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1889 lines
80 KiB
1889 lines
80 KiB
libpng.txt - A description on how to use and modify libpng |
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libpng version 1.0.1 March 9, 1998 |
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Updated and distributed by Glenn Randers-Pehrson |
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<randeg@alumni.rpi.edu> |
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Copyright (c) 1998, Glenn Randers-Pehrson |
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For conditions of distribution and use, see copyright |
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notice in png.h. |
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based on: |
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libpng 1.0 beta 6 version 0.96 May 28, 1997 |
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Updated and distributed by Andreas Dilger |
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Copyright (c) 1996, 1997 Andreas Dilger |
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libpng 1.0 beta 2 - version 0.88 January 26, 1996 |
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For conditions of distribution and use, see copyright |
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notice in png.h. Copyright (c) 1995, 1996 Guy Eric |
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Schalnat, Group 42, Inc. |
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Updated/rewritten per request in the libpng FAQ |
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Copyright (c) 1995 Frank J. T. Wojcik |
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December 18, 1995 && January 20, 1996 |
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Introduction |
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This file describes how to use and modify the PNG reference library |
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(known as libpng) for your own use. There are five sections to this |
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file: introduction, structures, reading, writing, and modification and |
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configuration notes for various special platforms. In addition to this |
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file, example.c is a good starting point for using the library, as |
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it is heavily commented and should include everything most people |
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will need. We assume that libpng is already installed; see the |
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INSTALL file for instructions on how to install libpng. |
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Libpng was written as a companion to the PNG specification, as a way |
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of reducing the amount of time and effort it takes to support the PNG |
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file format in application programs. The PNG specification is available |
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as RFC 2083 <ftp://ftp.uu.net/graphics/png/documents/> and as a |
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W3C Recommendation <http://www.w3.org/TR/REC.png.html>. Some |
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additional chunks are described in the special-purpose public chunks |
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documents at <ftp://ftp.uu.net/graphics/png/documents/>. Other information |
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about PNG can be found at the PNG home page, <http://www.cdrom.com/pub/png/>. |
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Most users will not have to modify the library significantly; advanced |
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users may want to modify it more. All attempts were made to make it as |
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complete as possible, while keeping the code easy to understand. |
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Currently, this library only supports C. Support for other languages |
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is being considered. |
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Libpng has been designed to handle multiple sessions at one time, |
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to be easily modifiable, to be portable to the vast majority of |
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machines (ANSI, K&R, 16-, 32-, and 64-bit) available, and to be easy |
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to use. The ultimate goal of libpng is to promote the acceptance of |
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the PNG file format in whatever way possible. While there is still |
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work to be done (see the TODO file), libpng should cover the |
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majority of the needs of its users. |
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Libpng uses zlib for its compression and decompression of PNG files. |
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The zlib compression utility is a general purpose utility that is |
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useful for more than PNG files, and can be used without libpng. |
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See the documentation delivered with zlib for more details. |
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You can usually find the source files for the zlib utility wherever you |
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find the libpng source files. |
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Libpng is thread safe, provided the threads are using different |
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instances of the structures. Each thread should have its own |
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png_struct and png_info instances, and thus its own image. |
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Libpng does not protect itself against two threads using the |
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same instance of a structure. |
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II. Structures |
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There are two main structures that are important to libpng, png_struct |
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and png_info. 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 libpng function call. |
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The png_info structure is designed to provide information about the |
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PNG file. At one time, the fields of png_info were intended to be |
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directly accessible to the user. However, this tended to cause problems |
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with applications using dynamically loaded libraries, and as a result |
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a set of interface functions for png_info was developed. The fields |
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of png_info are still available for older applications, but it is |
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suggested that applications use the new interfaces if at all possible. |
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The png.h header file is an invaluable reference for programming with libpng. |
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And while I'm on the topic, make sure you include the libpng header file: |
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#include <png.h> |
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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. |
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If you are intending to keep the file pointer open for use in libpng, |
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you must ensure you don't read more than 8 bytes from the beginning |
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of the file, and you also have to make a call to png_set_sig_bytes_read() |
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with the number of bytes you read from the beginning. Libpng will |
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then only check the bytes (if any) that your program didn't read. |
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(*): If you are not using the standard I/O functions, you will need |
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to replace them with custom functions. See the discussion under |
