1995-07-20 07:43:20 +00:00
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libpng.txt - a description on how to use and modify libpng
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1996-01-10 08:56:49 +00:00
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libpng 1.0 beta 2 - version 0.86
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For conditions of distribution and use, see copyright notice in png.h
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Copyright (c) 1995, 1996 Guy Eric Schalnat, Group 42, Inc.
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January 10, 1996
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Updated/rewritten per request in the libpng FAQ
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Copyright (c) 1995 Frank J. T. Wojcik
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December 18, 1995
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I. Introduction
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1995-07-20 07:43:20 +00:00
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This file describes how to use and modify the PNG reference library
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1996-01-10 08:56:49 +00:00
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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. Other then 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.
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1995-07-20 07:43:20 +00:00
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Libpng was written as a companion to the PNG specification, as a
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way to reduce the amount of time and effort it takes to support
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the PNG file format in application programs. Most users will not
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have to modify the library significantly; advanced users may want
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1996-01-10 08:56:49 +00:00
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to modify it more. The library was coded for both kind of users.
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All attempts were made to make it as complete as possible, while
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1995-07-20 07:43:20 +00:00
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keeping the code easy to understand. Currently, this library
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only supports C. Support for other languages 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 bit, 32 bit) available, and to be easy to
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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.txt file), libpng should cover the
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majority of the needs of it's users.
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Libpng uses zlib for its compression and decompression of PNG files.
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The zlib compression utility is a general purpose utility that is
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1996-01-10 08:56:49 +00:00
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useful for more then 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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1995-07-20 07:43:20 +00:00
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1996-01-10 08:56:49 +00:00
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II. Structures
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1995-07-20 07:43:20 +00:00
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There are two main structures that are important to libpng, png_struct
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and png_info. The first, png_struct, is an internal structure that
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will not, for the most part, be used by the general user except as
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1995-12-19 09:22:19 +00:00
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the first variable passed to every png function call.
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1995-07-20 07:43:20 +00:00
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The png_info structure is designed to provide information about the
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1996-01-10 08:56:49 +00:00
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png file. All of its fields are intended to be examined or modified
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1995-07-20 07:43:20 +00:00
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by the user. See png.h for a good description of the png_info fields.
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1996-01-10 08:56:49 +00:00
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png.h is also an invaluable reference for programming with libpng.
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1995-07-20 07:43:20 +00:00
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1995-12-19 09:22:19 +00:00
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And while I'm on the topic, make sure you include the png header file:
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1995-07-20 07:43:20 +00:00
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#include <png.h>
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1996-01-10 08:56:49 +00:00
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III. Reading
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1995-07-20 07:43:20 +00:00
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Checking PNG files:
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1996-01-10 08:56:49 +00:00
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Libpng provides a simple check to see if a file is a PNG file. To
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1995-07-20 07:43:20 +00:00
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use it, pass in the first 1 to 8 bytes of the file, and it will return
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true or false (1 or 0) depending on whether the bytes could be part
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1996-01-10 08:56:49 +00:00
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of a PNG file. Of course, the more bytes you pass in, the greater
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1995-12-19 09:22:19 +00:00
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the accuracy of the prediction. If you pass in more then eight bytes,
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libpng will only look at the first eight bytes.
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1995-07-20 07:43:20 +00:00
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1995-12-19 09:22:19 +00:00
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fread(header, 1, number, fp);
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is_png = png_check_sig(header, number);
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1995-07-20 07:43:20 +00:00
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Reading PNG files:
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1996-01-10 08:56:49 +00:00
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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, briefly explaining the syntax and purpose of each one.
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See example.c and png.h for more detail. While Progressive reading
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is covered in the next section, you will still need some of the
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functions discussed in this section to read a PNG file.
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1995-12-19 09:22:19 +00:00
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1996-01-10 08:56:49 +00:00
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The first thing you need to do while reading a PNG file, aside from
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the standard I/O initialization, is to allocate and initialize
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png_struct and png_info. As these are both large, you may not want to
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store these on the stack, unless you have stack space to spare. Of
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course, you will want to check if malloc returns NULL.
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1995-07-20 07:43:20 +00:00
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1995-12-19 09:22:19 +00:00
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png_structp png_ptr = malloc(sizeof (png_struct));
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if (!png_ptr)
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return;
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png_infop info_ptr = malloc(sizeof (png_info));
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if (!info_ptr)
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{
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free(png_ptr);
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return;
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}
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1995-07-20 07:43:20 +00:00
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You may also want to do any i/o initialization here, before
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you get into libpng, so if it doesn't work, you don't have
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much to undo.
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1995-12-19 09:22:19 +00:00
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FILE *fp = fopen(file_name, "rb");
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if (!fp)
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{
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free(png_ptr);
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free(info_ptr);
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return;
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}
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1995-11-28 17:22:13 +00:00
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1996-01-10 08:56:49 +00:00
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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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Customizing libpng.
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1995-07-20 07:43:20 +00:00
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After you have these structures, you will need to set up the
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error handling. When libpng encounters an error, it expects to
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longjmp back to your routine. Therefore, you will need to call
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setjmp and pass the jmpbuf field of your png_struct. If you
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read the file from different routines, you will need to update
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the jmpbuf field every time you enter a new routine that will
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call a png_ function. See your documentation of setjmp/longjmp
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for your compiler for more information on setjmp/longjmp. See
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1996-01-10 08:56:49 +00:00
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the discussion on libpng error handling in the Customizing Libpng
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section below for more information on the libpng error handling.
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1995-12-19 09:22:19 +00:00
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If an error occurs, and libpng longjmp's back to your setjmp,
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you will want to call png_read_destroy() to free any memory.
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if (setjmp(png_ptr->jmpbuf))
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{
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png_read_destroy(png_ptr, info_ptr, (png_info *)0);
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/* free pointers before returning, if necessary */
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free(png_ptr);
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free(info_ptr);
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fclose(fp);
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return;
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}
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1995-07-20 07:43:20 +00:00
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1996-01-10 08:56:49 +00:00
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Next, you will need to call png_info_init() and png_read_init().
