LIBPNG

NAME
SYNOPSIS
DESCRIPTION
LIBPNG.TXT
I. Introduction
II. Structures
III. Reading
IV. Writing
V. Modifying/Customizing libpng:
VI. Changes to Libpng from version 0.88
VII. Y2K Compliance in libpng
NOTE
SEE ALSO
AUTHORS
COPYRIGHT NOTICE:

NAME

libpng − Portable Network Graphics (PNG) Reference Library 1.0.5 - October 15, 1999

SYNOPSIS

#include <png.h>

int png_check_sig (png_bytep sig, int num);

void png_chunk_error (png_structp png_ptr, png_const_charp error);

void png_chunk_warning (png_structp png_ptr, png_const_charp message);

void png_convert_from_struct_tm (png_timep ptime, struct tm FAR * ttime);

void png_convert_from_time_t (png_timep ptime, time_t ttime);

png_charp png_convert_to_rfc1123 (png_structp png_ptr, png_timep ptime);

png_infop png_create_info_struct (png_structp png_ptr);

png_structp png_create_read_struct (png_const_charp user_png_ver, voidp error_ptr, png_error_ptr error_fn, png_error_ptr warn_fn);

png_structp png_create_read_struct_2(png_const_charp user_png_ver, png_voidp error_ptr, png_error_ptr error_fn, png_error_ptr warn_fn, png_voidp mem_ptr, png_malloc_ptr malloc_fn, png_free_ptr free_fn);

png_structp png_create_write_struct (png_const_charp user_png_ver, voidp error_ptr, png_error_ptr error_fn, png_error_ptr warn_fn);

png_structp png_create_write_struct_2(png_const_charp user_png_ver, png_voidp error_ptr, png_error_ptr error_fn, png_error_ptr warn_fn, png_voidp mem_ptr, png_malloc_ptr malloc_fn, png_free_ptr free_fn);

int png_debug(int level, png_const_charp message);

int png_debug1(int level, png_const_charp message, p1);

int png_debug2(int level, png_const_charp message, p1, p2);

void png_destroy_info_struct (png_structp png_ptr, png_infopp info_ptr_ptr);

void png_destroy_read_struct (png_structpp png_ptr_ptr, png_infopp info_ptr_ptr, png_infopp end_info_ptr_ptr);

void png_destroy_write_struct (png_structpp png_ptr_ptr, png_infopp info_ptr_ptr);

void png_error (png_structp png_ptr, png_const_charp error);

void png_free (png_structp png_ptr, png_voidp ptr);

void png_free_default(png_structp png_ptr, png_voidp ptr);

png_byte png_get_bit_depth (png_structp png_ptr, png_infop info_ptr);

png_uint_32 png_get_bKGD (png_structp png_ptr, png_infop info_ptr, png_color_16p *background);

png_byte png_get_channels (png_structp png_ptr, png_infop info_ptr);

png_uint_32 png_get_cHRM (png_structp png_ptr, png_infop info_ptr, double *white_x, double *white_y, double *red_x, double *red_y, double *green_x, double *green_y, double *blue_x, double *blue_y);

png_byte png_get_color_type (png_structp png_ptr, png_infop info_ptr);

png_byte png_get_compression_type (png_structp png_ptr, png_infop info_ptr);

png_byte png_get_copyright (png_structp png_ptr);

png_voidp png_get_error_ptr (png_structp png_ptr);

png_byte png_get_filter_type (png_structp png_ptr, png_infop info_ptr);

png_uint_32 png_get_gAMA (png_structp png_ptr, png_infop info_ptr, double *file_gamma);

png_byte png_get_header_version (png_structp png_ptr);

png_uint_32 png_get_hIST (png_structp png_ptr, png_infop info_ptr, png_uint_16p *hist);

png_uint_32 png_get_IHDR (png_structp png_ptr, png_infop info_ptr, png_uint_32 *width, png_uint_32 *height, int *bit_depth, int *color_type, int *interlace_type, int *compression_type, int *filter_type);

png_uint_32 png_get_image_height (png_structp png_ptr, png_infop info_ptr);

png_uint_32 png_get_image_width (png_structp png_ptr, png_infop info_ptr);

png_byte png_get_interlace_type (png_structp png_ptr, png_infop info_ptr);

png_voidp png_get_io_ptr (png_structp png_ptr);

png_voidp png_get_mem_ptr(png_structp png_ptr);

png_uint_32 png_get_oFFs (png_structp png_ptr, png_infop info_ptr, png_uint_32 *offset_x, png_uint_32 *offset_y, int *unit_type);

png_uint_32 png_get_pCAL (png_structp png_ptr, png_infop info_ptr, png_charp *purpose, png_int_32 *X0, png_int_32 *X1, int *type, int *nparams, png_charp *units, png_charpp *params);

png_uint_32 png_get_pHYs (png_structp png_ptr, png_infop info_ptr, png_uint_32 *res_x, png_uint_32 *res_y, int *unit_type);

float png_get_pixel_aspect_ratio (png_structp png_ptr, png_infop info_ptr);

png_uint_32 png_get_pixels_per_meter (png_structp png_ptr, png_infop info_ptr);

png_voidp png_get_progressive_ptr (png_structp png_ptr);

png_uint_32 png_get_PLTE (png_structp png_ptr, png_infop info_ptr, png_colorp *palette, int *num_palette);

png_byte png_get_rgb_to_gray_status (png_structp png_ptr)

png_uint_32 png_get_rowbytes (png_structp png_ptr, png_infop info_ptr);

