-0.95.
|
||
|
|
|
||
|
|
The png_permit_empty_plte() was removed. It has been deprecated
|
||
|
|
since libpng-1.0.9. Use png_permit_mng_features() instead.
|
||
|
|
|
||
|
|
We removed the obsolete stub functions png_get_mmx_flagmask(),
|
||
|
|
png_set_mmx_thresholds(), png_get_asm_flags(),
|
||
|
|
png_get_mmx_bitdepth_threshold(), png_get_mmx_rowbytes_threshold(),
|
||
|
|
png_set_asm_flags(), and png_mmx_supported()
|
||
|
|
|
||
|
|
We removed the obsolete png_check_sig(), png_memcpy_check(), and
|
||
|
|
png_memset_check() functions. Instead use !png_sig_cmp(), memcpy(),
|
||
|
|
and memset(), respectively.
|
||
|
|
|
||
|
|
The function png_set_gray_1_2_4_to_8() was removed. It has been
|
||
|
|
deprecated since libpng-1.0.18 and 1.2.9, when it was replaced with
|
||
|
|
png_set_expand_gray_1_2_4_to_8() because the former function also
|
||
|
|
expanded any tRNS chunk to an alpha channel.
|
||
|
|
|
||
|
|
Macros for png_get_uint_16, png_get_uint_32, and png_get_int_32
|
||
|
|
were added and are used by default instead of the corresponding
|
||
|
|
functions. Unfortunately,
|
||
|
|
from libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro (but not the
|
||
|
|
function) incorrectly returned a value of type png_uint_32.
|
||
|
|
|
||
|
|
We changed the prototype for png_malloc() from
|
||
|
|
png_malloc(png_structp png_ptr, png_uint_32 size)
|
||
|
|
to
|
||
|
|
png_malloc(png_structp png_ptr, png_alloc_size_t size)
|
||
|
|
|
||
|
|
This also applies to the prototype for the user replacement malloc_fn().
|
||
|
|
|
||
|
|
The png_calloc() function was added and is used in place of
|
||
|
|
of "png_malloc(); memset();" except in the case in png_read_png()
|
||
|
|
where the array consists of pointers; in this case a "for" loop is used
|
||
|
|
after the png_malloc() to set the pointers to NULL, to give robust.
|
||
|
|
behavior in case the application runs out of memory part-way through
|
||
|
|
the process.
|
||
|
|
|
||
|
|
We changed the prototypes of png_get_compression_buffer_size() and
|
||
|
|
png_set_compression_buffer_size() to work with png_size_t instead of
|
||
|
|
png_uint_32.
|
||
|
|
|
||
|
|
Support for numbered error messages was removed by default, since we
|
||
|
|
never got around to actually numbering the error messages. The function
|
||
|
|
png_set_strip_error_numbers() was removed from the library by default.
|
||
|
|
|
||
|
|
The png_zalloc() and png_zfree() functions are no longer exported.
|
||
|
|
The png_zalloc() function no longer zeroes out the memory that it
|
||
|
|
allocates.
|
||
|
|
|
||
|
|
Support for dithering was disabled by default in libpng-1.4.0, because
|
||
|
|
it has not been well tested and doesn't actually "dither".
|
||
|
|
The code was not
|
||
|
|
removed, however, and could be enabled by building libpng with
|
||
|
|
PNG_READ_DITHER_SUPPORTED defined. In libpng-1.4.2, this support
|
||
|
|
was reenabled, but the function was renamed png_set_quantize() to
|
||
|
|
reflect more accurately what it actually does. At the same time,
|
||
|
|
the PNG_DITHER_[RED,GREEN_BLUE]_BITS macros were also renamed to
|
||
|
|
PNG_QUANTIZE_[RED,GREEN,BLUE]_BITS, and PNG_READ_DITHER_SUPPORTED
|
||
|
|
was renamed to PNG_READ_QUANTIZE_SUPPORTED.
|
||
|
|
|
||
|
|
We removed the trailing '.' from the warning and error messages.
|
||
|
|
|
||
|
|
X. Changes to Libpng from version 1.4.x to 1.5.x
|
||
|
|
|
||
|
|
From libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro (but not the
|
||
|
|
function) incorrectly returned a value of type png_uint_32.