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1995-07-20 07:43:20 +00:00
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These functions make sure all the fields are initialized to useful
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values, and, in the case of png_read_init(), and allocate any memory
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needed for internal uses. You must call png_info_init() first, as
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png_read_init() could do a longjmp, and if the info is not initialized,
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the png_read_destroy() could try to png_free() random addresses, which
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would be bad.
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png_info_init(info_ptr);
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png_read_init(png_ptr);
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1995-07-20 07:43:20 +00:00
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Now you need to set up the input code. The default for libpng is
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to use the C function fread(). If you use this, you will need to
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pass a valid FILE * in the function png_init_io(). Be sure that
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1995-12-19 09:22:19 +00:00
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the file is opened in binary mode. If you wish to handle reading
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data in another way, see the discussion on png i/o handling in the
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Customizing Libpng section below.
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1995-07-20 07:43:20 +00:00
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1995-12-19 09:22:19 +00:00
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png_init_io(png_ptr, fp);
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1995-07-20 07:43:20 +00:00
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You are now ready to read all the file information up to the actual
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image data. You do this with a call to png_read_info().
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png_read_info(png_ptr, info_ptr);
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The png_info structure is now filled in with all the data necessary
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to read the file. Some of the more important parts of the png_info are:
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width - holds the width of the file
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height - holds the height of the file
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bit_depth - holds the bit depth of one of the image channels
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color_type - describes the channels and what they mean
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(see the PNG_COLOR_TYPE_ macros for more information)
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channels - number of channels of info for the color type
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pixel_depth - bits per pixel, the result of multiplying the bit_depth
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times the channels
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rowbytes - number of bytes needed to hold a row
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interlace_type - currently 0 for none, 1 for interlaced
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valid - this details which optional chunks were found in the
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file to see if a chunk was present, AND valid with the
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appropriate PNG_INFO_<chunk name> define.
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These are also important, but their validity depends on whether a
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corresponding chunk exists. Use valid (see above) to ensure that what
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you're doing with these values makes sense.
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palette - the palette for the file
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num_palette - number of entries in the palette
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gamma - the gamma the file is written at
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sig_bit - the number of significant bits
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for the gray, red, green, and blue channels, whichever are
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appropriate for the given color type.
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sig_bit_number - number of channels
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trans_values - transparent pixel for non-paletted images
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trans - array of transparent entries for paletted images
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number_trans - number of transparent entries
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hist - histogram of palette
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text - text comments in the file.
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num_text - number of comments
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1995-07-20 07:43:20 +00:00
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for more information, see the png_info definition in png.h and the
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PNG specification for chunk contents. Be careful with trusting
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rowbytes, as some of the transformations could increase the space
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needed to hold a row (expand, rgbx, xrgb, graph_to_rgb, etc.).
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1996-01-10 08:56:49 +00:00
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See png_update_info(), below.
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1995-07-20 07:43:20 +00:00
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A quick word about text and num_text. PNG stores comments in
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keyword/text pairs, one pair per chunk. While there are
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suggested keywords, there is no requirement to restrict the use
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to these strings. There is a requirement to have at least one
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character for a keyword. It is strongly suggested that keywords
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be sensible to humans (that's the point), so don't use abbreviations.
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1995-12-19 09:22:19 +00:00
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See the png specification for more details. There is no requirement
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to have text after the keyword.
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Keywords are restricted to 80 characters without leading or trailing
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1996-01-10 08:56:49 +00:00
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spaces, but spaces are allowed within the keyword It is possible to
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have the same keyword any number of times. The text field is an
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1995-12-19 09:22:19 +00:00
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array of png_text structures, each holding pointer to a keyword
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1995-07-20 07:43:20 +00:00
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and a pointer to a text string. Only the text string may be null.
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The keyword/text pairs are put into the array in the order that
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they are received. However, some or all of the text chunks may be
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after the image, so to make sure you have read all the text chunks,
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don't mess with these until after you read the stuff after the image.
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This will be mentioned again below in the discussion that goes with
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png_read_end().
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After you've read the file information, you can set up the library to
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handle any special transformations of the image data. The various
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ways to transform the data will be described in the order that they
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occur. This is important, as some of these change the color type
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and bit depth of the data, and some others only work on certain
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color types and bit depths. Even though each transformation should
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check to see if it has data that it can do somthing with, you should
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make sure to only enable a transformation if it will be valid for
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the data. For example, don't swap red and blue on grayscale data.
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1996-01-10 08:56:49 +00:00
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This transforms bit depths of less than 8 to 8 bits, changes paletted
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1995-07-20 07:43:20 +00:00
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images to rgb, and adds an alpha channel if there is transparency
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information in a tRNS chunk. This is probably most useful on grayscale
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images with bit depths of 2 or 4 and tRNS chunks.
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if (info_ptr->color_type == PNG_COLOR_TYPE_PALETTE &&
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info_ptr->bit_depth < 8)
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png_set_expand(png_ptr);
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1995-07-20 07:43:20 +00:00
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1995-12-19 09:22:19 +00:00
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if (info_ptr->color_type == PNG_COLOR_TYPE_GRAY &&
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1995-07-20 07:43:20 +00:00
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info_ptr->bit_depth < 8)
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png_set_expand(png_ptr);
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if (info_ptr->valid & PNG_INFO_tRNS)
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png_set_expand(png_ptr);
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This handles alpha and transparency by replacing it with a background
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value. If there was a valid one in the file, you can use it if you
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want. However, you can replace it with your own if you want also. If
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there wasn't one in the file, you must supply a color. If libpng is
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doing gamma correction, you will need to tell libpng where the
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background came from so it can do the appropriate gamma correction.
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If you are modifying the color data with png_set_expand(), you must
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indicate whether the background needs to be expanded. See the
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function definition in png.h for more details.