png_uint_32 png_get_sBIT (png_structp png_ptr, png_infop info_ptr, png_color_8p *sig_bit);

png_bytep png_get_signature (png_structp png_ptr, png_infop info_ptr);

png_uint_32 png_get_sRGB (png_structp png_ptr, png_infop info_ptr, int *intent);

png_uint_32 png_get_text (png_structp png_ptr, png_infop info_ptr, png_textp *text_ptr, int *num_text);

png_uint_32 png_get_tIME (png_structp png_ptr, png_infop info_ptr, png_timep *mod_time);

png_uint_32 png_get_tRNS (png_structp png_ptr, png_infop info_ptr, png_bytep *trans, int *num_trans, png_color_16p *trans_values);

png_voidp png_get_user_transform_ptr (png_structp png_ptr);

png_uint_32 png_get_valid (png_structp png_ptr, png_infop info_ptr, png_uint_32 flag);

png_uint_32 png_get_x_offset_microns (png_structp png_ptr, png_infop info_ptr);

png_uint_32 png_get_x_offset_pixels (png_structp png_ptr, png_infop info_ptr);

png_uint_32 png_get_x_pixels_per_meter (png_structp png_ptr, png_infop info_ptr);

png_uint_32 png_get_y_offset_microns (png_structp png_ptr, png_infop info_ptr);

png_uint_32 png_get_y_offset_pixels (png_structp png_ptr, png_infop info_ptr);

png_uint_32 png_get_y_pixels_per_meter (png_structp png_ptr, png_infop info_ptr);

void png_info_init (png_infop info_ptr);

void png_init_io (png_structp png_ptr, FILE *fp);

png_voidp png_malloc (png_structp png_ptr, png_uint_32 size);

png_voidp png_malloc_default(png_structp png_ptr, png_uint_32 size);

voidp png_memcpy (png_voidp s1, png_voidp s2, png_size_t size);

png_voidp png_memcpy_check (png_structp png_ptr, png_voidp s1, png_voidp s2, png_uint_32 size);

voidp png_memset (png_voidp s1, int value, png_size_t size);

png_voidp png_memset_check (png_structp png_ptr, png_voidp s1, int value, png_uint_32 size);

void png_permit_empty_plte (png_structp png_ptr, int empty_plte_permitted);

void png_process_data (png_structp png_ptr, png_infop info_ptr, png_bytep buffer, png_size_t buffer_size);

void png_progressive_combine_row (png_structp png_ptr, png_bytep old_row, png_bytep new_row);

void png_read_destroy (png_structp png_ptr, png_infop info_ptr, png_infop end_info_ptr);

void png_read_end (png_structp png_ptr, png_infop info_ptr);

void png_read_image (png_structp png_ptr, png_bytepp image);

void png_read_info (png_structp png_ptr, png_infop info_ptr);

void png_read_row (png_structp png_ptr, png_bytep row, png_bytep display_row);

void png_read_rows (png_structp png_ptr, png_bytepp row, png_bytepp display_row, png_uint_32 num_rows);

void png_read_update_info (png_structp png_ptr, png_infop info_ptr);

void png_set_background (png_structp png_ptr, png_color_16p background_color, int background_gamma_code, int need_expand, double background_gamma);

void png_set_bgr (png_structp png_ptr);

void png_set_bKGD (png_structp png_ptr, png_infop info_ptr, png_color_16p background);

void png_set_cHRM (png_structp png_ptr, png_infop info_ptr, double white_x, double white_y, double red_x, double red_y, double green_x, double green_y, double blue_x, double blue_y);

void png_set_compression_level (png_structp png_ptr, int level);

void png_set_compression_mem_level (png_structp png_ptr, int mem_level);

void png_set_compression_method (png_structp png_ptr, int method);

void png_set_compression_strategy (png_structp png_ptr, int strategy);

void png_set_compression_window_bits (png_structp png_ptr, int window_bits);

void png_set_crc_action (png_structp png_ptr, int crit_action, int ancil_action);

void png_set_dither (png_structp png_ptr, png_colorp palette, int num_palette, int maximum_colors, png_uint_16p histogram, int full_dither);

void png_set_error_fn (png_structp png_ptr, png_voidp error_ptr, png_error_ptr error_fn, png_error_ptr warning_fn);

void png_set_expand (png_structp png_ptr);

void png_set_filler (png_structp png_ptr, png_uint_32 filler, int flags);

void png_set_filter (png_structp png_ptr, int method, int filters);

void png_set_filter_heuristics (png_structp png_ptr, int heuristic_method, int num_weights, png_doublep filter_weights, png_doublep filter_costs);

void png_set_flush (png_structp png_ptr, int nrows);

void png_set_gamma (png_structp png_ptr, double screen_gamma, double default_file_gamma);

void png_set_gAMA (png_structp png_ptr, png_infop info_ptr, double file_gamma);

void png_set_gray_1_2_4_to_8(png_structp png_ptr);

void png_set_gray_to_rgb (png_structp png_ptr);

void png_set_hIST (png_structp png_ptr, png_infop info_ptr, png_uint_16p hist);

int png_set_interlace_handling (png_structp png_ptr);

void png_set_invert_alpha (png_structp png_ptr);

void png_set_invert_mono (png_structp png_ptr);

void png_set_IHDR (png_structp png_ptr, png_infop info_ptr, png_uint_32 width, png_uint_32 height, int bit_depth, int color_type, int interlace_type, int compression_type, int filter_type);

