1302 lines
58 KiB
Plaintext
1302 lines
58 KiB
Plaintext
libpng.txt - a description on how to use and modify libpng
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libpng 1.0 beta 3 - version 0.89
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Updated and distributed by Andreas Dilger <adilger@enel.ucalgary.ca>,
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based on:
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libpng 1.0 beta 2 - version 0.88
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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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May 24, 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 && 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.
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Libpng was written as a companion to the PNG specification, as a way
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to reduce the amount of time and effort it takes to support the PNG
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file format in application programs. Most users will not have to
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modify the library significantly; advanced users may want to modify it
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more. All attempts were made to make it as complete as possible,
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while 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 pngtodo.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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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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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. The first, png_struct, is an internal structure that
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will not, for the most part, be used by a user except as the first
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variable passed to every png function call.
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The png_info structure is designed to provide information about the
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png file. All of its fields are intended to be examined or modified
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by the user. See png.h for a good description of the png_info fields.
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png.h is also an invaluable reference for programming with libpng.
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And while I'm on the topic, make sure you include the png header file:
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#include <png.h>
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III. Reading
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Reading PNG files:
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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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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, and it will
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return true or false (1 or 0) depending on whether the bytes could be
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part of a PNG file. Of course, the more bytes you pass in, the
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greater the accuracy of the prediction. If you pass in more then
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eight bytes, libpng will only look at the first eight bytes. However,
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because libpng automatically checks the file header, this is not often
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necessary, and you should pass a newly opened file pointer to libpng
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when reading a file.
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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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FILE *fp = fopen(file_name, "rb");
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if (!fp)
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{
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return;
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}
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fread(header, 1, number, fp);
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is_png = png_check_sig(header, number);
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if (!is_png)
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{
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return;
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}
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fclose(fp);
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Next, png_struct and png_info need to be allocated and initialized.
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In order to ensure that the size of these structures is correct even
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with a dynamically linked libpng, there are functions to initialize
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and allocate the structures. We also pass the library version, and
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optionally pointers to error handling functions (these can be NULL
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if the default error handlers are to be used). See the section on
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Changes to Libpng below regarding the old initialization functions.
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png_structp png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING,
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(void *)user_error_ptr, user_error_fn, user_warning_fn);
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if (!png_ptr)
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return;
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png_infop info_ptr = png_create_info_struct(png_ptr);
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if (!info_ptr)
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{
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png_destroy_read_struct(&png_ptr, (png_infopp)NULL, (png_infopp)NULL);
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return;
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}
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png_infop end_info = png_create_info_struct(png_ptr);
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if (!end_info)
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{
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png_destroy_read_struct(&png_ptr, &info_ptr, (png_infopp)NULL);
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return;
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}
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The error handling routines passed to png_create_read_struct()
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are only necessary if you are not using the libpng supplied
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error handling functions. When libpng encounters an error,
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it expects to longjmp back to your routine. Therefore, you
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will need to call setjmp and pass the jmpbuf field of your
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png_struct. If you read the file from different routines, you
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will need to update the jmpbuf field every time you enter a new
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routine that will call a png_ function. See your documentation
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of setjmp/longjmp for your compiler for more information on
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setjmp/longjmp. See the discussion on libpng error handling
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in the Customizing Libpng section below for more information on
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the libpng error handling. If an error occurs, and libpng
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longjmp's back to your setjmp, you will want to call
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png_destroy_read_struct() to free any memory.
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if (setjmp(png_ptr->jmpbuf))
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{
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png_destroy_read_struct(&png_ptr, &info_ptr, &end_info);
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fclose(fp);
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return;
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}
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Now you need to set up the input code. The default for libpng is to
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use the C function fread(). If you use this, you will need to pass a
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valid FILE * in the function png_init_io(). Be sure that the file is
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opened in binary mode. Again, if you wish to handle reading data in
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another way, see the discussion on libpng I/O handling in the Customizing
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Libpng section below.
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png_init_io(png_ptr, fp);
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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 info_ptr 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
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bit_depth 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
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the 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 (PNG_INFO_PLTE)
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num_palette - number of entries in the palette
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gamma - the gamma the file is written at (PNG_INFO_gAMA)
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sig_bit - the number of significant bits (PNG_INFO_sBIT)
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for the gray, red, green, and blue channels, whichever
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are appropriate for the given color type.