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1995-12-19 09:22:19 +00:00
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png_color_16 my_background;
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if (info_ptr->valid & PNG_INFO_bKGD)
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png_set_backgrond(png_ptr, &(info_ptr->background),
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1995-12-19 09:22:19 +00:00
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PNG_BACKGROUND_GAMMA_FILE, 1, 1.0);
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1995-07-20 07:43:20 +00:00
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else
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png_set_background(png_ptr, &my_background,
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PNG_BACKGROUND_GAMMA_SCREEN, 0, 1.0);
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1995-07-20 07:43:20 +00:00
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This handles gamma transformations of the data. Pass both the file
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gamma and the desired screen gamma. If the file does not have a
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gamma value, you can pass one anyway if you wish. Note that file
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gammas are inverted from screen gammas. See the discussions on
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gamma in the PNG specification for more information. It is strongly
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recommended that viewers support gamma correction.
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1995-07-20 07:43:20 +00:00
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if (info_ptr->valid & PNG_INFO_gAMA)
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png_set_gamma(png_ptr, screen_gamma, info_ptr->gamma);
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1995-12-19 09:22:19 +00:00
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else
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png_set_gamma(png_ptr, screen_gamma, 0.45);
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1995-07-20 07:43:20 +00:00
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PNG can have files with 16 bits per channel. If you only can handle
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8 bits per channel, this will strip the pixels down to 8 bit.
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if (info_ptr->bit_depth == 16)
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png_set_strip_16(png_ptr);
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1996-01-10 08:56:49 +00:00
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If you need to reduce an rgb file to a paletted file (perhaps because
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a paletted file has more entries then will fit on your screen)
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png_set_dither() will do that. Note that this is a simple match
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dither, that merely finds the closest color available. This should
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work fairly well with optimized palettes, and fairly badly with linear
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|
|
color cubes. If you pass a palette that is larger then
|
|
|
|
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,
|
|
|
|
it will use it to make intelligent choices when reducing the palette.
|
|
|
|
If there is no histogram, it may not do as good a job.
|
|
|
|
|
|
|
|
This function will be rewritten and/or replaced in libpng 0.9, which
|
|
|
|
will have full two pass dithering with optimized palettes.
|
1995-07-20 07:43:20 +00:00
|
|
|
|
1995-12-19 09:22:19 +00:00
|
|
|
if (info_ptr->color_type & PNG_COLOR_MASK_COLOR)
|
1995-07-20 07:43:20 +00:00
|
|
|
{
|
|
|
|
if (info_ptr->valid & PNG_INFO_PLTE)
|
|
|
|
png_set_dither(png_ptr, info_ptr->palette,
|
|
|
|
info_ptr->num_palette, max_screen_colors,
|
|
|
|
info_ptr->histogram);
|
|
|
|
else
|
|
|
|
{
|
1995-12-19 09:22:19 +00:00
|
|
|
png_color std_color_cube[MAX_SCREEN_COLORS] =
|
1995-07-20 07:43:20 +00:00
|
|
|
{ ... colors ... };
|
|
|
|
|
|
|
|
png_set_dither(png_ptr, std_color_cube, MAX_SCREEN_COLORS,
|
|
|
|
MAX_SCREEN_COLORS, NULL);
|
|
|
|
}
|
|
|
|
}
|
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
PNG files describe monocrome as black is zero and white is one. The
|
|
|
|
following code will reverse this (make black be one and white be zero):
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
if (info_ptr->bit_depth == 1 &&
|
|
|
|
info_ptr->color_type == PNG_COLOR_GRAY)
|
|
|
|
png_set_invert(png_ptr);
|
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
PNG files have possible bit depths of 1, 2, 4, 8, and 16. However,
|
1995-07-20 07:43:20 +00:00
|
|
|
they also provide a way to describe the true bit depth of the image.
|
1996-01-10 08:56:49 +00:00
|
|
|
It is then required that values be "scaled" or "shifted" up to the bit
|
|
|
|
depth used in the file. See the PNG specification for details. This
|
|
|
|
code reduces the pixels back down to the true bit depth:
|
1995-07-20 07:43:20 +00:00
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
if (info_ptr->valid & PNG_INFO_sBIT)
|
|
|
|
png_set_shift(png_ptr, &(info_ptr->sig_bit));
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
|
1996-01-10 08:56:49 +00:00
|
|
|
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:
|
1995-07-20 07:43:20 +00:00
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
if (info_ptr->bit_depth < 8)
|
|
|
|
png_set_packing(png_ptr);
|
1995-07-20 07:43:20 +00:00
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
PNG files store 3 color pixels in red, green, blue order. This code
|
|
|
|
changes the storage of the pixels to blue, green, red:
|
1995-07-20 07:43:20 +00:00
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
if (info_ptr->color_type == PNG_COLOR_TYPE_RGB ||
|
1995-12-19 09:22:19 +00:00
|
|
|
info_ptr->color_type == PNG_COLOR_TYPE_RGB_ALPHA)
|
1996-01-10 08:56:49 +00:00
|
|
|
png_set_bgr(png_ptr);
|
1995-07-20 07:43:20 +00:00
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
For some uses, you may want a gray-scale image to be represented as
|
|
|
|
rgb. This code will do that conversion:
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
if (info_ptr->color_type == PNG_COLOR_TYPE_GRAY ||
|
|
|
|
info_ptr->color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
|
1995-12-19 09:22:19 +00:00
|
|
|
png_set_gray_to_rgb(png_ptr);
|
1995-07-20 07:43:20 +00:00
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
PNG files store 16 bit pixels in network byte order (big-endian,
|
|
|
|
ie. most significant bits first). This code chages the storage to the
|
|
|
|
other way (little-endian, ie. least significant bits first, eg. the
|
|
|
|
way PCs store them):
|
1995-07-20 07:43:20 +00:00
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
if (info_ptr->bit_depth == 16)
|
|
|
|
png_set_swap(png_ptr);
|
1995-07-20 07:43:20 +00:00
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
PNG files store rgb pixels packed into 3 bytes. This code packs them
|
|
|
|
into 4 bytes:
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
if (info_ptr->bit_depth == 8 &&
|
|
|
|
info_ptr->color_type == PNG_COLOR_TYPE_RGB)
|
1995-09-26 10:22:39 +00:00
|
|
|
png_set_filler(png_ptr, filler_byte, PNG_FILLER_BEFORE);
|
1995-07-20 07:43:20 +00:00
|
|
|
|
1995-09-26 10:22:39 +00:00
|
|
|
where filler_byte is the 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.