|
||
|
|
|
||
|
|
Checking for invalid palette index on read or write was added at libpng
|
||
|
|
1.5.10. When an invalid index is found, libpng issues a benign error.
|
||
|
|
This is enabled by default but can be disabled in each png_ptr with
|
||
|
|
|
||
|
|
png_set_check_for_invalid_index(png_ptr, allowed);
|
||
|
|
|
||
|
|
allowed - one of
|
||
|
|
0: disable
|
||
|
|
1: enable
|
||
|
|
|
||
|
|
A. Changes that affect users of libpng
|
||
|
|
|
||
|
|
There are no substantial API changes between the non-deprecated parts of
|
||
|
|
the 1.4.5 API and the 1.5.0 API, however the ability to directly access
|
||
|
|
the main libpng control structures, png_struct and png_info, deprecated
|
||
|
|
in earlier versions of libpng, has been completely removed from
|
||
|
|
libpng 1.5.
|
||
|
|
|
||
|
|
We no longer include zlib.h in png.h. Applications that need access
|
||
|
|
to information in zlib.h will need to add the '#include "zlib.h"'
|
||
|
|
directive. It does not matter whether it is placed prior to or after
|
||
|
|
the '"#include png.h"' directive.
|
||
|
|
|
||
|
|
We moved the png_strcpy(), png_strncpy(), png_strlen(), png_memcpy(),
|
||
|
|
png_memcmp(), png_sprintf, and png_memcpy() macros into a private
|
||
|
|
header file (pngpriv.h) that is not accessible to applications.
|
||
|
|
|
||
|
|
In png_get_iCCP, the type of "profile" was changed from png_charpp
|
||
|
|
to png_bytepp, and in png_set_iCCP, from png_charp to png_const_bytep.
|
||
|
|
|
||
|
|
There are changes of form in png.h, including new and changed macros to
|
||
|
|
declare parts of the API. Some API functions with arguments that are
|
||
|
|
pointers to data not modified within the function have been corrected to
|
||
|
|
declare these arguments with PNG_CONST.
|
||
|
|
|
||
|
|
Much of the internal use of C macros to control the library build has also
|
||
|
|
changed and some of this is visible in the exported header files, in
|
||
|
|
particular the use of macros to control data and API elements visible
|
||
|
|
during application compilation may require significant revision to
|
||
|
|
application code. (It is extremely rare for an application to do this.)
|
||
|
|
|
||
|
|
Any program that compiled against libpng 1.4 and did not use deprecated
|
||
|
|
features or access internal library structures should compile and work
|
||
|
|
against libpng 1.5, except for the change in the prototype for
|
||
|
|
png_get_iCCP() and png_set_iCCP() API functions mentioned above.
|
||
|
|
|
||
|
|
libpng 1.5.0 adds PNG_ PASS macros to help in the reading and writing of
|
||
|
|
interlaced images. The macros return the number of rows and columns in
|
||
|
|
each pass and information that can be used to de-interlace and (if
|
||
|
|
absolutely necessary) interlace an image.
|
||
|
|
|
||
|
|
libpng 1.5.0 adds an API png_longjmp(png_ptr, value). This API calls
|
||
|
|
the application-provided png_longjmp_ptr on the internal, but application
|
||
|
|
initialized, longjmp buffer. It is provided as a convenience to avoid
|
||
|
|
the need to use the png_jmpbuf macro, which had the unnecessary side
|
||
|
|
effect of resetting the internal png_longjmp_ptr value.
|
||
|
|
|
||
|
|
libpng 1.5.0 includes a complete fixed point API. By default this is
|
||
|
|
present along with the corresponding floating point API. In general the
|
||
|
|
fixed point API is faster and smaller than the floating point one because
|
||
|
|
the PNG file format used fixed point, not floating point. This applies
|
||
|
|
even if the library uses floating point in internal calculations. A new
|
||
|
|
macro, PNG_FLOATING_ARITHMETIC_SUPPORTED, reveals whether the library
|
||
|
|
uses floating point arithmetic (the default) or fixed point arithmetic
|
||
|
|
internally for performance critical calculations such as gamma correction.