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trans_values - transparent pixel for non-paletted images (PNG_INFO_tRNS)
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trans - array of transparent entries for paletted images
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num_trans - number of transparent entries
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hist - histogram of palette (PNG_INFO_hIST)
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mod_time - time image was last modified (PNG_VALID_tIME)
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background - background color (PNG_VALID_bKGD)
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text - text comments in the file.
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num_text - number of comments
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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, gray_to_rgb, etc.).
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See png_update_info(), below.
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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 suggested
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keywords, there is no requirement to restrict the use to these
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strings. There is a requirement to have at least one character for a
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keyword. It is strongly suggested that keywords be sensible to humans
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(that's the point), so don't use abbreviations. See the png
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specification for more details. There is also no requirement to have
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text after the keyword.
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Keywords should be limited to 80 characters without leading or trailing
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spaces, but non-consecutive spaces are allowed within the keyword. It is
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possible to have the same keyword any number of times. The text field
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is an array of png_text structures, each holding pointer to a keyword
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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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should occur. This is important, as some of these change the color
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type and/or bit depth of the data, and some others only work on
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certain color types and bit depths. Even though each transformation
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checks 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 the
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data. For example, don't swap red and blue on grayscale data.
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Data will be decoded into the supplied row buffers packed into bytes
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unless the library has been told to transform it into another format.
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For example, 4 bit/pixel paletted or grayscale data will be returned
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2 pixels/byte with the leftmost pixel in the high-order bits of the
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byte, unless png_set_packing() is called. 8-bit RGB data will be stored
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in RGBRGBRGB format unless png_set_filler() is called to insert filler
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bytes, either before or after each RGB triplet. 16-bit RGB data will
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be returned RRGGBBRRGGBB, with the most significant byte of the color
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value first, unless png_set_strip_16() is called to transform it to
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regular RGBRGB triplets.
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The following code transforms grayscale images of less than 8 to 8 bits,
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changes paletted images to RGB, and adds a full alpha channel if there is
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transparency information in a tRNS chunk. This is most useful on
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grayscale images with bit depths of 2 or 4 or if there is a multiple-image
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viewing application that wishes to treat all images in the same way.
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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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if (info_ptr->color_type == PNG_COLOR_TYPE_GRAY &&
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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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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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PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
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they can, resulting in, for example, 8 pixels per byte for 1 bit
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files. This code expands to 1 pixel per byte without changing the
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values of the pixels:
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if (info_ptr->bit_depth < 8)
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png_set_packing(png_ptr);
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PNG files have possible bit depths of 1, 2, 4, 8, and 16. It is then
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required that values be "scaled" or "shifted" up to the bit depth used
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in the file (ie from 5 bits/sample in the range [0,31] to 8 bits/sample
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in the range [0, 255]). However, they also provide a way to describe
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the true bit depth of the image. See the PNG specification for details.
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This call reduces the pixels back down to the true bit depth:
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if (info_ptr->valid & PNG_INFO_sBIT)
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png_set_shift(png_ptr, &(info_ptr->sig_bit));
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PNG files store 3 color pixels in red, green, blue order. This code
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changes the storage of the pixels to blue, green, red:
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if (info_ptr->color_type == PNG_COLOR_TYPE_RGB ||
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info_ptr->color_type == PNG_COLOR_TYPE_RGB_ALPHA)
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png_set_bgr(png_ptr);
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PNG files store RGB pixels packed into 3 bytes. This code expands them
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into 4 bytes for windowing systems that need them in this format:
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if (info_ptr->bit_depth == 8 &&
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info_ptr->color_type == PNG_COLOR_TYPE_RGB)
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png_set_filler(png_ptr, filler_byte, PNG_FILLER_BEFORE);
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where filler_byte is the number to fill with, and the location is
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either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether
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you want the filler before the RGB or after.