void png_set_mem_fn(png_structp png_ptr, png_voidp mem_ptr, png_malloc_ptr malloc_fn, png_free_ptr free_fn);

void png_set_oFFs (png_structp png_ptr, png_infop info_ptr, png_uint_32 offset_x, png_uint_32 offset_y, int unit_type);

void png_set_packing (png_structp png_ptr);

void png_set_packswap (png_structp png_ptr);

void png_set_palette_to_rgb(png_structp png_ptr);

void png_set_pCAL (png_structp png_ptr, png_infop info_ptr, png_charp purpose, png_int_32 X0, png_int_32 X1, int type, int nparams, png_charp units, png_charpp params);

void png_set_pHYs (png_structp png_ptr, png_infop info_ptr, png_uint_32 res_x, png_uint_32 res_y, int unit_type);

void png_set_progressive_read_fn (png_structp png_ptr, png_voidp progressive_ptr, png_progressive_info_ptr info_fn, png_progressive_row_ptr row_fn, png_progressive_end_ptr end_fn);

void png_set_PLTE (png_structp png_ptr, png_infop info_ptr, png_colorp palette, int num_palette);

void png_set_read_fn (png_structp png_ptr, png_voidp io_ptr, png_rw_ptr read_data_fn);

void png_set_read_status_fn (png_structp png_ptr, png_read_status_ptr read_row_fn);

void png_set_read_user_transform_fn (png_structp png_ptr, png_user_transform_ptr read_user_transform_fn);

void png_set_rgb_to_gray (png_structp png_ptr, int error_action);

void png_set_sBIT (png_structp png_ptr, png_infop info_ptr, png_color_8p sig_bit);

void png_set_shift (png_structp png_ptr, png_color_8p true_bits);

void png_set_sig_bytes (png_structp png_ptr, int num_bytes);

void png_set_sRGB (png_structp png_ptr, png_infop info_ptr, int intent);

void png_set_sRGB_gAMA_and_cHRM (png_structp png_ptr, png_infop info_ptr, int intent);

void png_set_strip_16 (png_structp png_ptr);

void png_set_strip_alpha (png_structp png_ptr);

void png_set_swap (png_structp png_ptr);

void png_set_swap_alpha (png_structp png_ptr);

void png_set_text (png_structp png_ptr, png_infop info_ptr, png_textp text_ptr, int num_text);

void png_set_tIME (png_structp png_ptr, png_infop info_ptr, png_timep mod_time);

void png_set_tRNS (png_structp png_ptr, png_infop info_ptr, png_bytep trans, int num_trans, png_color_16p trans_values);

void png_set_tRNS_to_alpha(png_structp png_ptr);

void png_set_user_transform_info (png_structp png_ptr, png_voidp user_transform_ptr, int user_transform_depth, int user_transform_channels);

void png_set_write_fn (png_structp png_ptr, png_voidp io_ptr, png_rw_ptr write_data_fn, png_flush_ptr output_flush_fn);

void png_set_write_status_fn (png_structp png_ptr, png_write_status_ptr write_row_fn);

void png_set_write_user_transform_fn (png_structp png_ptr, png_user_transform_ptr write_user_transform_fn);

int png_sig_cmp (png_bytep sig, png_size_t start, png_size_t num_to_check);

void png_start_read_image (png_structp png_ptr);

void png_warning (png_structp png_ptr, png_const_charp message);

void png_write_chunk (png_structp png_ptr, png_bytep chunk_name, png_bytep data, png_size_t length);

void png_write_chunk_data (png_structp png_ptr, png_bytep data, png_size_t length);

void png_write_chunk_end (png_structp png_ptr);

void png_write_chunk_start (png_structp png_ptr, png_bytep chunk_name, png_uint_32 length);

void png_write_destroy (png_structp png_ptr);

void png_write_destroy_info (png_infop info_ptr);

void png_write_end (png_structp png_ptr, png_infop info_ptr);

void png_write_flush (png_structp png_ptr);

void png_write_image (png_structp png_ptr, png_bytepp image);

void png_write_info (png_structp png_ptr, png_infop info_ptr);

void png_write_row (png_structp png_ptr, png_bytep row);

void png_write_rows (png_structp png_ptr, png_bytepp row, png_uint_32 num_rows);

DESCRIPTION

The libpng library supports encoding, decoding, and various manipulations of the Portable Network Graphics (PNG) format image files. It uses the zlib(3) compression library. Following is a copy of the libpng.txt file that accompanies libpng.

LIBPNG.TXT

libpng.txt - A description on how to use and modify libpng

libpng version 1.0.5 - October 15, 1999 Updated and distributed by Glenn Randers-Pehrson <randeg@alum.rpi.edu> Copyright (c) 1998, 1999 Glenn Randers-Pehrson For conditions of distribution and use, see copyright notice in png.h.

based on:

libpng 1.0 beta 6 version 0.96 May 28, 1997 Updated and distributed by Andreas Dilger Copyright (c) 1996, 1997 Andreas Dilger

libpng 1.0 beta 2 - version 0.88 January 26, 1996 For conditions of distribution and use, see copyright notice in png.h. Copyright (c) 1995, 1996 Guy Eric Schalnat, Group 42, Inc.

Updated/rewritten per request in the libpng FAQ Copyright (c) 1995 Frank J. T. Wojcik December 18, 1995 && January 20, 1996

*

I. Introduction

This file describes how to use and modify the PNG reference library (known as libpng) for your own use. There are five sections to this file: introduction, structures, reading, writing, and modification and configuration notes for various special platforms. In addition to this file, example.c is a good starting point for using the library, as it is heavily commented and should include everything most people will need. We assume that libpng is already installed; see the INSTALL file for instructions on how to install libpng.