|
||
|
|
In some cases, the gamma calculations may produce slightly different
|
||
|
|
results. This has changed the results in png_rgb_to_gray and in alpha
|
||
|
|
composition (png_set_background for example). This applies even if the
|
||
|
|
original image was already linear (gamma == 1.0) and, therefore, it is
|
||
|
|
not necessary to linearize the image. This is because libpng has *not*
|
||
|
|
been changed to optimize that case correctly, yet.
|
||
|
|
|
||
|
|
Fixed point support for the sCAL chunk comes with an important caveat;
|
||
|
|
the sCAL specification uses a decimal encoding of floating point values
|
||
|
|
and the accuracy of PNG fixed point values is insufficient for
|
||
|
|
representation of these values. Consequently a "string" API
|
||
|
|
(png_get_sCAL_s and png_set_sCAL_s) is the only reliable way of reading
|
||
|
|
arbitrary sCAL chunks in the absence of either the floating point API or
|
||
|
|
internal floating point calculations.
|
||
|
|
|
||
|
|
Applications no longer need to include the optional distribution header
|
||
|
|
file pngusr.h or define the corresponding macros during application
|
||
|
|
build in order to see the correct variant of the libpng API. From 1.5.0
|
||
|
|
application code can check for the corresponding _SUPPORTED macro:
|
||
|
|
|
||
|
|
#ifdef PNG_INCH_CONVERSIONS_SUPPORTED
|
||
|
|
/* code that uses the inch conversion APIs. */
|
||
|
|
#endif
|
||
|
|
|
||
|
|
This macro will only be defined if the inch conversion functions have been
|
||
|
|
compiled into libpng. The full set of macros, and whether or not support
|
||
|
|
has been compiled in, are available in the header file pnglibconf.h.
|
||
|
|
This header file is specific to the libpng build. Notice that prior to
|
||
|
|
1.5.0 the _SUPPORTED macros would always have the default definition unless
|
||
|
|
reset by pngusr.h or by explicit settings on the compiler command line.
|
||
|
|
These settings may produce compiler warnings or errors in 1.5.0 because
|
||
|
|
of macro redefinition.
|
||
|
|
|
||
|
|
From libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro (but not the
|
||
|
|
function) incorrectly returned a value of type png_uint_32. libpng 1.5.0
|
||
|
|
is consistent with the implementation in 1.4.5 and 1.2.x (where the macro
|
||
|
|
did not exist.)
|
||
|
|
|
||
|
|
Applications can now choose whether to use these macros or to call the
|
||
|
|
corresponding function by defining PNG_USE_READ_MACROS or
|
||
|
|
PNG_NO_USE_READ_MACROS before including png.h. Notice that this is
|
||
|
|
only supported from 1.5.0 -defining PNG_NO_USE_READ_MACROS prior to 1.5.0
|
||
|
|
will lead to a link failure.
|
||
|
|
|
||
|
|
Prior to libpng-1.5.4, the zlib compressor used the same set of parameters
|
||
|
|
when compressing the IDAT data and textual data such as zTXt and iCCP.
|
||
|
|
In libpng-1.5.4 we reinitialized the zlib stream for each type of data.
|
||
|
|
We added five png_set_text_*() functions for setting the parameters to
|
||
|
|
use with textual data.
|
||
|
|
|
||
|
|
Prior to libpng-1.5.4, the PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED
|
||
|
|
option was off by default, and slightly inaccurate scaling occurred.
|
||
|
|
This option can no longer be turned off, and the choice of accurate
|
||
|
|
or inaccurate 16-to-8 scaling is by using the new png_set_scale_16_to_8()
|
||
|
|
API for accurate scaling or the old png_set_strip_16_to_8() API for simple
|
||
|
|
chopping.
|
||
|
|
|
||
|
|
Prior to libpng-1.5.4, the png_set_user_limits() function could only be
|
||
|
|
used to reduce the width and height limits from the value of
|
||
|
|
PNG_USER_WIDTH_MAX and PNG_USER_HEIGHT_MAX, although this document said
|
||
|
|
that it could be used to override them. Now this function will reduce or
|
||
|
|
increase the limits.