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For some uses, you may want a gray-scale image to be represented as
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RGB. This code will do that conversion:
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if (info_ptr->color_type == PNG_COLOR_TYPE_GRAY ||
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info_ptr->color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
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png_set_gray_to_rgb(png_ptr);
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The following code handles alpha and transparency by replacing it with
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a background value. If there was a valid bKGD in the file, you can use
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it if you want. However, you can replace it with your own if you want
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also. If there wasn't one in the file, you must supply a color. If
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libpng is doing gamma correction, you will need to tell libpng where
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the background came from so it can do the appropriate gamma
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correction. If you have a grayscale and you are using png_set_expand()
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to change to a higher bit-depth you must indicate if the background gray
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needs to be expanded to the new bit-depth. Similarly, if you are reading
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a paletted image, you must indicate if you have supplied the background
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index that needs to be expanded to RGB values. You can always specify
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RGB color values directly when setting your background for paletted images.
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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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PNG_BACKGROUND_GAMMA_FILE, 1, 1.0);
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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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The following code handles gamma transformations of the data. Pass
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both the file gamma and the desired screen gamma. If the file does
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not have a gamma value, you can pass one anyway if you wish. Note
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that file gammas are inverted from screen gammas. See the discussions
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on gamma in the PNG specification for more information. It is
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strongly recommended that viewers support gamma correction.
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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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else
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png_set_gamma(png_ptr, screen_gamma, 0.45);
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If you need to reduce an RGB file to a paletted file, or if a paletted
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file has more entries then will fit on your screen, png_set_dither()
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will do that. Note that this is a simple match dither that merely
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finds the closest color available. This should work fairly well with
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optimized palettes, and fairly badly with linear color cubes. If you
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pass a palette that is larger then maximum_colors, the file will
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reduce the number of colors in the palette so it will fit into
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maximum_colors. If there is a histogram, it will use it to make
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more intelligent choices when reducing the palette. If there is no
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histogram, it may not do as good a job.
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if (info_ptr->color_type & PNG_COLOR_MASK_COLOR)
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{
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if (info_ptr->valid & PNG_INFO_PLTE)
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{
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png_set_dither(png_ptr, info_ptr->palette,
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info_ptr->num_palette, max_screen_colors,
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info_ptr->histogram, 1);
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}
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else
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{
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png_color std_color_cube[MAX_SCREEN_COLORS] =
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{ ... colors ... };
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png_set_dither(png_ptr, std_color_cube, MAX_SCREEN_COLORS,
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MAX_SCREEN_COLORS, NULL,0);
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}
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}
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PNG files describe monochrome as black being zero and white being one.
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The following code will reverse this (make black be one and white be
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zero):
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if (info_ptr->bit_depth == 1 &&
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info_ptr->color_type == PNG_COLOR_GRAY)
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png_set_invert_mono(png_ptr);
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PNG files store 16 bit pixels in network byte order (big-endian,
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ie. most significant bits first). This code chages the storage to the
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other way (little-endian, ie. least significant bits first, eg. the
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way PCs store them):
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if (info_ptr->bit_depth == 16)
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png_set_swap(png_ptr);
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The last thing to handle is interlacing; this is covered in detail below,
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but you must call the function here.
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if (info_ptr->interlace_type)
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number_passes = png_set_interlace_handling(png_ptr);
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After setting the transformations, libpng can update your png_info
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structure to reflect any transformations you've requested with this
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call. This is most useful to update the info structures rowbytes
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field, so you can use it to allocate your image memory. This function
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will also update your palette with the correct display gamma and
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background if these have been given with the calls above.
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png_read_update_info(png_ptr, info_ptr);
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After you call png_read_update_info(), you can allocate any
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memory you need to hold the image. The row data is simply
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raw byte data for all forms of images. As the actual allocation
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varies among applications, no example will be given. If you
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are allocating one large chunk, you will need to build an
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array of pointers to each row, as it will be needed for some
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of the functions below.
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After you've allocated memory, you can read the image data.
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The simplest way to do this is in one function call. If you are
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allocating enough memory to hold the whole image, you can just
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call png_read_image() and libpng will read in all the image data
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and put it in the memory area supplied. You will need to pass in
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an array of pointers to each row.
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This function automatically handles interlacing, so you don't need
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to call png_set_interlace_handling() or call this function multiple
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times, or any of that other stuff necessary with png_read_rows().