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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, 0, 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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Next, png_struct and png_info need to be allocated and initialized. In |
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order to ensure that the size of these structures is correct even with a |
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dynamically linked libpng, there are functions to initialize and |
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allocate the structures. We also pass the library version, optional |
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pointers to error handling functions, and a pointer to a data struct for |
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use by the error functions, if necessary (the pointer and functions can |
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be NULL if the default error handlers are to be used). See the section |
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on Changes to Libpng below regarding the old initialization functions. |
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png_structp png_ptr = png_create_read_struct |
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(PNG_LIBPNG_VER_STRING, (void *)user_error_ptr, |
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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, |
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(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, |
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(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() are only |
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necessary if you are not using the libpng supplied error handling |
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functions. When libpng encounters an error, it expects to longjmp back |
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to your routine. Therefore, you will need to call setjmp and pass the |
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jmpbuf field of your png_struct. If you read the file from different |
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routines, you will need to update the jmpbuf field every time you enter |
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a new routine that will call a png_ function. |
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See your documentation of setjmp/longjmp for your compiler for more |
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handling in the Customizing Libpng section below for more information on |
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the libpng error handling. If an error occurs, and libpng longjmp's |
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back to your setjmp, you will want to call png_destroy_read_struct() to |
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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, |
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&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. If you wish to handle reading data in another |
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way, you need not call the png_init_io() function, but you must then |
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implement the libpng I/O methods discussed in the Customizing Libpng |
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section below. |
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png_init_io(png_ptr, fp); |
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If you had previously opened the file and read any of the signature from |
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the beginning in order to see if this was a PNG file, you need to let |
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libpng know that there are some bytes missing from the start of the file. |
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png_set_sig_bytes(png_ptr, number); |
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At this point, you can set up a callback function that will be |
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called after each row has been read, which you can use to control |
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a progress meter or the like. It's demonstrated in pngtest.c. |
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You must supply a function |
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void read_row_callback(png_ptr, png_uint_32 row, int pass); |
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{ |
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/* put your code here */ |
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} |
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(You can give it another name that you like instead of "read_row_callback") |
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To inform libpng about your function, use |
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png_set_read_status_fn(png_ptr, read_row_callback); |
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In PNG files, the alpha channel in an image is the level of opacity. |
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If you need the alpha channel in an image to be the level of transparency |
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instead of opacity, you can invert the alpha channel (or the tRNS chunk |
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data) after it's read, so that 0 is fully opaque and 255 (in 8-bit or |
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paletted images) or 65535 (in 16-bit images) is fully transparent, with |
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png_set_invert_alpha(png_ptr); |
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This has to appear here rather than later with the other transformations |
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because the tRNS chunk data must be modified in the case of paletted images. |
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If your image is not a paletted image, the tRNS data (which in such cases |
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represents a single color to be rendered as transparent) won't be changed. |
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Finally, you can write your own transformation function if none of |
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the existing ones meets your needs. This is done by setting a callback |
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with |
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png_set_read_user_transform_fn(png_ptr, |
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read_transform_fn); |
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You must supply the function |
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void read_transform_fn(png_ptr ptr, row_info_ptr |
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row_info, png_bytep data) |
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See pngtest.c for a working example. Your function will be called |
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after all of the other transformations have been processed. |
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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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Functions are used to get the information from the info_ptr: |
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png_get_IHDR(png_ptr, info_ptr, &width, &height, |
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&bit_depth, &color_type, &interlace_type, |
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&compression_type, &filter_type); |
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width - holds the width of the image |
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in pixels (up to 2^31). |
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height - holds the height of the image |
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in pixels (up to 2^31). |
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bit_depth - holds the bit depth of one of the |
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image channels. (valid values are |
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1, 2, 4, 8, 16 and depend also on |
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the color_type. See also |
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significant bits (sBIT) below). |
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color_type - describes which color/alpha channels |
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are present. |
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PNG_COLOR_TYPE_GRAY |
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(bit depths 1, 2, 4, 8, 16) |