|
||
|
|
|
||
|
|
Starting in libpng-1.5.10, the user limits can be set en masse with the
|
||
|
|
configuration option PNG_SAFE_LIMITS_SUPPORTED. If this option is enabled,
|
||
|
|
a set of "safe" limits is applied in pngpriv.h. These can be overridden by
|
||
|
|
application calls to png_set_user_limits(), png_set_user_chunk_cache_max(),
|
||
|
|
and/or png_set_user_malloc_max() that increase or decrease the limits. Also,
|
||
|
|
in libpng-1.5.10 the default width and height limits were increased
|
||
|
|
from 1,000,000 to 0x7ffffff (i.e., made unlimited). Therefore, the
|
||
|
|
limits are now
|
||
|
|
default safe
|
||
|
|
png_user_width_max 0x7fffffff 1,000,000
|
||
|
|
png_user_height_max 0x7fffffff 1,000,000
|
||
|
|
png_user_chunk_cache_max 0 (unlimited) 128
|
||
|
|
png_user_chunk_malloc_max 0 (unlimited) 8,000,000
|
||
|
|
|
||
|
|
B. Changes to the build and configuration of libpng
|
||
|
|
|
||
|
|
Details of internal changes to the library code can be found in the CHANGES
|
||
|
|
file and in the GIT repository logs. These will be of no concern to the vast
|
||
|
|
majority of library users or builders, however the few who configure libpng
|
||
|
|
to a non-default feature set may need to change how this is done.
|
||
|
|
|
||
|
|
There should be no need for library builders to alter build scripts if
|
||
|
|
these use the distributed build support - configure or the makefiles -
|
||
|
|
however users of the makefiles may care to update their build scripts
|
||
|
|
to build pnglibconf.h where the corresponding makefile does not do so.
|
||
|
|
|
||
|
|
Building libpng with a non-default configuration has changed completely.
|
||
|
|
The old method using pngusr.h should still work correctly even though the
|
||
|
|
way pngusr.h is used in the build has been changed; however, library
|
||
|
|
builders will probably want to examine the changes to take advantage of
|
||
|
|
new capabilities and to simplify their build system.
|
||
|
|
|
||
|
|
B.1 Specific changes to library configuration capabilities
|
||
|
|
|
||
|
|
The library now supports a complete fixed point implementation and can
|
||
|
|
thus be used on systems that have no floating point support or very
|
||
|
|
limited or slow support. Previously gamma correction, an essential part
|
||
|
|
of complete PNG support, required reasonably fast floating point.
|
||
|
|
|
||
|
|
As part of this the choice of internal implementation has been made
|
||
|
|
independent of the choice of fixed versus floating point APIs and all the
|
||
|
|
missing fixed point APIs have been implemented.
|
||
|
|
|
||
|
|
The exact mechanism used to control attributes of API functions has
|
||
|
|
changed. A single set of operating system independent macro definitions
|
||
|
|
is used and operating system specific directives are defined in
|
||
|
|
pnglibconf.h
|
||
|
|
|
||
|
|
As part of this the mechanism used to choose procedure call standards on
|
||
|
|
those systems that allow a choice has been changed. At present this only
|
||
|
|
affects certain Microsoft (DOS, Windows) and IBM (OS/2) operating systems
|
||
|
|
running on Intel processors. As before, PNGAPI is defined where required
|
||
|
|
to control the exported API functions; however, two new macros, PNGCBAPI
|
||
|
|
and PNGCAPI, are used instead for callback functions (PNGCBAPI) and
|
||
|
|
(PNGCAPI) for functions that must match a C library prototype (currently
|
||
|
|
only png_longjmp_ptr, which must match the C longjmp function.) The new
|
||
|
|
approach is documented in pngconf.h
|
||
|
|
|
||
|
|
Despite these changes, libpng 1.5.0 only supports the native C function
|
||
|
|
calling standard on those platforms tested so far (__cdecl on Microsoft
|
||
|
|
Windows). This is because the support requirements for alternative
|
||
|
|
calling conventions seem to no longer exist. Developers who find it
|
||
|
|
necessary to set PNG_API_RULE to 1 should advise the mailing list
|
||
|
|
(png-mng-implement) of this and library builders who use Openwatcom and
|
||
|
|
therefore set PNG_API_RULE to 2 should also contact the mailing list.
|
||
|
|
|
||
|
|
A new test program, pngvalid, is provided in addition to pngtest.