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png_read_image(png_ptr, row_pointers);
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where row_pointers is:
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png_bytep row_pointers[height];
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You can point to void or char or whatever you use for pixels.
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If you don't want to read int the whole image at once, you can
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use png_read_rows() instead. If there is no interlacing (check
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info_ptr->interlace_type), this is simple:
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|
|
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
|
|
row_pointers:
|
|
|
|
png_bytep row_pointers = row;
|
|
png_read_row(png_ptr, &row_pointers, NULL);
|
|
|
|
If the file is interlaced (info_ptr->interlace_type != 0), things get
|
|
somewhat harder. The only currently (as of 6/96 -- PNG
|
|
Specification version 1.0) defined interlacing scheme for PNG files
|
|
(info_ptr->interlace_type == 1) is a someewhat complicated 2D interlace
|
|
scheme, known as Adam7, that breaks down an image into seven smaller
|
|
images of varying size, based on an 8x8 grid.
|
|
|
|
libpng can fill out those images or it can give them to you "as is".
|
|
If you want them 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. 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.
|
|
|
|
If you don't want libpng to handle the interlacing details, just call
|
|
png_read_rows() seven times to read in all seven images. Each of the
|
|
images are valid images by themselves, or they can be combined on an
|
|
8x8 grid to form a single image (although if you intend to combine them
|
|
you would be far better off using the libpng interlace handling).
|
|
|
|
The first pass will return an image 1/8 as wide as the entire image
|
|
(every 8th column starting in column 0) and 1/8 as high as the original
|
|
(every 8th row starting in row 0), the second will be 1/8 as wide
|
|
(starting in column 4) and 1/8 as high (also starting in row 0). The
|
|
third pass will be 1/4 as wide (every 4th pixel starting in row 0) and
|
|
1/8 as high (every 8th row starting in row 4), and the fourth pass will
|
|
be 1/4 as wide and 1/4 as high (every 4th column starting in column 2,
|
|
and every 4th row starting in row 0). The fifth pass will return an
|
|
image 1/2 as wide, and 1/4 as high (starting at column 0 and row 2),
|
|
while the sixth pass will be 1/2 as wide and 1/2 as high as the original
|
|
(starting in column 1 and row 0). The seventh and final pass will be as
|
|
wide as the original, and 1/2 as high, containing all of the odd
|
|
numbered scanlines. Phew!
|
|
|
|
If you want libpng to expand the images, call this before calling
|
|
png_start_read_image() or png_read_update_info():
|
|
|
|
if (info_ptr->interlace_type)
|
|
number_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.
|
|
|
|
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.
|
|
|
|
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);
|
|
|
|
After you are finished reading the image, 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
|
|
info_ptr pointer from the png_read_info() call, or you can pass a
|
|
separate png_info struct if you want to keep the comments from
|
|
before and after the image separate. If you are not interested, you
|
|
can pass NULL.
|
|
|
|
png_read_end(png_ptr, end_info);
|
|
|
|
When you are done, you can free all memory allocated by libpng like this:
|
|
|
|
png_destroy_read_struct(&png_ptr, &info_ptr, &end_info);
|
|
|
|
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,
|
|
(void *)user_error_ptr, user_error_fn, user_warning_fn);
|
|
if (!png_ptr)
|
|
return -1;
|
|
info_ptr = png_create_info_struct(png_ptr);
|
|
if (!info_ptr)
|
|
{
|
|
png_destroy_read_struct(&png_ptr, (png_infopp)NULL, (png_infopp)NULL);
|
|
return -1;
|
|
}
|
|
|
|
if (setjmp(png_ptr->jmpbuf))
|
|
{
|
|
png_destroy_read_struct(&png_ptr, &info_ptr, (png_infopp)NULL);
|
|
return -1;
|
|
}
|
|
|
|
/* 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 a NULL parameter. 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 recieve blocks of data */
|
|
int
|
|
process_data(png_bytep buffer, png_uint_32 length)
|
|
{
|
|
if (setjmp(png_ptr->jmpbuf))
|
|
{
|
|
png_destroy_read_struct(&png_ptr, &info_ptr, (png_infopp)NULL);
|
|
return -1;
|
|
}
|
|
|
|
/* 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);
|
|
return 0;
|
|
}
|
|
|
|
/* This function is called (as set by png_set_progressive_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 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.