|
1995-07-20 07:43:20 +00:00
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
The last thing to handle is interlacing; this is covered in detail below,
|
|
|
|
but you must call the function here.
|
1995-09-26 10:22:39 +00:00
|
|
|
|
|
|
|
if (info_ptr->interlace_type)
|
|
|
|
number_passes = png_set_interlace_handling(png_ptr);
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
After setting the transformations, you can update your palette by
|
|
|
|
calling png_start_read_image(). This function is provided for those
|
|
|
|
who need an updated palette before they read the image data. If you
|
|
|
|
don't call this function, the library will automatically call it
|
|
|
|
before it reads the first row.
|
|
|
|
|
1995-12-19 09:22:19 +00:00
|
|
|
png_start_read_image(png_ptr);
|
1995-07-20 07:43:20 +00:00
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
libpng can update your png_info structure to reflect any
|
|
|
|
transformations you've requested with this call. This is most useful
|
|
|
|
to update the info structures rowbytes field, so you can use it to
|
|
|
|
allocate your image memory. This function calls
|
1995-09-26 10:22:39 +00:00
|
|
|
png_start_read_image(), so you don't have to call both of them.
|
|
|
|
|
1995-12-19 09:22:19 +00:00
|
|
|
png_read_update_info(png_ptr, info_ptr);
|
1995-09-26 10:22:39 +00:00
|
|
|
|
|
|
|
After you call png_read_update_info(), you can allocate any
|
|
|
|
memory you need to hold the image. As the actual allocation
|
|
|
|
varies among applications, no example will be given. If you
|
|
|
|
are allocating one large chunk, you may find it useful to
|
|
|
|
build an array of pointers to each row, as it will be needed
|
|
|
|
for some of the functions below.
|
|
|
|
|
|
|
|
After you've allocated memory, you can read the image data.
|
1995-07-20 07:43:20 +00:00
|
|
|
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() 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:
|
|
|
|
|
1995-12-19 09:22:19 +00:00
|
|
|
png_bytep row_pointers[height];
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
You can point to void or char or whatever you use for pixels.
|
|
|
|
|
|
|
|
If you don't want to read the whole image in at once, you can
|
|
|
|
use png_read_rows() instead. If there is no interlacing (check
|
|
|
|
info_ptr->interlace_type), this is simple:
|
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
png_read_rows(png_ptr, row_pointers, NULL, number_of_rows);
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
row_pointers is the same as in the png_read_image() call.
|
|
|
|
|
|
|
|
If you are just calling one row at a time, you can do this for
|
|
|
|
row_pointers:
|
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
png_bytep row_pointers = row;
|
|
|
|
|
|
|
|
png_read_rows(png_ptr, &row_pointers, NULL, 1);
|
|
|
|
|
|
|
|
If the file is interlaced (info_ptr->interlace_type != 0), things get
|
|
|
|
a good deal harder. The only currently (as of 12/95) defined
|
|
|
|
interlacing scheme for PNG files (info_ptr->interlace_type == 1) is a
|
|
|
|
complicated interlace scheme, known as Adam7, that breaks down an
|
|
|
|
image into seven smaller images of varying size. libpng will fill out
|
|
|
|
those images or it will give them to you "as is". If you want to fill
|
|
|
|
them 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. This results in a blocky image for the first pass,
|
|
|
|
which gradually smoothes out as more pixels are read. The other
|
|
|
|
method is the "sparkle" method, where pixels are draw 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.
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
If you don't want libpng to handle the interlacing details, just
|
|
|
|
call png_read_rows() the correct number of times to read in all
|
|
|
|
seven images. See the PNG specification for more details on the
|
|
|
|
interlacing scheme.
|
|
|
|
|
1995-09-26 10:22:39 +00:00
|
|
|
If you want libpng to expand the images, call this above:
|
1995-07-20 07:43:20 +00:00
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
if (info_ptr->interlace_type)
|
|
|
|
number_passes = png_set_interlace_handling(png_ptr);
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
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,
|
|
|
|
when it will return one.
|
|
|
|
|
|
|
|
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_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.
|
|
|
|
|
1995-12-19 09:22:19 +00:00
|
|
|
png_read_rows(png_ptr, row_pointers, NULL, number_of_rows);
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
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.
|
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
png_read_rows(png_ptr, NULL, row_pointers, number_of_rows);
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
After you are finished reading the image, you can finish reading
|
|
|
|
the file. If you are interested in comments or time, you should
|
|
|
|
pass the png_info pointer from the png_read_info() call. If you
|
|
|
|
are not interested, you can pass NULL.
|
|
|
|
|
|
|
|
png_read_end(png_ptr, info_ptr);
|
|
|
|
|
|
|
|
When you are done, you can free all memory used by libpng like this:
|
|
|
|
|
|
|
|
png_read_destroy(png_ptr, info_ptr, (png_info *)0);
|
|
|
|
|
|
|
|
After that, you can discard the structures, or reuse them another
|
|
|
|
read or write. For a more compact example of reading a PNG image,
|
|
|
|
see the file example.c.
|
|
|
|
|
|
|
|
|
1995-12-19 09:22:19 +00:00
|
|
|
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 second 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;
|
|
|
|
|
|
|
|
int
|
|
|
|
initialize_png_reader()
|
|
|
|
{
|
|
|
|
png_ptr = malloc(sizeof (png_struct));
|
|
|
|
if (!png_ptr)
|
|
|
|
return -1;
|
|
|
|
info_ptr = malloc(sizeof (png_info));
|
|
|
|
if (!info_ptr)
|
|
|
|
{
|
|
|
|
free(png_ptr);
|
|
|
|
return -1;
|
|
|
|
}
|
|
|
|
|
|
|
|
if (setjmp(png_ptr->jmpbuf))
|
|
|
|
{
|
|
|
|
png_read_destroy(png_ptr, info_ptr, (png_info *)0);
|
|
|
|
/* free pointers before returning, if necessary */
|
|
|
|
free(png_ptr);
|
|
|
|
free(info_ptr);
|
|
|
|
return -1;
|
|
|
|
}
|
|
|
|
|
|
|
|
png_info_init(info_ptr);
|
|
|
|
png_read_init(png_ptr);
|
|
|
|
|
|
|
|
/* this one's new. You will need to provide all three
|
|
|
|
function callbacks, even if you aren't using them all.