|
||
|
|
pngvalid validates the arithmetic accuracy of the gamma correction
|
||
|
|
calculations and includes a number of validations of the file format.
|
||
|
|
A subset of the full range of tests is run when "make check" is done
|
||
|
|
(in the 'configure' build.) pngvalid also allows total allocated memory
|
||
|
|
usage to be evaluated and performs additional memory overwrite validation.
|
||
|
|
|
||
|
|
Many changes to individual feature macros have been made. The following
|
||
|
|
are the changes most likely to be noticed by library builders who
|
||
|
|
configure libpng:
|
||
|
|
|
||
|
|
1) All feature macros now have consistent naming:
|
||
|
|
|
||
|
|
#define PNG_NO_feature turns the feature off
|
||
|
|
#define PNG_feature_SUPPORTED turns the feature on
|
||
|
|
|
||
|
|
pnglibconf.h contains one line for each feature macro which is either:
|
||
|
|
|
||
|
|
#define PNG_feature_SUPPORTED
|
||
|
|
|
||
|
|
if the feature is supported or:
|
||
|
|
|
||
|
|
/*#undef PNG_feature_SUPPORTED*/
|
||
|
|
|
||
|
|
if it is not. Library code consistently checks for the 'SUPPORTED' macro.
|
||
|
|
It does not, and libpng applications should not, check for the 'NO' macro
|
||
|
|
which will not normally be defined even if the feature is not supported.
|
||
|
|
The 'NO' macros are only used internally for setting or not setting the
|
||
|
|
corresponding 'SUPPORTED' macros.
|
||
|
|
|
||
|
|
Compatibility with the old names is provided as follows:
|
||
|
|
|
||
|
|
PNG_INCH_CONVERSIONS turns on PNG_INCH_CONVERSIONS_SUPPORTED
|
||
|
|
|
||
|
|
And the following definitions disable the corresponding feature:
|
||
|
|
|
||
|
|
PNG_SETJMP_NOT_SUPPORTED disables SETJMP
|
||
|
|
PNG_READ_TRANSFORMS_NOT_SUPPORTED disables READ_TRANSFORMS
|
||
|
|
PNG_NO_READ_COMPOSITED_NODIV disables READ_COMPOSITE_NODIV
|
||
|
|
PNG_WRITE_TRANSFORMS_NOT_SUPPORTED disables WRITE_TRANSFORMS
|
||
|
|
PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED disables READ_ANCILLARY_CHUNKS
|
||
|
|
PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED disables WRITE_ANCILLARY_CHUNKS
|
||
|
|
|
||
|
|
Library builders should remove use of the above, inconsistent, names.
|
||
|
|
|
||
|
|
2) Warning and error message formatting was previously conditional on
|
||
|
|
the STDIO feature. The library has been changed to use the
|
||
|
|
CONSOLE_IO feature instead. This means that if CONSOLE_IO is disabled
|
||
|
|
the library no longer uses the printf(3) functions, even though the
|
||
|
|
default read/write implementations use (FILE) style stdio.h functions.
|
||
|
|
|
||
|
|
3) Three feature macros now control the fixed/floating point decisions:
|
||
|
|
|
||
|
|
PNG_FLOATING_POINT_SUPPORTED enables the floating point APIs
|
||
|
|
|
||
|
|
PNG_FIXED_POINT_SUPPORTED enables the fixed point APIs; however, in
|
||
|
|
practice these are normally required internally anyway (because the PNG
|
||
|
|
file format is fixed point), therefore in most cases PNG_NO_FIXED_POINT
|
||
|
|
merely stops the function from being exported.
|
||
|
|
|
||
|
|
PNG_FLOATING_ARITHMETIC_SUPPORTED chooses between the internal floating
|
||
|
|
point implementation or the fixed point one. Typically the fixed point
|
||
|
|
implementation is larger and slower than the floating point implementation
|
||
|
|
on a system that supports floating point, however it may be faster on a
|
||
|
|
system which lacks floating point hardware and therefore uses a software
|
||
|
|
emulation.
|
||
|
|
|
||
|
|
4) Added PNG_{READ,WRITE}_INT_FUNCTIONS_SUPPORTED. This allows the
|
||
|
|
functions to read and write ints to be disabled independently of
|
||
|
|
PNG_USE_READ_MACROS, which allows libpng to be built with the functions
|
||
|
|
even though the default is to use the macros - this allows applications
|
||
|
|
to choose at app buildtime whether or not to use macros (previously
|
||
|
|
impossible because the functions weren't in the default build.)