|
|
|
|
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.
|
|
*/
|
|
}
|
|
|
|
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.
|
|
|
|
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;
|
|
}
|
|
|
|
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.
|
|
|
|
png_structp png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING,
|
|
(void *)user_error_ptr, user_error_fn, user_warning_fn);
|
|
if (!png_ptr)
|
|
return;
|
|
|
|
png_infop info_ptr = png_create_info_struct(png_ptr);
|
|
if (!info_ptr)
|
|
{
|
|
png_destroy_write_struct(&png_ptr, (png_infopp)NULL);
|
|
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
|
|
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_ptr->jmpbuf))
|
|
{
|
|
png_destroy_write_struct(&png_ptr, &info_ptr);
|
|
fclose(fp);
|
|
return;
|
|
}
|
|
|
|
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. 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);
|
|
|
|
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 value is '0' (as of the
|
|
06/96 PNG specification. The third parameter is a flag that indicates
|
|
which filter type(s) are to be tested for each scanline. See the
|
|
Compression Library for details on the specific filter types.
|
|
|
|
|
|
/* turn on or off filtering, and/or choose specific filters */
|
|
png_set_filter(png_ptr, 0,
|
|
PNG_FILTER_NONE | PNG_FILTER_SUB | PNG_FILTER_PAETH);
|
|
|
|
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 for details on the compression levels.
|
|
|
|
/* set the zlib compression level */
|
|
png_set_compression_level(png_ptr, Z_BEST_COMPRESSION);
|
|
|
|
/* set other zlib parameters */
|
|
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);
|
|
|
|
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 1.0, 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:
|
|
|
|
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
|
|
interlace_type - allowed values are 0 for none, 1 for interlaced
|
|
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, logical OR '|' valid with
|
|
the appropriate PNG_INFO_<chunk name> define.
|
|
palette - the palette for the file (PNG_INFO_PLTE)
|
|
num_palette - number of entries in the palette
|
|
gamma - the gamma the file is written at (PNG_INFO_gAMA)
|
|
sig_bit - the number of significant bits (PNG_INFO_sBIT)
|
|
for the gray, red, green, and blue channels, whichever
|
|
are appropriate for the given color type.
|
|
trans_values - transparent pixel for non-paletted images (PNG_INFO_tRNS)
|
|
trans - array of transparent entries for paletted images
|
|
num_trans - number of transparent entries
|
|
hist - histogram of palette (PNG_INFO_hIST)
|
|
mod_time - time image was last modified (PNG_VALID_tIME)
|
|
background - background color (PNG_VALID_bKGD)
|
|
text - text comments in the file.
|
|
num_text - number of comments
|
|
|
|
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
|
|
around 1000 bytes, it is not worth compressing it.
|
|
|
|
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
|
|
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 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 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 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
|
|
instead of your local time. Note that the year number is the full
|
|
year (ie 1996, rather than 96), and that months start with 1.
|
|
|
|
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);
|
|
|
|
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 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.
|
|
|
|
PNG files store RGB pixels packed into 3 bytes. This code tells
|
|
the library to expect input data with 4 bytes per pixel
|
|
|
|
png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);
|
|
|
|
where the 0 is the value that will be put in the 4th byte, and the
|
|
location is either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending
|
|
upon whether the filler byte 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 get the original data if desired.
|
|
|
|
/* Do this before png_write_info() */
|
|
info_ptr->valid |= PNG_INFO_sBIT;
|
|
|
|
/* Set the true bit depth of the image data */
|
|
if (info_ptr->color_type & PNG_COLOR_MASK_COLOR)
|
|
{
|
|
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;
|
|
}
|
|
|
|
If the data is stored in the row buffer in a bit depth other than
|
|
one supported by PNG (ie 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, &(info_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, ie. least significant bits
|
|
first, eg. the way PCs store them):
|
|
|
|
png_set_swap(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(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()ls 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 noticably (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.
|
|
|
|
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:
|
|
|
|
png_bytef *row_pointers[height];
|
|
|
|
You can point to void or char or whatever you use for pixels.