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PNG_COLOR_TYPE_GRAY_ALPHA |
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(bit depths 8, 16) |
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PNG_COLOR_TYPE_PALETTE |
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(bit depths 1, 2, 4, 8) |
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PNG_COLOR_TYPE_RGB |
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(bit_depths 8, 16) |
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PNG_COLOR_TYPE_RGB_ALPHA |
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(bit_depths 8, 16) |
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PNG_COLOR_MASK_PALETTE |
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PNG_COLOR_MASK_COLOR |
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PNG_COLOR_MASK_ALPHA |
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filter_type - (must be PNG_FILTER_TYPE_BASE |
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for PNG 1.0) |
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compression_type - (must be PNG_COMPRESSION_TYPE_BASE |
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for PNG 1.0) |
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interlace_type - (PNG_INTERLACE_NONE or |
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PNG_INTERLACE_ADAM7) |
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Any or all of interlace_type, compression_type, of |
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filter_type can be |
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NULL if you are not interested in their values. |
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channels = png_get_channels(png_ptr, info_ptr); |
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channels - number of channels of info for the |
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color type (valid values are 1 (GRAY, |
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PALETTE), 2 (GRAY_ALPHA), 3 (RGB), |
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4 (RGB_ALPHA or RGB + filler byte)) |
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rowbytes = png_get_rowbytes(png_ptr, info_ptr); |
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rowbytes - number of bytes needed to hold a row |
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signature = png_get_signature(png_ptr, info_ptr); |
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signature - holds the signature read from the |
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file (if any). The data is kept in |
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the same offset it would be if the |
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whole signature were read (i.e. if an |
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application had already read in 4 |
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bytes of signature before starting |
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libpng, the remaining 4 bytes would |
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be in signature[4] through signature[7] |
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(see png_set_sig_bytes())). |
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width = png_get_image_width(png_ptr, |
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info_ptr); |
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height = png_get_image_height(png_ptr, |
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info_ptr); |
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bit_depth = png_get_bit_depth(png_ptr, |
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info_ptr); |
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color_type = png_get_color_type(png_ptr, |
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info_ptr); |
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filter_type = png_get_filter_type(png_ptr, |
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info_ptr); |
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compression_type = png_get_compression_type(png_ptr, |
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info_ptr); |
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interlace_type = png_get_interlace_type(png_ptr, |
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info_ptr); |
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These are also important, but their validity depends on whether the chunk |
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has been read. The png_get_valid(png_ptr, info_ptr, PNG_INFO_<chunk>) and |
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png_get_<chunk>(png_ptr, info_ptr, ...) functions return non-zero if the |
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data has been read, or zero if it is missing. The parameters to the |
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png_get_<chunk> are set directly if they are simple data types, or a pointer |
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into the info_ptr is returned for any complex types. |
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png_get_PLTE(png_ptr, info_ptr, &palette, |
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&num_palette); |
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palette - the palette for the file |
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(array of png_color) |
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num_palette - number of entries in the palette |
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png_get_gAMA(png_ptr, info_ptr, &gamma); |
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gamma - the gamma the file is written |
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at (PNG_INFO_gAMA) |
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png_get_sRGB(png_ptr, info_ptr, &srgb_intent); |
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srgb_intent - the rendering intent (PNG_INFO_sRGB) |
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The presence of the sRGB chunk |
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means that the pixel data is in the |
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sRGB color space. This chunk also |
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implies specific values of gAMA and |
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cHRM. |
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png_get_sBIT(png_ptr, info_ptr, &sig_bit); |
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sig_bit - the number of significant bits for |
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(PNG_INFO_sBIT) each of the gray, |
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red, green, and blue channels, |
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whichever are appropriate for the |
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given color type (png_color_16) |
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png_get_tRNS(png_ptr, info_ptr, &trans, &num_trans, |
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&trans_values); |
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trans - array of transparent entries for |
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palette (PNG_INFO_tRNS) |
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trans_values - transparent pixel for non-paletted |
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images (PNG_INFO_tRNS) |
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num_trans - number of transparent entries |
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(PNG_INFO_tRNS) |
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png_get_hIST(png_ptr, info_ptr, &hist); |
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(PNG_INFO_hIST) |
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hist - histogram of palette (array of |
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png_color_16) |
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png_get_tIME(png_ptr, info_ptr, &mod_time); |
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mod_time - time image was last modified |
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(PNG_VALID_tIME) |
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png_get_bKGD(png_ptr, info_ptr, &background); |
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background - background color (PNG_VALID_bKGD) |
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num_text = png_get_text(png_ptr, info_ptr, &text_ptr); |
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text_ptr - array of png_text holding image |
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comments |
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text_ptr[i]->key - keyword for comment. |
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text_ptr[i]->text - text comments for current |
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keyword. |