|
||
|
|
|
||
|
|
B.2 Changes to the configuration mechanism
|
||
|
|
|
||
|
|
Prior to libpng-1.5.0 library builders who needed to configure libpng
|
||
|
|
had either to modify the exported pngconf.h header file to add system
|
||
|
|
specific configuration or had to write feature selection macros into
|
||
|
|
pngusr.h and cause this to be included into pngconf.h by defining
|
||
|
|
PNG_USER_CONFIG. The latter mechanism had the disadvantage that an
|
||
|
|
application built without PNG_USER_CONFIG defined would see the
|
||
|
|
unmodified, default, libpng API and thus would probably fail to link.
|
||
|
|
|
||
|
|
These mechanisms still work in the configure build and in any makefile
|
||
|
|
build that builds pnglibconf.h, although the feature selection macros
|
||
|
|
have changed somewhat as described above. In 1.5.0, however, pngusr.h is
|
||
|
|
processed only once, when the exported header file pnglibconf.h is built.
|
||
|
|
pngconf.h no longer includes pngusr.h, therefore pngusr.h is ignored after the
|
||
|
|
build of pnglibconf.h and it is never included in an application build.
|
||
|
|
|
||
|
|
The rarely used alternative of adding a list of feature macros to the
|
||
|
|
CFLAGS setting in the build also still works, however the macros will be
|
||
|
|
copied to pnglibconf.h and this may produce macro redefinition warnings
|
||
|
|
when the individual C files are compiled.
|
||
|
|
|
||
|
|
All configuration now only works if pnglibconf.h is built from
|
||
|
|
scripts/pnglibconf.dfa. This requires the program awk. Brian Kernighan
|
||
|
|
(the original author of awk) maintains C source code of that awk and this
|
||
|
|
and all known later implementations (often called by subtly different
|
||
|
|
names - nawk and gawk for example) are adequate to build pnglibconf.h.
|
||
|
|
The Sun Microsystems (now Oracle) program 'awk' is an earlier version
|
||
|
|
and does not work; this may also apply to other systems that have a
|
||
|
|
functioning awk called 'nawk'.
|
||
|
|
|
||
|
|
Configuration options are now documented in scripts/pnglibconf.dfa. This
|
||
|
|
file also includes dependency information that ensures a configuration is
|
||
|
|
consistent; that is, if a feature is switched off dependent features are
|
||
|
|
also removed. As a recommended alternative to using feature macros in
|
||
|
|
pngusr.h a system builder may also define equivalent options in pngusr.dfa
|
||
|
|
(or, indeed, any file) and add that to the configuration by setting
|
||
|
|
DFA_XTRA to the file name. The makefiles in contrib/pngminim illustrate
|
||
|
|
how to do this, and a case where pngusr.h is still required.
|
||
|
|
|
||
|
|
XI. Detecting libpng
|
||
|
|
|
||
|
|
The png_get_io_ptr() function has been present since libpng-0.88, has never
|
||
|
|
changed, and is unaffected by conditional compilation macros. It is the
|
||
|
|
best choice for use in configure scripts for detecting the presence of any
|
||
|
|
libpng version since 0.88. In an autoconf "configure.in" you could use
|
||
|
|
|
||
|
|
AC_CHECK_LIB(png, png_get_io_ptr, ...
|
||
|
|
|
||
|
|
XII. Source code repository
|
||
|
|
|
||
|
|
Since about February 2009, version 1.2.34, libpng has been under "git" source
|
||
|
|
control. The git repository was built from old libpng-x.y.z.tar.gz files
|
||
|
|
going back to version 0.70. You can access the git repository (read only)
|
||
|
|
at
|
||
|
|
|
||
|
|
git://libpng.git.sourceforge.net/gitroot/libpng
|
||
|
|
|
||
|
|
or you can browse it via "gitweb" at
|
||
|
|
|
||
|
|
http://libpng.git.sourceforge.net/git/gitweb.cgi?p=libpng
|
||
|
|
|
||
|
|
Patches can be sent to glennrp at users.sourceforge.net or to
|
||
|
|
png-mng-implement at lists.sourceforge.net or you can upload them to
|
||
|
|
the libpng bug tracker at
|
||
|
|
|
||
|
|
http://libpng.sourceforge.net
|
||
|
|
|
||
|
|
We also accept patches built from the tar or zip distributions, and
|
||
|
|
simple verbal discriptions of bug fixes, reported either to the
|
||
|
|
SourceForge bug tracker, to the png-mng-implement at lists.sf.net
|
||
|
|
mailing list, or directly to glennrp.