|
1996-01-10 08:56:49 +00:00
|
|
|
You can use any void pointer as the user_ptr, and
|
1995-12-19 09:22:19 +00:00
|
|
|
retrieve the pointer from inside the callbacks using
|
|
|
|
the function png_get_progressive_ptr(png_ptr); */
|
1996-01-10 08:56:49 +00:00
|
|
|
png_set_progressive_read_fn(png_ptr, user_ptr,
|
1995-12-19 09:22:19 +00:00
|
|
|
info_callback, row_callback, end_callback);
|
|
|
|
|
|
|
|
return 0;
|
|
|
|
}
|
|
|
|
|
|
|
|
int
|
|
|
|
process_data(png_bytep buffer, png_uint_32 length)
|
|
|
|
{
|
|
|
|
if (setjmp(png_ptr->jmpbuf))
|
|
|
|
{
|
|
|
|
png_read_destroy(png_ptr, info_ptr, (png_info *)0);
|
|
|
|
free(png_ptr);
|
|
|
|
free(info_ptr);
|
|
|
|
return -1;
|
|
|
|
}
|
|
|
|
|
|
|
|
/* this one's new also. Simply give it a chunk of data
|
|
|
|
from the file stream (in order, of course). On Segmented
|
|
|
|
machines, don't give it any more then 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, but 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. */
|
|
|
|
png_process_data(png_ptr, info_ptr, buffer, length);
|
|
|
|
return 0;
|
|
|
|
}
|
|
|
|
|
|
|
|
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.
|
|
|
|
}
|
|
|
|
|
|
|
|
row_callback(png_structp png_ptr, png_bytep new_row,
|
|
|
|
png_uint_32 row_num, int pass)
|
|
|
|
{
|
|
|
|
this function is called for every row in the image. If the
|
|
|
|
image is interlacing, 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.
|
|
|
|
|
|
|
|
For the non-NULL rows of interlaced images, 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:
|
|
|
|
|
|
|
|
png_progressive_combine_row(png_ptr, old_row, new_row);
|
|
|
|
|
|
|
|
where old_row is what was displayed for previous rows. 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.
|
|
|
|
}
|
|
|
|
|
|
|
|
end_callback(png_structp png_ptr, png_infop info)
|
|
|
|
{
|
|
|
|
this function is called when 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.
|
|
|
|
}
|
|
|
|
|
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
IV. Writing
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
Much of this is very similar to reading. However, everything of
|
|
|
|
importance is repeated here, so you don't have to constantly look
|
1996-01-10 08:56:49 +00:00
|
|
|
back up in the reading section to understand writing.
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
The first thing you need to do while writing a PNG file is to allocate
|
|
|
|
and initialize png_struct and png_info. As these are both large, you
|
|
|
|
may not want to store these on the stack, unless you have stack space
|
|
|
|
to spare.
|
|
|
|
|
1995-12-19 09:22:19 +00:00
|
|
|
png_structp png_ptr = malloc(sizeof (png_struct));
|
|
|
|
if (!png_ptr)
|
|
|
|
return;
|
|
|
|
png_infop info_ptr = malloc(sizeof (png_info));
|
|
|
|
if (!info_ptr)
|
1995-07-20 07:43:20 +00:00
|
|
|
{
|
|
|
|
free(png_ptr);
|
|
|
|
return;
|
|
|
|
}
|
|
|
|
|
|
|
|
You may also want to do any i/o initialization here, before
|
|
|
|
you get into libpng, so if it doesn't work, you don't have
|
|
|
|
much to undo.
|
|
|
|
|
|
|
|
FILE *fp = fopen(file_name, "wb");
|
|
|
|
if (!fp)
|
|
|
|
{
|
|
|
|
free(png_ptr);
|
1995-12-19 09:22:19 +00:00
|
|
|
free(info_ptr);
|
1995-07-20 07:43:20 +00:00
|
|
|
return;
|
|
|
|
}
|
|
|
|
|
|
|
|
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 jmpbuf field of your png_struct. If you
|
|
|
|
write the file from different routines, you will need to update
|
|
|
|
the jmpbuf field every time you enter a new routine that will
|
|
|
|
call a png_ function. See your documentation of setjmp/longjmp
|
1995-12-19 09:22:19 +00:00
|
|
|
for your compiler for more information on setjmp/longjmp. See
|
1996-01-10 08:56:49 +00:00
|
|
|
the discussion on libpng error handling in the Customizing Libpng
|
|
|
|
section below for more information on the libpng error handling.
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
if (setjmp(png_ptr->jmpbuf))
|
|
|
|
{
|
|
|
|
png_write_destroy(png_ptr);
|
1995-12-19 09:22:19 +00:00
|
|
|
/* free pointers before returning. Make sure you clean up
|
1995-07-20 07:43:20 +00:00
|
|
|
anything else you've done. */
|
|
|
|
free(png_ptr);
|
|
|
|
free(info_ptr);
|
1995-12-19 09:22:19 +00:00
|
|
|
fclose(fp);
|
1995-07-20 07:43:20 +00:00
|
|
|
return;
|
|
|
|
}
|
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
Next, you will need to call png_info_init() and png_write_init().
|
1995-07-20 07:43:20 +00:00
|
|
|
These functions make sure all the fields are initialized to useful
|
|
|
|
values, and, in the case of png_write_init(), allocate any memory
|
|
|
|
needed for internal uses. Do png_info_init() first, so if
|
|
|
|
png_write_init() longjmps, you know info_ptr is valid, so you
|
|
|
|
don't free random memory pointers, which would be bad.