Libpng was written as a companion to the PNG specification, as a way of reducing the amount of time and effort it takes to support the PNG file format in application programs.

The PNG-1.2 specification is available at <http://www.cdrom.com/png>.

The PNG-1.0 specification is available as RFC 2083 <ftp://ftp.uu.net/graphics/png/documents/> and as a W3C Recommendation <http://www.w3.org/TR/REC.png.html>. Some additional chunks are described in the special-purpose public chunks documents at <ftp://ftp.uu.net/graphics/png/documents/>.

Other information about PNG, and the latest version of libpng, can be found at the PNG home page, <http://www.cdrom.com/pub/png/>.

Most users will not have to modify the library significantly; advanced users may want to modify it more. All attempts were made to make it as complete as possible, while keeping the code easy to understand. Currently, this library only supports C. Support for other languages is being considered.

Libpng has been designed to handle multiple sessions at one time, to be easily modifiable, to be portable to the vast majority of machines (ANSI, K&R, 16-, 32-, and 64-bit) available, and to be easy to use. The ultimate goal of libpng is to promote the acceptance of the PNG file format in whatever way possible. While there is still work to be done (see the TODO file), libpng should cover the majority of the needs of its users.

Libpng uses zlib for its compression and decompression of PNG files. Further information about zlib, and the latest version of zlib, can be found at the zlib home page, <http://www.cdrom.com/pub/infozip/zlib/>. The zlib compression utility is a general purpose utility that is useful for more than PNG files, and can be used without libpng. See the documentation delivered with zlib for more details. You can usually find the source files for the zlib utility wherever you find the libpng source files.

Libpng is thread safe, provided the threads are using different instances of the structures. Each thread should have its own png_struct and png_info instances, and thus its own image. Libpng does not protect itself against two threads using the same instance of a structure.

II. Structures

There are two main structures that are important to libpng, png_struct and png_info. The first, png_struct, is an internal structure that will not, for the most part, be used by a user except as the first variable passed to every libpng function call.

The png_info structure is designed to provide information about the PNG file. At one time, the fields of png_info were intended to be directly accessible to the user. However, this tended to cause problems with applications using dynamically loaded libraries, and as a result a set of interface functions for png_info was developed. The fields of png_info are still available for older applications, but it is suggested that applications use the new interfaces if at all possible.

The png.h header file is an invaluable reference for programming with libpng. And while I’m on the topic, make sure you include the libpng header file:

#include <png.h>

III. Reading

Reading PNG files:

We’ll now walk you through the possible functions to call when reading in a PNG file, briefly explaining the syntax and purpose of each one. See example.c and png.h for more detail. While Progressive reading is covered in the next section, you will still need some of the functions discussed in this section to read a PNG file.

You will want to do the I/O initialization(*) before you get into libpng, so if it doesn’t work, you don’t have much to undo. Of course, you will also want to insure that you are, in fact, dealing with a PNG file. Libpng provides a simple check to see if a file is a PNG file. To use it, pass in the first 1 to 8 bytes of the file, and it will return true or false (1 or 0) depending on whether the bytes could be part of a PNG file. Of course, the more bytes you pass in, the greater the accuracy of the prediction.

If you are intending to keep the file pointer open for use in libpng, you must ensure you don’t read more than 8 bytes from the beginning of the file, and you also have to make a call to png_set_sig_bytes_read() with the number of bytes you read from the beginning. Libpng will then only check the bytes (if any) that your program didn’t read.

(*): If you are not using the standard I/O functions, you will need to replace them with custom functions. See the discussion under Customizing libpng.

FILE *fp = fopen(file_name, "rb"); if (!fp) { return; } fread(header, 1, number, fp); is_png = !png_sig_cmp(header, 0, number); if (!is_png) { return; }

Next, png_struct and png_info need to be allocated and initialized. In order to ensure that the size of these structures is correct even with a dynamically linked libpng, there are functions to initialize and allocate the structures. We also pass the library version, optional pointers to error handling functions, and a pointer to a data struct for use by the error functions, if necessary (the pointer and functions can be NULL if the default error handlers are to be used). See the section on Changes to Libpng below regarding the old initialization functions.

png_structp png_ptr = png_create_read_struct (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, user_error_fn, user_warning_fn); if (!png_ptr) return;

png_infop info_ptr = png_create_info_struct(png_ptr); if (!info_ptr) { png_destroy_read_struct(&png_ptr, (png_infopp)NULL, (png_infopp)NULL); return; }

png_infop end_info = png_create_info_struct(png_ptr); if (!end_info) { png_destroy_read_struct(&png_ptr, &info_ptr, (png_infopp)NULL); return; }

If you want to use your own memory allocation routines, define PNG_USER_MEM_SUPPORTED and use png_create_read_struct_2() instead of png_create_read_struct():

png_structp png_ptr = png_create_read_struct_2 (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, user_error_fn, user_warning_fn, (png_voidp) user_mem_ptr, user_malloc_fn, user_free_fn);

The error handling routines passed to png_create_read_struct() and the memory alloc/free routines passed to png_create_struct_2() are only necessary if you are not using the libpng supplied error handling and memory alloc/free functions.