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text_ptr[i]->compression - type of compression used |
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on "text" PNG_TEXT_COMPRESSION_NONE |
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or PNG_TEXT_COMPRESSION_zTXt |
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num_text - number of comments |
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png_get_oFFs(png_ptr, info_ptr, &offset_x, &offset_y, |
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&unit_type); |
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offset_x - positive offset from the left edge |
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of the screen |
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offset_y - positive offset from the top edge |
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of the screen |
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unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER |
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png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y, |
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&unit_type); |
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res_x - pixels/unit physical resolution in |
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x direction |
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res_y - pixels/unit physical resolution in |
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x direction |
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unit_type - PNG_RESOLUTION_UNKNOWN, |
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PNG_RESOLUTION_METER |
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The data from the pHYs chunk can be retrieved in several convenient |
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forms: |
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res_x = png_get_x_pixels_per_meter(png_ptr, |
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info_ptr) |
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res_y = png_get_y_pixels_per_meter(png_ptr, |
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info_ptr) |
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res_x_and_y = png_get_pixels_per_meter(png_ptr, |
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info_ptr) |
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aspect_ratio = png_get_pixel_aspect_ratio(png_ptr, |
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info_ptr) |
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(Each of these returns 0 [signifying "unknown"] if |
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the data is not present or if res_x is 0; |
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res_x_and_y is 0 if res_x != res_y) |
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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, filler, gray_to_rgb, etc.). |
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See png_read_update_info(), below. |
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A quick word about text_ptr and num_text. PNG stores comments in |
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keyword/text pairs, one pair per chunk, with no limit on the number |
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of text chunks, and a 2^31 byte limit on their size. While there are |
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suggested keywords, there is no requirement to restrict the use to these |
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strings. It is strongly suggested that keywords and text be sensible |
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to humans (that's the point), so don't use abbreviations. Non-printing |
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symbols are not allowed. See the PNG specification for more details. |
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There is also no requirement to have text after the keyword. |
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Keywords should be limited to 79 Latin-1 characters without leading or |
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trailing spaces, but non-consecutive spaces are allowed within the |
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keyword. It is possible to have the same keyword any number of times. |
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The text_ptr is an array of png_text structures, each holding pointer |
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to a keyword and a pointer to a text string. Only the text string may |
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be null. The keyword/text pairs are put into the array in the order |
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that 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 header information, you can set up the library |
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to 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 |
|
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 something with, you should |
|
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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|
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The colors used for the background and transparency values should be |
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supplied in the same format/depth as the current image data. They |
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are stored in the same format/depth as the image data in a bKGD or tRNS |
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chunk, so this is what libpng expects for this data. The colors are |
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transformed to keep in sync with the image data when an application |
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calls the png_read_update_info() routine (see below). |
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|
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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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|
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The following code transforms grayscale images of less than 8 to 8 bits, |
|
changes paletted images to RGB, and adds a full alpha channel if there is |
|
transparency information in a tRNS chunk. This is most useful on |
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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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|
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if (color_type == PNG_COLOR_TYPE_PALETTE && |
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bit_depth <= 8) png_set_expand(png_ptr); |
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|
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if (color_type == PNG_COLOR_TYPE_GRAY && |
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bit_depth < 8) png_set_expand(png_ptr); |
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if (png_get_valid(png_ptr, info_ptr, |
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PNG_INFO_tRNS)) png_set_expand(png_ptr); |
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PNG can have files with 16 bits per channel. If you only can handle |
|
8 bits per channel, this will strip the pixels down to 8 bit. |
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|
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if (bit_depth == 16) |
|
png_set_strip_16(png_ptr); |
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|
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The png_set_background() function tells libpng to composite images |
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with alpha or simple transparency against the supplied background |
|
color. If the PNG file contains a bKGD chunk (PNG_INFO_bKGD valid), |
|
you may use this color, or supply another color more suitable for |
|
the current display (e.g., the background color from a web page). You |
|
need to tell libpng whether the color is in the gamma space of the |
|
display (PNG_BACKGROUND_GAMMA_SCREEN for colors you supply), the file |
|
(PNG_BACKGROUND_GAMMA_FILE for colors from the bKGD chunk), or one |
|
that is neither of these gammas (PNG_BACKGROUND_GAMMA_UNIQUE - I don't |
|
know why anyone would use this, but it's here). |
|
|
|
If, for some reason, you don't need the alpha channel on an image, |
|
and you want to remove it rather than combining it with the background |
|
(but the image author certainly had in mind that you *would* combine |
|
it with the background, so that's what you should probably do): |
|
|
|
if (color_type & PNG_COLOR_MASK_ALPHA) |
|
png_set_strip_alpha(png_ptr); |
|
|
|
PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as |
|
they can, resulting in, for example, 8 pixels per byte for 1 bit |