|
|
|
|
|
|
|
|
png_info_init(info_ptr);
|
1996-01-10 08:56:49 +00:00
|
|
|
png_write_init(png_ptr);
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
Now you need to set up the input 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. If you wish to handle writing
|
1995-12-19 09:22:19 +00:00
|
|
|
data in another way, see the discussion on png i/o handling in the
|
1995-07-20 07:43:20 +00:00
|
|
|
Customizing Libpng section below.
|
|
|
|
|
|
|
|
png_init_io(png_ptr, fp);
|
|
|
|
|
1995-09-26 10:22:39 +00:00
|
|
|
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 certain special cases, like if you need to
|
1995-12-19 09:22:19 +00:00
|
|
|
write png files extremely fast and are willing to give up some
|
1995-09-26 10:22:39 +00:00
|
|
|
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, as it has been
|
|
|
|
carefully tuned to deliver the best speed/compression ratio.
|
|
|
|
See the compression library for more details.
|
|
|
|
|
|
|
|
/* turn on or off filtering (1 or 0) */
|
1995-12-19 09:22:19 +00:00
|
|
|
png_set_filtering(png_ptr, 1);
|
1995-09-26 10:22:39 +00:00
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
/* compression level (0 - none, 6 - default, 9 - maximum) */
|
1995-12-19 09:22:19 +00:00
|
|
|
png_set_compression_level(png_ptr, Z_DEFAULT_COMPRESSION);
|
1995-09-26 10:22:39 +00:00
|
|
|
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);
|
|
|
|
|
1995-07-20 07:43:20 +00:00
|
|
|
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. See png_write_end() for more information on that. If you
|
|
|
|
wish to write them before the image, fill them in now. If you want to
|
|
|
|
wait until after the data, don't fill them until png_write_end(). For
|
1996-01-10 08:56:49 +00:00
|
|
|
all the fields in png_info, 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:
|
|
|
|
|
|
|
|
width - holds the width of the file
|
|
|
|
height - holds the height of the file
|
|
|
|
bit_depth - holds the bit depth of one of the image channels
|
|
|
|
color_type - describes the channels and what they mean
|
|
|
|
see the PNG_COLOR_TYPE_ defines for more information
|
1995-07-20 07:43:20 +00:00
|
|
|
interlace_type - currently 0 for none, 1 for interlaced
|
1996-01-10 08:56:49 +00:00
|
|
|
valid - this describes which optional chunks to write to the
|
|
|
|
file. Note that if you are writing a
|
|
|
|
PNG_COLOR_TYPE_PALETTE file, the PLTE chunk is not
|
|
|
|
optional, but must still be marked for writing. To
|
|
|
|
mark chunks for writing, OR valid with the appropriate
|
|
|
|
PNG_INFO_<chunk name> define.
|
|
|
|
palette - the palette for the file
|
|
|
|
num_palette - number of entries in the palette
|
|
|
|
gamma - the gamma the file is written at
|
|
|
|
sig_bit - the number of significant bits
|
|
|
|
for the gray, red, green, and blue channels, whichever are
|
|
|
|
appropriate for the given color type.
|
|
|
|
sig_bit_number - number of channels
|
|
|
|
trans_values - transparent pixel for non-paletted images
|
|
|
|
trans - array of transparent entries for paletted images
|
|
|
|
number_trans - number of transparent entries
|
|
|
|
hist - histogram of palette
|
|
|
|
text - text comments in the file.
|
|
|
|
num_text - number of comments
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
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.
|
|
|
|
If you want, you can use max_text to hold the size of the array, but
|
|
|
|
libpng ignores it for writing (it does use it for reading). Each
|
|
|
|
png_text structure holds a keyword-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 -1. Until text gets
|
|
|
|
arount 1000 bytes, it is not worth compressing it.
|
|
|
|
|
|
|
|
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 recomendations
|
|
|
|
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 can not contain NUL characters,
|
|
|
|
and should not contain control characters. Text in general should
|
|
|
|
not contain control characters. 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.
|
|
|
|
|
1995-12-19 09:22:19 +00:00
|
|
|
PNG supports modification time via the png_time structure. Two
|
1995-07-20 07:43:20 +00:00
|
|
|
conversion routines are proved, 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
|
1996-01-10 08:56:49 +00:00
|
|
|
instead of your local time. Note that the year number is the full
|
|
|
|
year (ie 1996, rather than 96)
|
|
|
|
|
|
|
|
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 on line 75, unless png_write_flush is
|
|
|
|
called earlier. If nrows is too small (less than about 10 lines) the
|
|
|
|
image compression may decrease dramatically (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.
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
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().
|
|
|
|
|
1995-12-19 09:22:19 +00:00
|
|
|
png_write_info(png_ptr, info_ptr);
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
After you've read 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
|
|
|
|
occur. This is important, as some of these change the color type
|
|
|
|
and bit depth of the data, and some others only work on certain
|
|
|
|
color types and bit depths. Even though each transformation should
|
|
|
|
check to see if it has data that it can do somthing 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.
|
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
PNG files store rgb pixels packed into 3 bytes. This code tells
|
|
|
|
the library to use 4 bytes per pixel
|
1995-07-20 07:43:20 +00:00
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);
|
1995-07-20 07:43:20 +00:00
|
|
|
|
1995-09-26 10:22:39 +00:00
|
|
|
where the 0 is not used for writing, and the location is either
|
|
|
|
PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether you
|
|
|
|
want the filler before the rgb or after.