|
|
|
|
If you can'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
|
|
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 6/96 -- PNG Specification
|
|
version 1.0) defined interlacing scheme for PNG files is a
|
|
compilcated interlace scheme, known as 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.
|
|
|
|
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 seven sub-images.
|
|
|
|
If you want libpng to build the sub-images, call this before you start
|
|
writing any rows:
|
|
|
|
number_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_passes times.
|
|
|
|
png_write_rows(png_ptr, row_pointers, number_of_rows);
|
|
|
|
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. If you have written text at
|
|
the beginning and are not writing more at the end, you should set
|
|
info_ptr->num_text = 0, or the text will be written again here.
|
|
|
|
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);
|
|
|
|
You must free any data you allocated for info_ptr, such as comments,
|
|
palette, or histogram, before the call to png_destroy_write_struct();
|
|
|
|
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.
|
|
|
|
All of the memory allocation, input/output, and error handling in
|
|
libpng goes through callbacks which are user setable. The default
|
|
routines are in pngmem.c, pngrio.c, pngwio.c, and pngerror.c respectively.
|
|
To change these functions, call the approprate png_set_???_fn() function.
|
|
|
|
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 than that. If
|
|
your pointers can't access more then 64K at a 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.
|
|
|
|
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
|
|
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
|
|
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);
|
|
|
|
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);
|
|
|
|
Supplying NULL for the read, write, or flush functions sets them back
|
|
to using the default C stream functions. 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 it's caller. Currently, this is handled via
|
|
setjmp() and longjmp(), but you could change this to do things like
|
|
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 via
|
|
fprintf() unless the library is compiled with PNG_NO_STDIO defined. 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 change
|
|
these 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_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.
|
|
|
|
If you need to read or write custom chunks, you will need to get deeper
|
|
into the libpng code, as a mechanism has not yet been supplied for user
|
|
callbacks with custom chunks. 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 similarly. 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. More details can be found in the
|
|
comments inside the code. A way of handling unknown chunks in a generic
|
|
method, potentially via callback functions, would be best.
|
|
|
|
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 may need to change the png_large_malloc() and png_large_free()
|
|
routines in pngmem.c, as these are requred to allocate 64K, although
|
|
there is already support for many of the common DOS compilers. 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.
|
|
|
|
Configuring for DOS:
|
|
|
|
For DOS users which 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
|
|
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
|
|
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 *.
|
|
|
|
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 compliers,
|
|
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/delete
|
|
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, which includes pngconf.h.
|
|
|
|
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), but if
|
|
speed is not critical it is possible to configure it for maximum
|
|
compression (Z_BEST_COMPRESSION = 9) to generate smaller PNG files.
|
|
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:
|
|
|
|
png_set_compression_mem_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).
|
|
|
|
png_set_compression_mem_level(png_ptr, level);
|
|
|
|
If you want to control whether libpng uses filtering or not, you
|
|
can call this function. Filtering is enabled by default for RGB
|
|
and grayscale images (with and without alpha), and for 8-bit
|
|
paletted images, but not for paletted 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 1.0 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 '|' 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.
|
|
|
|
png_set_filter(png_ptr, method, filters);
|
|
|
|
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.
|
|
|
|
png_set_compression_strategy(png_ptr, strategy);
|
|
png_set_compression_window_bits(png_ptr, window_bits);
|
|
png_set_compression_method(png_ptr, method);
|
|
|
|
Except for png_set_filter(), all of these are just controlling zlib,
|
|
so see the zlib documentation (zlib.h and zconf.h) for more information.
|
|
|
|
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 _SUPPORT. If you are
|
|
never going to use an ability, you can change the #define to #undef
|
|
before recompiling libpng and 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 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 which are actually used will be loaded into memory.
|
|
|
|
Changes to Libpng from version 0.88 to version 0.89
|
|
|
|
It should be noted that version 0.89 of libpng is not distributed by
|
|
the original author, Guy Schalnat, but rather Andreas Dilger, although
|
|
all of the copyright messages have been left in Guy's name.
|
|
|
|
The old libpng functions png_read_init(), png_write_init() and
|
|
png_info_init() still exist in the 0.89 version of the library, as
|
|
do png_read_destroy() and png_write_destroy(). 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 which 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 the new library.
|
|
|