|
files. This code expands to 1 pixel per byte without changing the |
|
values of the pixels: |
|
|
|
if (bit_depth < 8) |
|
png_set_packing(png_ptr); |
|
|
|
PNG files have possible bit depths of 1, 2, 4, 8, and 16. All pixels |
|
stored in a PNG image have been "scaled" or "shifted" up to the next |
|
higher possible bit depth (e.g. from 5 bits/sample in the range [0,31] to |
|
8 bits/sample in the range [0, 255]). However, it is also possible to |
|
convert the PNG pixel data back to the original bit depth of the image. |
|
This call reduces the pixels back down to the original bit depth: |
|
|
|
png_color_16p sig_bit; |
|
|
|
if (png_get_sBIT(png_ptr, info_ptr, &sig_bit)) |
|
png_set_shift(png_ptr, sig_bit); |
|
|
|
PNG files store 3-color pixels in red, green, blue order. This code |
|
changes the storage of the pixels to blue, green, red: |
|
|
|
if (color_type == PNG_COLOR_TYPE_RGB || |
|
color_type == PNG_COLOR_TYPE_RGB_ALPHA) |
|
png_set_bgr(png_ptr); |
|
|
|
PNG files store RGB pixels packed into 3 bytes. This code expands them |
|
into 4 bytes for windowing systems that need them in this format: |
|
|
|
if (bit_depth == 8 && color_type == |
|
PNG_COLOR_TYPE_RGB) png_set_filler(png_ptr, |
|
filler, PNG_FILLER_BEFORE); |
|
|
|
where "filler" 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. This transformation |
|
does not affect images that already have full alpha channels. |
|
|
|
If you are reading an image with an alpha channel, and you need the |
|
data as ARGB instead of the normal PNG format RGBA: |
|
|
|
if (color_type == PNG_COLOR_TYPE_RGB_ALPHA) |
|
png_set_swap_alpha(png_ptr); |
|
|
|
For some uses, you may want a grayscale image to be represented as |
|
RGB. This code will do that conversion: |
|
|
|
if (color_type == PNG_COLOR_TYPE_GRAY || |
|
color_type == PNG_COLOR_TYPE_GRAY_ALPHA) |
|
png_set_gray_to_rgb(png_ptr); |
|
|
|
If you have a grayscale and you are using png_set_expand() to change to |
|
a higher bit-depth you must indicate if the supplied background gray |
|
is supplied in the original file bit depth (need_expand = 1) or in the |
|
expanded bit depth (need_expand = 0). Similarly, if you are reading |
|
a paletted image, you must indicate if you have supplied the background |
|
as a palette index that needs to be expanded (need_expand = 1). You can |
|
also specify an RGB triplet that isn't in the palette when setting your |
|
background for a paletted image. |
|
|
|
png_color_16 my_background; |
|
png_color_16p image_background; |
|
|
|
if (png_get_bKGD(png_ptr, info_ptr, |
|
&image_background)) |
|
png_set_background(png_ptr, image_background), |
|
PNG_BACKGROUND_GAMMA_FILE, 1, 1.0); |
|
else |
|
png_set_background(png_ptr, &my_background, |
|
PNG_BACKGROUND_GAMMA_SCREEN, 0, 1.0); |
|
|
|
To properly display PNG images on any kind of system, the application needs |
|
to know what the display gamma is. Ideally, the user will know this, and |
|
the application will allow them to set it. One method of allowing the user |
|
to set the display gamma separately for each system is to check for the |
|
DISPLAY_GAMMA and VIEWING_GAMMA environment variables or for a SCREEN_GAMMA |
|
environment variable, which will hopefully be correctly set. |
|
|
|
Note that display_gamma is the gamma of your display, while screen_gamma is |
|
the overall gamma correction required to produce pleasing results, |
|
which depends on the lighting conditions in the surrounding environment. |
|
Screen_gamma is display_gamma/viewing_gamma, where viewing_gamma is |
|
the amount of additional gamma correction needed to compensate for |
|
a (viewing_gamma=1.25) environment. In a dim or brightly lit room, no |
|
compensation other than the display_gamma is needed (viewing_gamma=1.0). |
|
|
|
if (/* We have a user-defined screen |
|
gamma value */) |
|
{ |
|
screen_gamma = user_defined_screen_gamma; |
|
} |
|
/* One way that applications can share the same |
|
screen gamma value */ |
|
else if ((gamma_str = getenv("SCREEN_GAMMA")) |
|
!= NULL) |
|
{ |
|
screen_gamma = atof(gamma_str); |
|
} |
|
/* If we don't have another value */ |
|
else |
|
{ |
|
screen_gamma = 2.2; /* A good guess for a |
|
PC monitor in a bright office or a dim room */ |
|
screen_gamma = 2.0; /* A good guess for a |
|
PC monitor in a dark room */ |
|
screen_gamma = 1.7 or 1.0; /* A good |
|
guess for Mac systems */ |
|
} |
|
|
|
The png_set_gamma() function handles gamma transformations of the data. |
|
Pass both the file gamma and the current screen_gamma. If the file does |
|
not have a gamma value, you can pass one anyway if you have an idea what |
|
it is (usually 0.50 is a good guess for GIF images on PCs). Note |
|
that file gammas are inverted from screen gammas. See the discussions |
|
on gamma in the PNG specification for an excellent description of what |
|
gamma is, and why all applications should support it. It is strongly |
|
recommended that PNG viewers support gamma correction. |
|
|
|
if (png_get_gAMA(png_ptr, info_ptr, &gamma)) |
|
png_set_gamma(png_ptr, screen_gamma, gamma); |
|
else |
|
png_set_gamma(png_ptr, screen_gamma, 0.50); |
|
|
|
If you need to reduce an RGB file to a paletted file, or if a paletted |
|
file has more entries then will fit on your screen, png_set_dither() |
|
will do that. Note that this is a simple match dither that merely |
|
finds the closest color available. This should work fairly well with |
|
optimized palettes, and fairly badly with linear 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 |
|
more intelligent choices when reducing the palette. If there is no |
|
histogram, it may not do as good a job. |
|
|
|
if (color_type & PNG_COLOR_MASK_COLOR) |
|
{ |
|
if (png_get_valid(png_ptr, info_ptr, |
|
PNG_INFO_PLTE)) |
|
{ |
|
png_color_16p histogram; |
|
|
|
png_get_hIST(png_ptr, info_ptr, |
|
&histogram); |
|
png_set_dither(png_ptr, palette, num_palette, |
|
max_screen_colors, histogram, 1); |
|
} |
|
else |
|
{ |
|
png_color std_color_cube[MAX_SCREEN_COLORS] = |
|
{ ... colors ... }; |
|
|
|
png_set_dither(png_ptr, std_color_cube, |
|
MAX_SCREEN_COLORS, MAX_SCREEN_COLORS, |
|
NULL,0); |
|
} |
|
} |
|
|
|
PNG files describe monochrome as black being zero and white being one. |
|
The following code will reverse this (make black be one and white be |
|
zero): |
|
|
|
if (bit_depth == 1 && color_type == PNG_COLOR_GRAY) |
|
png_set_invert_mono(png_ptr); |
|
|
|
PNG files store 16 bit pixels in network byte order (big-endian, |
|
ie. most significant bits first). This code changes the storage to the |
|
other way (little-endian, i.e. least significant bits first, the |
|
way PCs store them): |
|
|
|
if (bit_depth == 16) |
|
png_set_swap(png_ptr); |
|
|
|
If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you |
|
need to change the order the pixels are packed into bytes, you can use: |
|
|
|
if (bit_depth < 8) |
|
png_set_packswap(png_ptr); |
|
|
|
The last thing to handle is interlacing; this is covered in detail below, |
|
but you must call the function here if you want libpng to handle expansion |
|
of the interlaced image. |
|
|
|
number_of_passes = png_set_interlace_handling(png_ptr); |
|
|
|
After setting the transformations, libpng can update your png_info |
|
structure to reflect any transformations you've requested with this |
|
call. This is most useful to update the info structure's rowbytes |
|
field so you can use it to allocate your image memory. This function |
|
will also update your palette with the correct screen_gamma and |
|
background if these have been given with the calls above. |
|
|
|
png_read_update_info(png_ptr, info_ptr); |
|
|
|
After you call png_read_update_info(), you can allocate any |
|
memory you need to hold the image. The row data is simply |
|
raw byte data for all forms of images. As the actual allocation |
|
varies among applications, no example will be given. If you |
|
are allocating one large chunk, you will need to build an |
|
array of pointers to each row, as it will be needed for some |
|
of the functions below. |
|
|
|
After you've allocated memory, you can read the image data. |
|
The simplest way to do this is in one function call. If you are |
|
allocating enough memory to hold the whole image, you can just |
|
call png_read_image() and libpng will read in all the image data |
|
and put it in the memory area supplied. You will need to pass in |
|
an array of pointers to each row. |
|
|
|
This function automatically handles interlacing, so you don't need |
|
to call png_set_interlace_handling() or call this function multiple |
|
times, or any of that other stuff necessary with png_read_rows(). |
|
|
|
png_read_image(png_ptr, row_pointers); |
|
|
|
where row_pointers is: |
|
|
|
png_bytep row_pointers[height]; |
|
|
|
You can point to void or char or whatever you use for pixels. |
|
|
|
If you don't want to read in the whole image at once, you can |
|
use png_read_rows() instead. If there is no interlacing (check |
|