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
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.
|
1996-01-10 08:56:49 +00:00
|
|
|
If the data is supplied at 1 pixel per byte, use this code, which will
|
|
|
|
correctly pack the values:
|
1995-07-20 07:43:20 +00:00
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
png_set_packing(png_ptr);
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
PNG files reduce possible bit depths to 1, 2, 4, 8, and 16. If your
|
|
|
|
data is of another bit depth, but is packed into the bytes correctly,
|
|
|
|
this will scale the values to appear to be the correct bit depth.
|
|
|
|
Make sure you write a sBIT chunk when you do this, so others, if
|
|
|
|
they want, can reduce the values down to their true depth.
|
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
/* do this before png_write_info() */
|
|
|
|
info_ptr->valid |= PNG_INFO_sBIT;
|
1995-07-20 07:43:20 +00:00
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
/* note that you can cheat and set all the values of
|
|
|
|
sig_bit to true_bit_depth if you want */
|
|
|
|
if (info_ptr->color_type & PNG_COLOR_MASK_COLOR)
|
|
|
|
{
|
1995-07-20 07:43:20 +00:00
|
|
|
info_ptr->sig_bit.red = true_bit_depth;
|
|
|
|
info_ptr->sig_bit.green = true_bit_depth;
|
|
|
|
info_ptr->sig_bit.blue = true_bit_depth;
|
|
|
|
}
|
|
|
|
else
|
|
|
|
{
|
|
|
|
info_ptr->sig_bit.gray = true_bit_depth;
|
|
|
|
}
|
|
|
|
|
|
|
|
if (info_ptr->color_type & PNG_COLOR_MASK_ALPHA)
|
|
|
|
{
|
|
|
|
info_ptr->sig_bit.alpha = true_bit_depth;
|
|
|
|
}
|
|
|
|
|
|
|
|
png_set_shift(png_ptr, &(info_ptr->sig_bit));
|
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
PNG files store 16 bit pixels in network byte order (big-endian,
|
|
|
|
ie. most significant bits first). This code would be used to supply
|
|
|
|
them the other way (little-endian, ie. least significant bits first,
|
|
|
|
eg. the way PCs store them):
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
png_set_swap(png_ptr);
|
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
PNG files store 3 color pixels in red, green, blue order. This code
|
|
|
|
would be used to supply the pixels as blue, green, red:
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
png_set_bgr(png_ptr);
|
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
PNG files describe monochrome as black being zero and white being
|
|
|
|
one. This code would be used to supply the pixels with this reversed
|
|
|
|
(black being one and white being zero):
|
1995-07-20 07:43:20 +00:00
|
|
|
|
1995-12-19 09:22:19 +00:00
|
|
|
png_set_invert(png_ptr);
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
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 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:
|
|
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|
|
1995-12-19 09:22:19 +00:00
|
|
|
png_bytef *row_pointers[height];
|
1995-07-20 07:43:20 +00:00
|
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|
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|
|
You can point to void or char or whatever you use for pixels.
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|
|
If you can't want to write the whole image at once, you can
|
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|
|
use png_write_rows() instead. If the file is not interlaced,
|
|
|
|
this is simple:
|
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|
|
png_write_rows(png_ptr, row_pointers, number_of_rows);
|
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|
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|
|
row_pointers is the same as in the png_write_image() call.
|
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|
|
If you are just calling one row at a time, you can do this for
|
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|
|
row_pointers:
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|
|
1995-12-19 09:22:19 +00:00
|
|
|
png_bytep row_pointers = row;
|
1995-07-20 07:43:20 +00:00
|
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|
|
png_write_rows(png_ptr, &row_pointers, 1);
|
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|
|
1996-01-10 08:56:49 +00:00
|
|
|
When the file is interlaced, things can get a good deal more
|
|
|
|
complicated. The only currently (as of 12/95) defined interlacing
|
|
|
|
scheme for PNG files is a compilcated interlace scheme, known as
|
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|
|
Adam7, 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.
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
If you don't want libpng to handle the interlacing details, just
|
|
|
|
call png_write_rows() the correct number of times to write all
|
|
|
|
seven sub-images.
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|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
If you want libpng to build the sub-images, call this before you start
|
|
|
|
writing any rows:
|
1995-07-20 07:43:20 +00:00
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
number_passes = png_set_interlace_handling(png_ptr);
|
1995-07-20 07:43:20 +00:00
|
|
|
|
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|
|
This will return the number of passes needed. Currently, this
|
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|
|
is seven, but may change if another interlace type is added.
|
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|
|
Then write the image number_passes times.
|
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|
|
1996-01-10 08:56:49 +00:00
|
|
|
png_write_rows(png_ptr, row_pointers, number_of_rows);
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
As some of these rows are not used, and thus return immediately,
|
|
|
|
you may want to read about interlacing in the PNG specification,
|
|
|
|
and only update the rows that are actually used.
|
|
|
|
|
|
|
|
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 the an appropriately filled png_info pointer. If you
|
|
|
|
are not interested, you can pass NULL. Be careful that you don't
|
|
|
|
write the same text or time chunks here as you did in png_write_info().
|
|
|
|
|
1995-12-19 09:22:19 +00:00
|
|
|
png_write_end(png_ptr, info_ptr);
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
When you are done, you can free all memory used by libpng like this:
|
|
|
|
|
|
|
|
png_write_destroy(png_ptr);
|
|
|
|
|
|
|
|
Any data you allocated for png_info, you must free yourself.
|
|
|
|
|
|
|
|
After that, you can discard the structures, or reuse them another
|
|
|
|
read or write. For a more compact example of writing a PNG image,
|
|
|
|
see the file example.c.
|
|
|
|
|
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
V. Modifying/Customizing libpng:
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
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.
|
|
|
|
|
1995-12-19 09:22:19 +00:00
|
|
|
All of the memory allocation, input/output, and error handling in libpng
|
|
|
|
goes through callbacks which are user setable. The default routines
|
|
|
|
are in pngerror.c, pngmem.c, and pngio.c. To change these functions,
|
1996-01-10 08:56:49 +00:00
|
|
|
call the approprate _fn function.
|
1995-12-19 09:22:19 +00:00
|
|
|
|
|
|
|
Memory allocation is done through the functions png_large_malloc(),
|
|
|
|
png_malloc(), png_realloc(), png_large_free(), and png_free().
|
|
|
|
These currently just call the standard C functions. The large
|
|
|
|
functions must handle exactly 64K, but they don't have to handle
|
|
|
|
more then that. If your pointers can't access more then 64K at a
|
1996-01-10 08:56:49 +00:00
|
|
|
time, you will want to set MAXSEG_64K in zlib.h. 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.