|
||
|
|
|
||
|
|
XIII. Coding style
|
||
|
|
|
||
|
|
Our coding style is similar to the "Allman" style, with curly
|
||
|
|
braces on separate lines:
|
||
|
|
|
||
|
|
if (condition)
|
||
|
|
{
|
||
|
|
action;
|
||
|
|
}
|
||
|
|
|
||
|
|
else if (another condition)
|
||
|
|
{
|
||
|
|
another action;
|
||
|
|
}
|
||
|
|
|
||
|
|
The braces can be omitted from simple one-line actions:
|
||
|
|
|
||
|
|
if (condition)
|
||
|
|
return (0);
|
||
|
|
|
||
|
|
We use 3-space indentation, except for continued statements which
|
||
|
|
are usually indented the same as the first line of the statement
|
||
|
|
plus four more spaces.
|
||
|
|
|
||
|
|
For macro definitions we use 2-space indentation, always leaving the "#"
|
||
|
|
in the first column.
|
||
|
|
|
||
|
|
#ifndef PNG_NO_FEATURE
|
||
|
|
# ifndef PNG_FEATURE_SUPPORTED
|
||
|
|
# define PNG_FEATURE_SUPPORTED
|
||
|
|
# endif
|
||
|
|
#endif
|
||
|
|
|
||
|
|
Comments appear with the leading "/*" at the same indentation as
|
||
|
|
the statement that follows the comment:
|
||
|
|
|
||
|
|
/* Single-line comment */
|
||
|
|
statement;
|
||
|
|
|
||
|
|
/* This is a multiple-line
|
||
|
|
* comment.
|
||
|
|
*/
|
||
|
|
statement;
|
||
|
|
|
||
|
|
Very short comments can be placed after the end of the statement
|
||
|
|
to which they pertain:
|
||
|
|
|
||
|
|
statement; /* comment */
|
||
|
|
|
||
|
|
We don't use C++ style ("//") comments. We have, however,
|
||
|
|
used them in the past in some now-abandoned MMX assembler
|
||
|
|
code.
|
||
|
|
|
||
|
|
Functions and their curly braces are not indented, and
|
||
|
|
exported functions are marked with PNGAPI:
|
||
|
|
|
||
|
|
/* This is a public function that is visible to
|
||
|
|
* application programmers. It does thus-and-so.
|
||
|
|
*/
|
||
|
|
void PNGAPI
|
||
|
|
png_exported_function(png_ptr, png_info, foo)
|
||
|
|
{
|
||
|
|
body;
|
||
|
|
}
|
||
|
|
|
||
|
|
The prototypes for all exported functions appear in png.h,
|
||
|
|
above the comment that says
|
||
|
|
|
||
|
|
/* Maintainer: Put new public prototypes here ... */
|
||
|
|
|
||
|
|
We mark all non-exported functions with "/* PRIVATE */"":
|
||
|
|
|
||
|
|
void /* PRIVATE */
|
||
|
|
png_non_exported_function(png_ptr, png_info, foo)
|
||
|
|
{
|
||
|
|
body;
|
||
|
|
}
|
||
|
|
|
||
|
|
The prototypes for non-exported functions (except for those in
|
||
|
|
pngtest) appear in
|
||
|
|
pngpriv.h
|
||
|
|
above the comment that says
|
||
|
|
|
||
|
|
/* Maintainer: Put new private prototypes here ^ and in libpngpf.3 */
|
||
|
|
|
||
|
|
To avoid polluting the global namespace, the names of all exported
|
||
|
|
functions and variables begin with "png_", and all publicly visible C
|
||
|
|
preprocessor macros begin with "PNG". We request that applications that
|
||
|
|
use libpng *not* begin any of their own symbols with either of these strings.