interlace_type == PNG_INTERLACE_NONE), this is simple: |
|
|
|
png_read_rows(png_ptr, row_pointers, NULL, |
|
number_of_rows); |
|
|
|
where row_pointers is the same as in the png_read_image() call. |
|
|
|
If you are doing this just one row at a time, you can do this with |
|
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 current (PNG Specification version 1.0) |
|
interlacing type for PNG is (interlace_type == PNG_INTERLACE_ADAM7) |
|
is a somewhat complicated 2D interlace scheme, known as Adam7, that |
|
breaks down an image into seven smaller images of varying size, based |
|
on an 8x8 grid. |
|
|
|
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 (the "rectangle" method). |
|
This results in a blocky image for the first pass, which gradually |
|
smooths out as more pixels are read. The other method is the "sparkle" |
|
method, where pixels are drawn only in their final locations, with the |
|
rest of the image remaining whatever colors they were initialized to |
|
before the start of the read. The first method usually looks better, |
|
but tends to be slower, as there are more pixels to put in the rows. |
|
|
|
If 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 is a valid image by itself, or they can all 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 column 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 (interlace_type == PNG_INTERLACE_ADAM7) |
|
number_of_passes |
|
= png_set_interlace_handling(png_ptr); |
|
|
|
This will return the number of passes needed. Currently, this |
|
is seven, but may change if another interlace type is added. |
|
This function can be called even if the file is not interlaced, |
|
where it will return one pass. |
|
|
|
If you are not going to display the image after each pass, but are |
|
going to wait until the entire image is read in, use the sparkle |
|
effect. This effect is faster and the end result of either method |
|
is exactly the same. If you are planning on displaying the image |
|
after each pass, the "rectangle" effect is generally considered the |
|
better looking one. |
|
|
|
If you only want the "sparkle" effect, just call png_read_rows() as |
|
normal, with the third parameter NULL. Make sure you make pass over |
|
the image number_of_passes times, and you don't change the data in the |
|
rows between calls. You can change the locations of the data, just |
|
not the data. Each pass only writes the pixels appropriate for that |
|
pass, and assumes the data from previous passes is still valid. |
|
|
|
png_read_rows(png_ptr, row_pointers, NULL, |
|
number_of_rows); |
|
|
|
If you only want the first effect (the rectangles), do the same as |
|
before except pass the row buffer in the third parameter, and leave |
|
the second parameter NULL. |
|
|
|
png_read_rows(png_ptr, NULL, row_pointers, |
|
number_of_rows); |
|
|
|
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 |
|
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 receive 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. If you are also reading, |
|
you won't want to name your read structure and your write structure |
|
both "png_ptr"; you can call them anything you like, such as |
|
"read_ptr" and "write_ptr". Look at pngtest.c, for example. |
|
|
|
png_structp png_ptr = png_create_write_struct |
|
(PNG_LIBPNG_VER_STRING, (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 output code. The default for libpng is to |
|
use the C function fwrite(). If you use this, you will need to pass a |
|
valid FILE * in the function png_init_io(). Be sure that the file is |
|
opened in binary mode. Again, if you wish to handle writing data in |
|
another way, see the discussion on libpng I/O handling in the Customizing |
|
Libpng section below. |
|
|
|
png_init_io(png_ptr, fp); |
|
|
|
At this point, you can set up a callback function that will be |
|
called after each row has been written, which you can use to control |
|
a progress meter or the like. It's demonstrated in pngtest.c. |
|
You must supply a function |
|
|
|
void write_row_callback(png_ptr, png_uint_32 row, int pass); |
|
{ |
|
/* put your code here */ |
|
} |
|
|
|
(You can give it another name that you like instead of "write_row_callback") |
|
|
|
To inform libpng about your function, use |
|
|
|
png_set_write_status_fn(png_ptr, write_row_callback); |
|
|
|
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 |
|
October 1996 PNG specification, version 1.0). 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: |
|
|
|
png_set_IHDR(png_ptr, info_ptr, width, height, |
|
bit_depth, color_type, interlace_type, |
|
compression_type, filter_type) |
|
width - holds the width of the image |
|
in pixels (up to 2^31). |
|
height - holds the height of the image |
|
in pixels (up to 2^31). |
|
bit_depth - holds the bit depth of one of the |
|
image channels. |
|
(valid values are 1, 2, 4, 8, 16 |
|
and depend also on the |
|
color_type. See also significant |
|
bits (sBIT) below). |
|
color_type - describes which color/alpha |
|
channels are present. |
|
PNG_COLOR_TYPE_GRAY |
|
(bit depths 1, 2, 4, 8, 16) |
|
PNG_COLOR_TYPE_GRAY_ALPHA |
|
(bit depths 8, 16) |
|
PNG_COLOR_TYPE_PALETTE |
|
(bit depths 1, 2, 4, 8) |
|
PNG_COLOR_TYPE_RGB |
|
(bit_depths 8, 16) |
|
PNG_COLOR_TYPE_RGB_ALPHA |
|
(bit_depths 8, 16) |
|
|
|
PNG_COLOR_MASK_PALETTE |
|
PNG_COLOR_MASK_COLOR |
|
PNG_COLOR_MASK_ALPHA |
|
|
|
interlace_type - PNG_INTERLACE_NONE or |
|
PNG_INTERLACE_ADAM7 |
|
compression_type - (must be |
|
PNG_COMPRESSION_TYPE_DEFAULT) |
|
filter_type - (must be PNG_FILTER_TYPE_DEFAULT) |
|
|
|
png_set_PLTE(png_ptr, info_ptr, palette, |
|
num_palette); |
|
palette - the palette for the file |
|
(array of png_color) |
|
num_palette - number of entries in the palette |
|
|
|
png_set_gAMA(png_ptr, info_ptr, gamma); |
|
gamma - the gamma the image was created |
|
at (PNG_INFO_gAMA) |
|
|
|
png_set_sRGB(png_ptr, info_ptr, srgb_intent); |
|
srgb_intent - the rendering intent |
|
(PNG_INFO_sRGB) The presence of |
|
the sRGB chunk means that the pixel |
|
data is in the sRGB color space. |
|
This chunk also implies specific |
|
values of gAMA and cHRM. Rendering |
|
intent is the CSS-1 property that |
|
has been defined by the International |
|
Color Consortium |
|
(http://www.color.org). |
|
It can be one of |
|
PNG_SRGB_INTENT_SATURATION, |
|
PNG_SRGB_INTENT_PERCEPTUAL, |
|
PNG_SRGB_INTENT_ABSOLUTE, or |
|
PNG_SRGB_INTENT_RELATIVE. |
|
|
|
|
|
png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr, |
|
srgb_intent); |
|
srgb_intent - the rendering intent |
|
(PNG_INFO_sRGB) The presence of the |
|
sRGB chunk means that the pixel |
|
data is in the sRGB color space. |
|
This function also causes gAMA and |
|
cHRM chunks with the specific values |
|
that are consistent with sRGB to be |
|
written. |
|
|
|
png_set_sBIT(png_ptr, info_ptr, sig_bit); |
|
sig_bit - the number of significant bits for |
|
(PNG_INFO_sBIT) each of the gray, red, |
|
green, and blue channels, whichever are |
|
appropriate for the given color type |
|
(png_color_16) |
|
|
|
png_set_tRNS(png_ptr, info_ptr, trans, num_trans, |
|
trans_values); |
|
trans - array of transparent entries for |
|
palette (PNG_INFO_tRNS) |
|
trans_values - transparent pixel for non-paletted |
|
images (PNG_INFO_tRNS) |
|
num_trans - number of transparent entries |
|
(PNG_INFO_tRNS) |
|
|
|
png_set_hIST(png_ptr, info_ptr, hist); |
|
(PNG_INFO_hIST) |
|
hist - histogram of palette (array of |
|
png_color_16) |
|
|
|
png_set_tIME(png_ptr, info_ptr, mod_time); |
|
mod_time - time image was last modified |
|
(PNG_VALID_tIME) |
|
|
|
png_set_bKGD(png_ptr, info_ptr, background); |
|
background - background color (PNG_VALID_bKGD) |
|
|
|
png_set_text(png_ptr, info_ptr, text_ptr, num_text); |
|
text_ptr - array of png_text holding image |
|
comments |
|
text_ptr[i]->key - keyword for comment. |
|
text_ptr[i]->text - text comments for current |
|
keyword. |
|
text_ptr[i]->compression - type of compression used |
|
on "text" PNG_TEXT_COMPRESSION_NONE or |
|
PNG_TEXT_COMPRESSION_zTXt |
|
num_text - number of comments in text_ptr |
|
|
|
png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y, |
|
unit_type); |
|
offset_x - positive offset from the left |
|
edge of the screen |
|
offset_y - positive offset from the top |
|
edge of the screen |
|
unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER |
|
|
|
png_set_pHYs(png_ptr, info_ptr, res_x, res_y, |
|
unit_type); |
|
res_x - pixels/unit physical resolution |
|
in x direction |
|
res_y - pixels/unit physical resolution |
|
in y direction |
|
unit_type - PNG_RESOLUTION_UNKNOWN, |
|
PNG_RESOLUTION_METER |
|
|
|
In PNG files, the alpha channel in an image is the level of opacity. |
|
If your data is supplied as a level of transparency, you can invert the |
|
alpha channel before you write it, so that 0 is fully transparent and 255 |
|
(in 8-bit or paletted images) or 65535 (in 16-bit images) is fully opaque, |
|
with |
|
|
|
png_set_invert_alpha(png_ptr); |
|