|
1995-12-19 09:22:19 +00:00
|
|
|
|
|
|
|
Input/Output in libpng is done throught 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
|
1996-01-10 08:56:49 +00:00
|
|
|
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. These
|
|
|
|
functions also provide a void pointer that can be retrieved via the function
|
1995-12-19 09:22:19 +00:00
|
|
|
png_get_io_ptr(). For example:
|
|
|
|
|
|
|
|
png_set_read_fn(png_structp png_ptr, voidp io_ptr,
|
|
|
|
png_rw_ptr read_data_fn)
|
|
|
|
|
|
|
|
png_set_write_fn(png_structp png_ptr, voidp io_ptr,
|
|
|
|
png_rw_ptr write_data_fn, png_flush_ptr output_flush_fn);
|
|
|
|
|
|
|
|
voidp io_ptr = png_get_io_ptr(png_ptr);
|
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
The replacement I/O functions should have prototypes as follows:
|
|
|
|
|
|
|
|
void user_read_data(png_structp png_ptr, png_bytep data,
|
|
|
|
png_uint_32 length);
|
|
|
|
void user_write_data(png_structp png_ptr, png_bytep data,
|
|
|
|
png_uint_32 length);
|
|
|
|
void user_flush_data(png_structp png_ptr);
|
|
|
|
|
1995-12-19 09:22:19 +00:00
|
|
|
Note that you can pass NULL for the flush function if you are not doing
|
1996-01-10 08:56:49 +00:00
|
|
|
flushing of the output data.
|
1995-12-19 09:22:19 +00:00
|
|
|
|
|
|
|
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 it's caller. Currently, this is handled via
|
|
|
|
setjmp() and longjmp(), but you could change this to do things like
|
1996-01-10 08:56:49 +00:00
|
|
|
exit() if you should wish. 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. If
|
|
|
|
you wish to change the behavior of the error functions, you will need to
|
|
|
|
set up your own message callbacks. You do this like the I/O callbacks above.
|
1995-11-28 17:22:13 +00:00
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
png_set_message_fn(png_structp png_ptr, png_voidp msg_ptr,
|
|
|
|
png_msg_ptr error_fn, png_msg_ptr warning_fn);
|
1995-11-28 17:22:13 +00:00
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
png_voidp msg_ptr = png_get_msg_ptr(png_ptr);
|
|
|
|
|
|
|
|
The replacement message functions should have parameters as follows:
|
|
|
|
|
|
|
|
void user_error_fn(png_struct png_ptr, png_const_charp error_msg);
|
|
|
|
void user_warning_fn(png_struct 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.
|
1995-11-28 17:22:13 +00:00
|
|
|
|
1995-07-20 07:43:20 +00:00
|
|
|
If you need to read or write custom chunks, you will need to get deeper
|
|
|
|
into the libpng code. First, read the PNG specification, and have
|
|
|
|
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 similar.
|
|
|
|
Second, check out the sections of libpng that read and write chunks.
|
|
|
|
Try to find a chunk that is similar to yours, and copy off of it.
|
1995-12-19 09:22:19 +00:00
|
|
|
More details can be found in the comments inside the code.
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
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 more simple 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:
|
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
You will probably need to change the png_large_malloc() and
|
1995-12-19 09:22:19 +00:00
|
|
|
png_large_free() routines in pngmem.c, as these are requred
|
1995-07-20 07:43:20 +00:00
|
|
|
to allocate 64K. Also, 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 accessable. So limit zlib
|
|
|
|
and libpng to 64K by defining MAXSEG_64K.
|
|
|
|
|
1995-12-19 09:22:19 +00:00
|
|
|
Configuring for Medium Model:
|
|
|
|
|
|
|
|
Libpng's support for medium model has been tested on most of the popular
|
1996-01-10 08:56:49 +00:00
|
|
|
complers. Make sure MAXSEG_64K gets defined, USE_FAR_KEYWORD gets
|
|
|
|
defined, and FAR gets defined to far in pngconf.h, and you should be
|
1995-12-19 09:22:19 +00:00
|
|
|
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 row's of data are defined as png_bytepp which is a
|
|
|
|
unsigned char far * far *
|
|
|
|
|
1995-07-20 07:43:20 +00:00
|
|
|
Configuring for gui/windowing platforms:
|
|
|
|
|
1995-12-19 09:22:19 +00:00
|
|
|
You will need to change the error message display in png_error() and
|
|
|
|
png_warning() to display a message instead of fprinting it to stderr.
|
|
|
|
You may want to write a single function to do this and call it something
|
|
|
|
like png_message(). On some compliers, you may have to change the
|
|
|
|
memory allocators (png_malloc, etc.).
|
|
|
|
|
1995-07-20 07:43:20 +00:00
|
|
|
|
|
|
|
Configuring for compiler xxx:
|
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
All includes for libpng are in pngconf.h. If you need to add/change/delete
|
1995-07-20 07:43:20 +00:00
|
|
|
an include, this is the place to do it. The includes that are not
|
|
|
|
needed outside libpng are protected by the PNG_INTERNAL definition,
|
|
|
|
which is only defined for those routines inside libpng itself. The
|
|
|
|
files in libpng proper only include png.h.
|
|
|
|
|
1995-09-26 10:22:39 +00:00
|
|
|
Removing unwanted object code:
|
|
|
|
|
1996-01-10 08:56:49 +00:00
|
|
|
There are a bunch of #define's in pngconf.h that control what parts of
|
1995-09-26 10:22:39 +00:00
|
|
|
libpng are compiled. All the defines end in _SUPPORT. If you are
|
1996-01-10 08:56:49 +00:00
|
|
|
never going to use an ability, you can change the #define to #undef and
|
1995-09-26 10:22:39 +00:00
|
|
|
save yourself code and data space. All the reading and writing
|
|
|
|
specific code are in seperate 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
|
1995-12-19 09:22:19 +00:00
|
|
|
both reading and writing, and always need to be included. The
|
|
|
|
progressive reader is in pngpread.c
|
|
|
|
|