|
||
|
|
|
||
|
|
We put a space after each comma and after each semicolon
|
||
|
|
in "for" statements, and we put spaces before and after each
|
||
|
|
C binary operator and after "for" or "while", and before
|
||
|
|
"?". We don't put a space between a typecast and the expression
|
||
|
|
being cast, nor do we put one between a function name and the
|
||
|
|
left parenthesis that follows it:
|
||
|
|
|
||
|
|
for (i = 2; i > 0; --i)
|
||
|
|
y[i] = a(x) + (int)b;
|
||
|
|
|
||
|
|
We prefer #ifdef and #ifndef to #if defined() and if !defined()
|
||
|
|
when there is only one macro being tested.
|
||
|
|
|
||
|
|
We prefer to express integers that are used as bit masks in hex format,
|
||
|
|
with an even number of lower-case hex digits (e.g., 0x00, 0xff, 0x0100).
|
||
|
|
|
||
|
|
We do not use the TAB character for indentation in the C sources.
|
||
|
|
|
||
|
|
Lines do not exceed 80 characters.
|
||
|
|
|
||
|
|
Other rules can be inferred by inspecting the libpng source.
|
||
|
|
|
||
|
|
XIV. Y2K Compliance in libpng
|
||
|
|
|
||
|
|
March 29, 2012
|
||
|
|
|
||
|
|
Since the PNG Development group is an ad-hoc body, we can't make
|
||
|
|
an official declaration.
|
||
|
|
|
||
|
|
This is your unofficial assurance that libpng from version 0.71 and
|
||
|
|
upward through 1.5.10 are Y2K compliant. It is my belief that earlier
|
||
|
|
versions were also Y2K compliant.
|
||
|
|
|
||
|
|
Libpng only has three year fields. One is a 2-byte unsigned integer that
|
||
|
|
will hold years up to 65535. The other two hold the date in text
|
||
|
|
format, and will hold years up to 9999.
|
||
|
|
|
||
|
|
The integer is
|
||
|
|
"png_uint_16 year" in png_time_struct.
|
||
|
|
|
||
|
|
The strings are
|
||
|
|
"png_charp time_buffer" in png_struct and
|
||
|
|
"near_time_buffer", which is a local character string in png.c.
|
||
|
|
|
||
|
|
There are seven time-related functions:
|
||
|
|
|
||
|
|
png_convert_to_rfc_1123() in png.c
|
||
|
|
(formerly png_convert_to_rfc_1152() in error)
|
||
|
|
png_convert_from_struct_tm() in pngwrite.c, called
|
||
|
|
in pngwrite.c
|
||
|
|
png_convert_from_time_t() in pngwrite.c
|
||
|
|
png_get_tIME() in pngget.c
|
||
|
|
png_handle_tIME() in pngrutil.c, called in pngread.c
|
||
|
|
png_set_tIME() in pngset.c
|
||
|
|
png_write_tIME() in pngwutil.c, called in pngwrite.c
|
||
|
|
|
||
|
|
All appear to handle dates properly in a Y2K environment. The
|
||
|
|
png_convert_from_time_t() function calls gmtime() to convert from system
|
||
|
|
clock time, which returns (year - 1900), which we properly convert to
|
||
|
|
the full 4-digit year. There is a possibility that applications using
|
||
|
|
libpng are not passing 4-digit years into the png_convert_to_rfc_1123()
|
||
|
|
function, or that they are incorrectly passing only a 2-digit year
|
||
|
|
instead of "year - 1900" into the png_convert_from_struct_tm() function,
|
||
|
|
but this is not under our control. The libpng documentation has always
|
||
|
|
stated that it works with 4-digit years, and the APIs have been
|
||
|
|
documented as such.
|
||
|
|
|
||
|
|
The tIME chunk itself is also Y2K compliant. It uses a 2-byte unsigned
|
||
|
|
integer to hold the year, and can hold years as large as 65535.
|
||
|
|
|
||
|
|
zlib, upon which libpng depends, is also Y2K compliant. It contains
|
||
|
|
no date-related code.
|
||
|
|
|
||
|
|
|
||
|
|
Glenn Randers-Pehrson
|
||
|
|
libpng maintainer
|
||
|
|
PNG Development Group
|