|
|
This must appear here instead of later with the other transformations |
|
because in the case of paletted images the tRNS chunk data has to |
|
be inverted before the tRNS chunk is written. If your image is not a |
|
paletted image, the tRNS data (which in such cases represents a single |
|
color to be rendered as transparent) won't be changed. |
|
|
|
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 PNG_TEXT_COMPRESSION_NONE. |
|
Until text gets around 1000 bytes, it is not worth compressing it. |
|
After the text has been written out to the file, the compression type |
|
is set to PNG_TEXT_COMPRESSION_NONE_WR or PNG_TEXT_COMPRESSION_zTXt_WR, |
|
so that it isn't written out again at the end (in case you are calling |
|
png_write_end() with the same struct. |
|
|
|
The keywords that are given in the PNG Specification are: |
|
|
|
Title Short (one line) title or |
|
caption for image |
|
Author Name of image's creator |
|
Description Description of image (possibly long) |
|
Copyright Copyright notice |
|
Creation Time Time of original image creation |
|
(usually RFC 1123 format, see below) |
|
Software Software used to create the image |
|
Disclaimer Legal disclaimer |
|
Warning Warning of nature of content |
|
Source Device used to create the image |
|
Comment Miscellaneous comment; conversion |
|
from other image format |
|
|
|
The keyword-text pairs work like this. Keywords should be short |
|
simple descriptions of what the comment is about. Some typical |
|
keywords are found in the PNG specification, as is some recommendations |
|
on keywords. You can repeat keywords in a file. You can even write |
|
some text before the image and some after. For example, you may want |
|
to put a description of the image before the image, but leave the |
|
disclaimer until after, so viewers working over modem connections |
|
don't have to wait for the disclaimer to go over the modem before |
|
they start seeing the image. Finally, keywords should be full |
|
words, not abbreviations. Keywords and text are in the ISO 8859-1 |
|
(Latin-1) character set (a superset of regular ASCII) and can not |
|
contain NUL characters, and should not contain control or other |
|
unprintable characters. To make the comments widely readable, stick |
|
with basic ASCII, and avoid machine specific character set extensions |
|
like the IBM-PC character set. The keyword must be present, but |
|
you can leave off the text string on non-compressed pairs. |
|
Compressed pairs must have a text string, as only the text string |
|
is compressed anyway, so the compression would be meaningless. |
|
|
|
PNG supports modification time via the png_time structure. Two |
|
conversion routines are 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 (e.g. 1998, rather than 98 - PNG is year 2000 compliant!), and |
|
that months start with 1. |
|
|
|
If you want to store the time of the original image creation, you should |
|
use a plain tEXt chunk with the "Creation Time" keyword. This is |
|
necessary because the "creation time" of a PNG image is somewhat vague, |
|
depending on whether you mean the PNG file, the time the image was |
|
created in a non-PNG format, a still photo from which the image was |
|
scanned, or possibly the subject matter itself. In order to facilitate |
|
machine-readable dates, it is recommended that the "Creation Time" |
|
tEXt chunk use RFC 1123 format dates (e.g. 22 May 1997 18:07:10 GMT"), |
|
although this isn't a requirement. Unlike the tIME chunk, the |
|
"Creation Time" tEXt chunk is not expected to be automatically changed |
|
by the software. To facilitate the use of RFC 1123 dates, a function |
|
png_convert_to_rfc1123(png_timep) is provided to convert from PNG |
|
time to an RFC 1123 format string. |
|
|
|
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 something with, you should |
|
make sure to only enable a transformation if it will be valid for the |
|
data. For example, don't swap red and blue on grayscale data. |
|
|
|
PNG files store RGB pixels packed into 3 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. |
|
|
|
/* Set the true bit depth of the image data */ |
|
if (color_type & PNG_COLOR_MASK_COLOR) |
|
{ |
|
sig_bit.red = true_bit_depth; |
|
sig_bit.green = true_bit_depth; |
|
sig_bit.blue = true_bit_depth; |
|
} |
|
else |
|
{ |
|
sig_bit.gray = true_bit_depth; |
|
} |
|
if (color_type & PNG_COLOR_MASK_ALPHA) |
|
{ |
|
sig_bit.alpha = true_bit_depth; |
|
} |
|
|
|
png_set_sBIT(png_ptr, info_ptr, &sig_bit); |
|
|
|
If the data is stored in the row buffer in a bit depth other than |
|
one supported by PNG (e.g. 3 bit data in the range 0-7 for a 4-bit PNG), |
|
this will scale the values to appear to be the correct bit depth as |
|
is required by PNG. |
|
|
|
png_set_shift(png_ptr, &sig_bit); |
|
|
|
PNG files store 16 bit pixels in network byte order (big-endian, |
|
ie. most significant bits first). This code would be used if they are |
|
supplied the other way (little-endian, i.e. least significant bits |
|
first, the way PCs store them): |
|
|
|
if (bit_depth > 8) |
|
png_set_swap(png_ptr); |
|
|
|
If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you |
|
need to change the order the pixels are packed into bytes, you can use: |
|
|
|
if (bit_depth < 8) |
|
png_set_packswap(png_ptr); |
|
|
|
PNG files store 3 color pixels in red, green, blue order. This code |
|
would be used if they are supplied as blue, green, red: |
|
|
|
png_set_bgr(png_ptr); |
|
|
|
PNG files describe monochrome as black being zero and white being |
|
one. This code would be used if the pixels are supplied with this reversed |
|
(black being one and white being zero): |
|
|
|
png_set_invert_mono(png_ptr); |
|
|
|
Finally, you can write your own transformation function if none of |
|
the existing ones meets your needs. This is done by setting a callback |
|
with |
|
|
|
png_set_write_user_transform_fn(png_ptr, |
|
write_transform_fn); |
|
|
|
You must supply the function |
|
|
|
void write_transform_fn(png_ptr ptr, row_info_ptr |
|
row_info, png_bytep data) |
|
|
|
See pngtest.c for a working example. Your function will be called |
|
before any of the other transformations have been processed. |
|
|
|
It is possible to have libpng flush any pending output, either manually, |
|
or automatically after a certain number of lines have been written. To |
|
flush the output stream a single time call: |
|
|
|
png_write_flush(png_ptr); |
|
|
|
and to have libpng flush the output stream periodically after a certain |
|
number of scanlines have been written, call: |
|
|
|
png_set_flush(png_ptr, nrows); |
|
|
|
Note that the distance between rows is from the last time png_write_flush() |
|
was called, or the first row of the image if it has never been called. |
|
So if you write 50 lines, and then png_set_flush 25, it will flush the |
|
output on the next scanline, and every 25 lines thereafter, unless |
|
png_write_flush() is called before 25 more lines have been written. |
|
If nrows is too small (less than about 10 lines for a 640 pixel wide |
|
RGB image) the image compression may decrease noticeably (although this |
|
may be acceptable for real-time applications). Infrequent flushing will |
|
only degrade the compression performance by a few percent over images |
|
that do not use flushing. |
|
|
|
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 February 1998 -- PNG Specification |
|
version 1.0, dated October 1996) defined interlacing scheme for PNG files |
|
is the "Adam7" interlace scheme, that breaks down an |
|
image into seven smaller images of varying size. libpng will build |
|
these images for you, or you can do them yourself. If you want to |
|
build them yourself, see the PNG specification for details of which |
|
pixels to write when. |
|
|
|
If you don't want libpng to handle the interlacing details, just |
|
use png_set_interlace_handling() and call png_write_rows() the |
|
correct number of times to write all seven sub-images. |
|
|
|
If you want libpng to build the sub-images, call this before you start |
|
writing any rows: |
|
|
|
number_of_passes = |
|
png_set_interlace_handling(png_ptr); |
|
|
|
This will return the number of passes needed. Currently, this |
|
is seven, but may change if another interlace type is added. |
|
|
|
Then write the complete image number_of_passes times. |
|
|
|
png_write_rows(png_ptr, row_pointers, |
|
number_of_rows); |
|
|
|
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 an appropriately filled png_info pointer. If you are not interested, |
|
you can pass NULL. |
|
|
|
png_write_end(png_ptr, info_ptr); |
|
|
|
When you are done, you can free all memory used by libpng like this: |
|
|
|
png_destroy_write_struct(&png_ptr, &info_ptr); |
|
|
|
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 settable. The default routines are |
|
in pngmem.c, pngrio.c, pngwio.c, and pngerror.c respectively. To change |
|
these functions, call the appropriate png_set_???_fn() function. |