When libpng encounters an error, it expects to longjmp back to your routine. Therefore, you will need to call setjmp and pass your png_ptr->jmpbuf. If you read 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 handling in the Customizing Libpng section below for more information on the libpng error handling. If an error occurs, and libpng longjmp’s back to your setjmp, you will want to call png_destroy_read_struct() to free any memory.

if (setjmp(png_ptr->jmpbuf)) { png_destroy_read_struct(&png_ptr, &info_ptr, &end_info); fclose(fp); return; }

Now you need to set up the input code. The default for libpng is to use the C function fread(). If you use this, you will need to pass a valid FILE * in the function png_init_io(). Be sure that the file is opened in binary mode. If you wish to handle reading data in another way, you need not call the png_init_io() function, but you must then implement the libpng I/O methods discussed in the Customizing Libpng section below.

png_init_io(png_ptr, fp);

If you had previously opened the file and read any of the signature from the beginning in order to see if this was a PNG file, you need to let libpng know that there are some bytes missing from the start of the file.

png_set_sig_bytes(png_ptr, number);

At this point, you can set up a callback function that will be called after each row has been read, which you can use to control a progress meter or the like. It’s demonstrated in pngtest.c. You must supply a function

void read_row_callback(png_ptr, png_uint_32 row, int pass); { /* put your code here */ }

(You can give it another name that you like instead of "read_row_callback")

To inform libpng about your function, use

png_set_read_status_fn(png_ptr, read_row_callback);

In PNG files, the alpha channel in an image is the level of opacity. If you need the alpha channel in an image to be the level of transparency instead of opacity, you can invert the alpha channel (or the tRNS chunk data) after it’s read, so that 0 is fully opaque and 255 (in 8-bit or paletted images) or 65535 (in 16-bit images) is fully transparent, with

png_set_invert_alpha(png_ptr);

This has to appear here rather than later with the other transformations because the tRNS chunk data must be modified in the case of paletted images. 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.

Finally, you can write your own transformation function if none of the existing ones meets your needs. This is done by setting a callback with

png_set_read_user_transform_fn(png_ptr, read_transform_fn);

You must supply the function

void read_transform_fn(png_ptr ptr, row_info_ptr row_info, png_bytep data)

See pngtest.c for a working example. Your function will be called after all of the other transformations have been processed.

You can also set up a pointer to a user structure for use by your callback function, and you can inform libpng that your transform function will change the number of channels or bit depth with the function

png_set_user_transform_info(png_ptr, user_ptr, user_depth, user_channels);

You can retrieve the pointer via the function png_get_user_transform_ptr(). For example:

voidp read_user_transform_ptr = png_get_user_transform_ptr(png_ptr);

You are now ready to read all the file information up to the actual image data. You do this with a call to png_read_info().

png_read_info(png_ptr, info_ptr);

Functions are used to get the information from the info_ptr:

png_get_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

filter_type - (must be PNG_FILTER_TYPE_BASE for PNG 1.0) compression_type - (must be PNG_COMPRESSION_TYPE_BASE for PNG 1.0) interlace_type - (PNG_INTERLACE_NONE or PNG_INTERLACE_ADAM7) Any or all of interlace_type, compression_type, of filter_type can be NULL if you are not interested in their values.

channels = png_get_channels(png_ptr, info_ptr); channels - number of channels of info for the color type (valid values are 1 (GRAY, PALETTE), 2 (GRAY_ALPHA), 3 (RGB), 4 (RGB_ALPHA or RGB + filler byte)) rowbytes = png_get_rowbytes(png_ptr, info_ptr); rowbytes - number of bytes needed to hold a row

signature = png_get_signature(png_ptr, info_ptr); signature - holds the signature read from the file (if any). The data is kept in the same offset it would be if the whole signature were read (i.e. if an application had already read in 4 bytes of signature before starting libpng, the remaining 4 bytes would be in signature[4] through signature[7] (see png_set_sig_bytes())).

width = png_get_image_width(png_ptr, info_ptr); height = png_get_image_height(png_ptr, info_ptr); bit_depth = png_get_bit_depth(png_ptr, info_ptr); color_type = png_get_color_type(png_ptr, info_ptr); filter_type = png_get_filter_type(png_ptr, info_ptr); compression_type = png_get_compression_type(png_ptr, info_ptr); interlace_type = png_get_interlace_type(png_ptr, info_ptr);

These are also important, but their validity depends on whether the chunk has been read. The png_get_valid(png_ptr, info_ptr, PNG_INFO_<chunk>) and png_get_<chunk>(png_ptr, info_ptr, ...) functions return non-zero if the data has been read, or zero if it is missing. The parameters to the png_get_<chunk> are set directly if they are simple data types, or a pointer into the info_ptr is returned for any complex types.

png_get_PLTE(png_ptr, info_ptr, &palette, &num_palette); palette - the palette for the file (array of png_color) num_palette - number of entries in the palette

png_get_gAMA(png_ptr, info_ptr, &gamma); gamma - the gamma the file is written at (PNG_INFO_gAMA)

png_get_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.

png_get_sBIT(png_ptr, info_ptr, &sig_bit); sig_bit - the number of significant bits for (PNG_INFO_sBIT) each of the gray, red, green, and blue channels, whichever are appropriate for the given color type (png_color_16)

png_get_tRNS(png_ptr, info_ptr, &trans, &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_get_hIST(png_ptr, info_ptr, &hist); (PNG_INFO_hIST) hist - histogram of palette (array of png_color_16)

png_get_tIME(png_ptr, info_ptr, &mod_time); mod_time - time image was last modified (PNG_VALID_tIME)

png_get_bKGD(png_ptr, info_ptr, &background); background - background color (PNG_VALID_bKGD)

num_text = png_get_text(png_ptr, info_ptr, &text_ptr); 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

png_get_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_get_pHYs(png_ptr, info_ptr, &res_x, &res_y, &unit_type); res_x - pixels/unit physical resolution in x direction res_y - pixels/unit physical resolution in x direction unit_type - PNG_RESOLUTION_UNKNOWN, PNG_RESOLUTION_METER