|
|
|
Memory allocation is done through the functions png_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 through png_read() and png_write(), |
|
which currently just call fread() and fwrite(). The FILE * is stored in |
|
png_struct and is initialized via png_init_io(). If you wish to change |
|
the method of I/O, the library supplies callbacks that you can set |
|
through the function png_set_read_fn() and png_set_write_fn() at run |
|
time, instead of calling the png_init_io() function. These functions |
|
also provide a void pointer that can be retrieved via the function |
|
png_get_io_ptr(). For example: |
|
|
|
png_set_read_fn(png_structp 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 its 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_structp png_ptr, |
|
png_const_charp error_msg); |
|
void user_warning_fn(png_structp png_ptr, |
|
png_const_charp warning_msg); |
|
|
|
The motivation behind using setjmp() and longjmp() is the C++ throw and |
|
catch exception handling methods. This makes the code much easier to write, |
|
as there is no need to check every return code of every function call. |
|
However, there are some uncertainties about the status of local variables |
|
after a longjmp, so the user may want to be careful about doing anything after |
|
setjmp returns non-zero besides returning itself. Consult your compiler |
|
documentation for more details. |
|
|
|
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 required 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 accessible. 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 |
|
compilers. Make sure MAXSEG_64K gets defined, USE_FAR_KEYWORD gets |
|
defined, and FAR gets defined to far in pngconf.h, and you should be |
|
all set. Everything in the library (except for zlib's structure) is |
|
expecting far data. You must use the typedefs with the p or pp on |
|
the end for pointers (or at least look at them and be careful). Make |
|
note that the 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 compilers, |
|
you may also have to change the memory allocators (png_malloc, etc.). |
|
|
|
Configuring for compiler xxx: |
|
|
|
All includes for libpng are in pngconf.h. If you need to add/change/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). Tests |
|
have shown that for a large majority of images, compression values in |
|
the range 3-6 compress nearly as well as higher levels, and do so much |
|
faster. For online applications it may be desirable to have maximum speed |
|
(Z_BEST_SPEED = 1). With versions of zlib after v0.99, you can also |
|
specify no compression (Z_NO_COMPRESSION = 0), but this would create |
|
files larger than just storing the raw bitmap. You can specify the |
|
compression level by calling: |
|
|
|
png_set_compression_level(png_ptr, level); |
|
|
|
Another useful one is to reduce the memory level used by the library. |
|
The memory level defaults to 8, but it can be lowered if you are |
|
short on memory (running DOS, for example, where you only have 640K). |
|
|
|
png_set_compression_mem_level(png_ptr, level); |
|
|
|
The other functions are for configuring zlib. They are not recommended |
|
for normal use and may result in writing an invalid PNG file. See |
|
zlib.h for more information on what these mean. |
|
|
|
png_set_compression_strategy(png_ptr, |
|
strategy); |
|
png_set_compression_window_bits(png_ptr, |
|
window_bits); |
|
png_set_compression_method(png_ptr, method); |
|
|
|
Controlling row filtering: |
|
|
|
If you want to control whether libpng uses filtering or not, which |
|
filters are used, and how it goes about picking row filters, you |
|
can call one of these functions. The selection and configuration |
|
of row filters can have a significant impact on the size and |
|
encoding speed and a somewhat lesser impact on the decoding speed |
|
of an image. Filtering is enabled by default for RGB and grayscale |
|
images (with and without alpha), 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. |
|
|
|
filters = PNG_FILTER_NONE | PNG_FILTER_SUB |
|
| PNG_FILTER_UP; |
|
png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE, |
|
filters); |
|
|
|
It is also possible to influence how libpng chooses from among the |
|
available filters. This is done in two ways - by telling it how |
|
important it is to keep the same filter for successive rows, and |
|
by telling it the relative computational costs of the filters. |
|
|
|
double weights[3] = {1.5, 1.3, 1.1}, |
|
costs[PNG_FILTER_VALUE_LAST] = |
|
{1.0, 1.3, 1.3, 1.5, 1.7}; |
|
|
|
png_set_filter_selection(png_ptr, |
|
PNG_FILTER_SELECTION_WEIGHTED, 3, |
|
weights, costs); |
|
|
|
The weights are multiplying factors which indicate to libpng that row |
|
should be the same for successive rows unless another row filter is that |
|
many times better than the previous filter. In the above example, if |
|
the previous 3 filters were SUB, SUB, NONE, the SUB filter could have a |
|
"sum of absolute differences" 1.5 x 1.3 times higher than other filters |
|
and still be chosen, while the NONE filter could have a sum 1.1 times |
|
higher than other filters and still be chosen. Unspecified weights are |
|
taken to be 1.0, and the specified weights should probably be declining |
|
like those above in order to emphasize recent filters over older filters. |
|
|
|
The filter costs specify for each filter type a relative decoding cost |
|
to be considered when selecting row filters. This means that filters |
|
with higher costs are less likely to be chosen over filters with lower |
|
costs, unless their "sum of absolute differences" is that much smaller. |
|
The costs do not necessarily reflect the exact computational speeds of |
|
the various filters, since this would unduly influence the final image |
|
size. |
|
|
|
Note that the numbers above were invented purely for this example and |
|
are given only to help explain the function usage. Little testing has |
|
been done to find optimum values for either the costs or the weights. |
|
|
|
Removing unwanted object code: |
|
|
|
There are a bunch of #define's in pngconf.h that control what parts of |
|
libpng are compiled. All the defines end in _SUPPORTED. If you are |
|
never going to use an ability, you can change the #define to #undef |
|
before recompiling libpng and save yourself code and data space. |
|
You can also turn a number of them off en masse with a compiler directive |
|
that defines PNG_READ[or WRITE]_TRANSFORMS_NOT_SUPPORTED, or |
|
PNG_READ[or WRITE]_ANCILLARY_CHUNKS_NOT_SUPPORTED, or all four, |
|
along with directives to turn on any of the capabilities that you do |
|
want. The PNG_READ[or WRITE]_TRANSFORMS_NOT_SUPPORTED directives disable |
|
the extra transformations but still leave the library fully capable of reading |
|
and writing PNG files with all known public chunks [except for sPLT]. |
|
Use of the PNG_READ[or WRITE]_ANCILLARY_CHUNKS_NOT_SUPPORTED directive |
|
produces a library that is incapable of reading or writing ancillary chunks. |
|
If you are not using the progressive reading capability, you can |
|
turn that off with PNG_PROGRESSIVE_READ_NOT_SUPPORTED (don't confuse |
|
this with the INTERLACING capability, which you'll still have). |
|
|
|
All the reading and writing specific code are in separate files, so the |
|
linker should only grab the files it needs. However, if you want to |
|
make sure, or if you are building a stand alone library, all the |
|
reading files start with pngr and all the writing files start with |
|
pngw. The files that don't match either (like png.c, pngtrans.c, etc.) |
|
are used for both reading and writing, and always need to be included. |
|
The progressive reader is in pngpread.c |
|
|
|
If you are creating or distributing a dynamically linked library (a .so |
|
or DLL file), you should not remove or disable any parts of the library, |
|
as this will cause applications linked with different versions of the |
|
library to fail if they call functions not available in your library. |
|
The size of the library itself should not be an issue, because only |
|
those sections which are actually used will be loaded into memory. |
|
|
|
|
|
Changes to Libpng from version 0.88 |
|
|
|
It should be noted that versions of libpng later than 0.96 are not |
|
distributed by the original libpng author, Guy Schalnat, nor by |
|
Andreas Dilger, who had taken over from Guy during 1996 and 1997, and |
|
distributed versions 0.89 through 0.96, but rather by another member |
|
of the original PNG Group, Glenn Randers-Pehrson. Guy and Andreas are |
|
still alive and well, but they have moved on to other things. |
|
|
|
The old libpng functions png_read_init(), png_write_init(), |
|
png_info_init(), png_read_destroy(), and png_write_destory() have been |
|
moved to PNG_INTERNAL in version 0.95 to discourage their use. 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 applications that try to use |
|
the old method. |
|
|
|
|