The data from the pHYs chunk can be retrieved in several convenient forms:

res_x = png_get_x_pixels_per_meter(png_ptr, info_ptr) res_y = png_get_y_pixels_per_meter(png_ptr, info_ptr) res_x_and_y = png_get_pixels_per_meter(png_ptr, info_ptr) aspect_ratio = png_get_pixel_aspect_ratio(png_ptr, info_ptr)

(Each of these returns 0 [signifying "unknown"] if the data is not present or if res_x is 0; res_x_and_y is 0 if res_x != res_y)

For more information, see the png_info definition in png.h and the PNG specification for chunk contents. Be careful with trusting rowbytes, as some of the transformations could increase the space needed to hold a row (expand, filler, gray_to_rgb, etc.). See png_read_update_info(), below.

A quick word about text_ptr and num_text. PNG stores comments in keyword/text pairs, one pair per chunk, with no limit on the number of text chunks, and a 2^31 byte limit on their size. While there are suggested keywords, there is no requirement to restrict the use to these strings. It is strongly suggested that keywords and text be sensible to humans (that’s the point), so don’t use abbreviations. Non-printing symbols are not allowed. See the PNG specification for more details. There is also no requirement to have text after the keyword.

Keywords should be limited to 79 Latin-1 characters without leading or trailing spaces, but non-consecutive spaces are allowed within the keyword. It is possible to have the same keyword any number of times. The text_ptr is an array of png_text structures, each holding pointer to a keyword and a pointer to a text string. Only the text string may be null. The keyword/text pairs are put into the array in the order that they are received. However, some or all of the text chunks may be after the image, so, to make sure you have read all the text chunks, don’t mess with these until after you read the stuff after the image. This will be mentioned again below in the discussion that goes with png_read_end().

After you’ve read the header information, you can set up the library to handle any special transformations of the image data. The various ways to transform the data will be described in the order that they should occur. This is important, as some of these change the color type and/or bit depth of the data, and some others only work on certain color types and bit depths. 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.

The colors used for the background and transparency values should be supplied in the same format/depth as the current image data. They are stored in the same format/depth as the image data in a bKGD or tRNS chunk, so this is what libpng expects for this data. The colors are transformed to keep in sync with the image data when an application calls the png_read_update_info() routine (see below).

Data will be decoded into the supplied row buffers packed into bytes unless the library has been told to transform it into another format. For example, 4 bit/pixel paletted or grayscale data will be returned 2 pixels/byte with the leftmost pixel in the high-order bits of the byte, unless png_set_packing() is called. 8-bit RGB data will be stored in RGB RGB RGB format unless png_set_filler() is called to insert filler bytes, either before or after each RGB triplet. 16-bit RGB data will be returned RRGGBB RRGGBB, with the most significant byte of the color value first, unless png_set_strip_16() is called to transform it to regular RGB RGB triplets, or png_set_filler() is called to insert filler bytes, either before or after each RRGGBB triplet. Similarly, 8-bit or 16-bit grayscale data can be modified with png_set_filler() or png_set_strip_16().

The following code transforms grayscale images of less than 8 to 8 bits, changes paletted images to RGB, and adds a full alpha channel if there is transparency information in a tRNS chunk. This is most useful on grayscale images with bit depths of 2 or 4 or if there is a multiple-image viewing application that wishes to treat all images in the same way.

if (color_type == PNG_COLOR_TYPE_PALETTE && bit_depth <= 8) png_set_palette_to_rgb(png_ptr);

if (color_type == PNG_COLOR_TYPE_GRAY && bit_depth < 8) png_set_gray_1_2_4_to_8(png_ptr);

if (png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS)) png_set_tRNS_to_alpha(png_ptr);

These three functions are actually aliases for png_set_expand(), added in libpng version 1.0.4, with the function names expanded to improve code readability. In some future version they may actually do different things.

PNG can have files with 16 bits per channel. If you only can handle 8 bits per channel, this will strip the pixels down to 8 bit.

if (bit_depth == 16) png_set_strip_16(png_ptr);

The png_set_background() function tells libpng to composite images 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 8 or 16-bit number to fill with, and the location is either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether you want the filler before the RGB or after. This transformation does not affect images that already have full alpha channels.

If you are reading an image with an alpha channel, and you need the data as ARGB instead of the normal PNG format RGBA:

if (color_type == PNG_COLOR_TYPE_RGB_ALPHA) png_set_swap_alpha(png_ptr);

For some uses, you may want a grayscale image to be represented as RGB. This code will do that conversion:

if (color_type == PNG_COLOR_TYPE_GRAY || color_type == PNG_COLOR_TYPE_GRAY_ALPHA) png_set_gray_to_rgb(png_ptr);

Conversely, you can convert an RGB or RGBA image to grayscale or grayscale with alpha. This is intended for conversion of images that really are gray (red == green == blue), so the function simply strips out the red and blue channels, leaving the green channel in the gray position.

if (color_type == PNG_COLOR_TYPE_RGB || color_type == PNG_COLOR_TYPE_RGB_ALPHA) png_set_rgb_to_gray(png_ptr, error_action, double red_weight, double green_weight);

error_action = 1: silently do the conversion error_action = 2: issue a warning if the original image has any pixel where red != green or red != blue error_action = 3: issue an error and abort the conversion if the original image has any pixel where red != green or red != blue

red_weight: weight of red component (NULL -> default 54/256) green_weight: weight of green component (NULL -> default 183/256)

If you have set error_action = 1 or 2, you can later check whether the image really was gray, after processing the image rows, with the png_get_rgb_to_gray_status(png_ptr) function. It will return a png_byte that is zero if the image was gray or 1 if there were any non-gray pixels. bKGD and sBIT data will be silently converted to grayscale, using the green channel data, regardless of the error_action setting.

With 0.0<=red_weight+green_weight<=1.0, the normalized graylevel is computed:

int rw = red_weight * 256; int gw = green_weight * 256; int bw = 256 - (rw + gw); gray = (rw*red + gw*green + bw*blue)/256;

The default values approximate those recommended in the Charles Poynton’s Color FAQ, <http://www.inforamp.net/~poynton/> Copyright (c) 1998-01-04 Charles Poynton poynton@inforamp.net

Y = 0.212671 * R + 0.715160 * G + 0.072169 * B

Libpng approximates this with

Y = 0.211 * R + 0.715 * G + 0.074 * B

which can be expressed with integers as

Y = (54 * R + 183 * G + 19 * B)/256

The calculation is done in a linear colorspace, if the image gamma is known.

If you have a grayscale and you are using png_set_expand_depth() or png_set_expand() to change to a higher bit-depth, you must either supply the background color as a gray value at the original file bit-depth (need_expand = 1) or else supply the background color as an RGB triplet at the final, expanded bit depth (need_expand = 0). Similarly, if you are reading a paletted image, you must either supply the background color as a palette index (need_expand = 1) or as an RGB triplet that may or may not be in the palette (need_expand = 0).

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 a SCREEN_GAMMA or DISPLAY_GAMMA environment variable, which will hopefully be correctly set.

Note that display_gamma is the overall gamma correction required to produce pleasing results, which depends on the lighting conditions in the surrounding environment. In a dim or brightly lit room, no compensation other than the physical gamma exponent of the monitor is needed, while in a dark room a slightly smaller exponent is better.

double gamma, screen_gamma;

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 = (double)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 0.45455 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, 0.45455);

If you need to reduce an RGB file to a paletted file, or if a paletted file has more entries then will fit on your screen, png_set_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 1.2) 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, (png_voidp)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 NULL parameters. Even when all three functions are NULL, you need to call png_set_progressive_read_fn(). You can use any struct as the user_ptr (cast to a void pointer for the function call), and retrieve the pointer from inside the callbacks using the function

png_get_progressive_ptr(png_ptr);

which will return a void pointer, which you have to cast appropriately. */ png_set_progressive_read_fn(png_ptr, (void *)user_ptr, info_callback, row_callback, end_callback);

return 0; }

/* A code fragment that you call as you receive blocks of data */ int process_data(png_bytep buffer, png_uint_32 length) { if (setjmp(png_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_read_fn() above) when enough data has been supplied so all of the header has been read. */ void info_callback(png_structp png_ptr, png_infop info) { /* Do any setup here, including setting any of the transformations mentioned in the Reading PNG files section. For now, you _must_ call either png_start_read_image() or png_read_update_info() after all the transformations are set (even if you don’t set any). You may start getting rows before png_process_data() returns, so this is your last chance to prepare for that. */ }

/* This 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 previously for the row. Note that the first pass (pass == 0, really) will completely cover the old row, so the rows do not have to be initialized. After the first pass (and only for interlaced images), you will have to pass the current row, and the function will combine the old row and the new row. */ }

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, (png_voidp)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; }

If you want to use your own memory allocation routines, define PNG_USER_MEM_SUPPORTED and use png_create_write_struct_2() instead of png_create_read_struct():

png_structp png_ptr = png_create_write_struct_2 (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, user_error_fn, user_warning_fn, (png_voidp) user_mem_ptr, user_malloc_fn, user_free_fn);

After you have these structures, you will need to set up the error handling. When libpng encounters an error, it expects to longjmp() back to your routine. Therefore, you will need to call setjmp() and pass the png_ptr->jmpbuf. 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; } ... 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 July 1999 PNG specification, version 1.2). 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 1.2, anyway). See png_write_end() and the latest PNG specification for more information on that. If you wish to write them before the image, fill them in now, and flag that data as being valid. If you want to wait until after the data, don’t fill them until png_write_end(). For all the fields in png_info and their data types, see png.h. For explanations of what the fields contain, see the PNG specification.

Some of the more important parts of the png_info are:

png_set_IHDR(png_ptr, info_ptr, width, height, bit_depth, color_type, interlace_type, compression_type, filter_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 or 6 bytes. This code tells the library to expand the input data to 4 or 8 bytes per pixel (or expand 1 or 2-byte grayscale data to 2 or 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 are processed.

You can also set up a pointer to a user structure for use by your callback function.

png_set_user_transform_info(png_ptr, user_ptr, 0, 0);

The user_channels and user_depth parameters of this function are ignored when writing; you can set them to zero as shown.

You can retrieve the pointer via the function png_get_user_transform_ptr(). For example:

voidp write_user_transform_ptr = png_get_user_transform_ptr(png_ptr);

It is possible to have libpng flush any pending output, either manually, or automatically after a certain number of lines have been written. To flush the output stream a single time call:

png_write_flush(png_ptr);

and to have libpng flush the output stream periodically after a certain number of scanlines have been written, call:

png_set_flush(png_ptr, nrows);

Note that the distance between rows is from the last time png_write_flush() was called, or the first row of the image if it has never been called. So if you write 50 lines, and then png_set_flush 25, it will flush the output on the next scanline, and every 25 lines thereafter, unless png_write_flush() is called before 25 more lines have been written. If nrows is too small (less than about 10 lines for a 640 pixel wide RGB image) the image compression may decrease noticeably (although this may be acceptable for real-time applications). Infrequent flushing will only degrade the compression performance by a few percent over images that do not use flushing.

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_byte *row_pointers[height];

You can point to void or char or whatever you use for pixels.

If you don’t want to write the whole image at once, you can use png_write_rows() instead. If the file is not interlaced, this is simple:

png_write_rows(png_ptr, row_pointers, number_of_rows);

row_pointers is the same as in the png_write_image() call.

If you are just writing one row at a time, you can do this with 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 August 1999 -- PNG Specification version 1.2, dated July 1999) defined interlacing scheme for PNG files is the "Adam7" interlace scheme, that breaks down an image into seven smaller images of varying size. libpng will build these images for you, or you can do them yourself. If you want to build them yourself, see the PNG specification for details of which pixels to write when.

If you don’t want libpng to handle the interlacing details, just use png_set_interlace_handling() and call png_write_rows() the correct number of times to write all 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 that are user settable. The default routines are in pngmem.c, pngrio.c, pngwio.c, and pngerror.c respectively. To change these functions, call the appropriate png_set_???_fn() function.

Memory allocation is done through the functions png_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 read_ptr, voidp read_io_ptr, png_rw_ptr read_data_fn)

png_set_write_fn(png_structp write_ptr, voidp write_io_ptr, png_rw_ptr write_data_fn, png_flush_ptr output_flush_fn);

voidp read_io_ptr = png_get_io_ptr(read_ptr); voidp write_io_ptr = png_get_io_ptr(write_ptr);

The replacement I/O functions 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 who only have access to the lower 640K, you will have to limit zlib’s memory usage via a png_set_compression_mem_level() call. See zlib.h or zconf.h in the zlib library for more information.

Configuring for Medium Model:

Libpng’s support for medium model has been tested on most of the popular compilers. Make sure MAXSEG_64K gets defined, USE_FAR_KEYWORD gets defined, and FAR gets defined to far in pngconf.h, and you should be all set. Everything in the library (except for zlib’s structure) is expecting far data. You must use the typedefs with the p or pp on the end for pointers (or at least look at them and be careful). Make note that the row’s of data are defined as png_bytepp, which is an unsigned char far * far *.

Configuring for gui/windowing platforms:

You will need to write new error and warning functions that use the GUI interface, as described previously, and set them to be the error and warning functions at the time that png_create_???_struct() is called, in order to have them available during the structure initialization. They can be changed later via png_set_error_fn(). On some compilers, you may also have to change the memory allocators (png_malloc, etc.).

Configuring for compiler xxx:

All includes for libpng are in pngconf.h. If you need to add/change/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), but not for paletted images nor for any images with bit depths less than 8 bits/pixel.

The ’method’ parameter sets the main filtering method, which is currently only ’0’ in the PNG 1.2 specification. The ’filters’ parameter sets which filter(s), if any, should be used for each scanline. Possible values are PNG_ALL_FILTERS and PNG_NO_FILTERS to turn filtering on and off, respectively.

Individual filter types are PNG_FILTER_NONE, PNG_FILTER_SUB, PNG_FILTER_UP, PNG_FILTER_AVG, PNG_FILTER_PAETH, which can be bitwise ORed together ’|’ 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 that indicate to libpng that the row filter should be the same for successive rows unless another row filter is that many times better than the previous filter. In the above example, if the previous 3 filters were SUB, SUB, NONE, the SUB filter could have a "sum of absolute differences" 1.5 x 1.3 times higher than other filters and still be chosen, while the NONE filter could have a sum 1.1 times higher than other filters and still be chosen. Unspecified weights are taken to be 1.0, and the specified weights should probably be declining like those above in order to emphasize recent filters over older filters.

The filter costs specify for each filter type a relative decoding cost to be considered when selecting row filters. This means that filters with higher costs are less likely to be chosen over filters with lower costs, unless their "sum of absolute differences" is that much smaller. The costs do not necessarily reflect the exact computational speeds of the various filters, since this would unduly influence the final image size.

Note that the numbers above were invented purely for this example and are given only to help explain the function usage. Little testing has been done to find optimum values for either the costs or the weights.

Removing unwanted object code:

There are a bunch of #define’s in pngconf.h that control what parts of libpng are compiled. All the defines end in _SUPPORTED. If you are never going to use a capability, you can change the #define to #undef before recompiling libpng and save yourself code and data space, or you can turn off individual capabilities with defines that begin with PNG_NO_.

You can also turn all of the transforms and ancillary chunk capabilities off en masse with compiler directives that define PNG_NO_READ[or WRITE]_TRANSFORMS, or PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS, or all four, along with directives to turn on any of the capabilities that you do want. The PNG_NO_READ[or WRITE]_TRANSFORMS directives disable the extra transformations but still leave the library fully capable of reading and writing PNG files with all known public chunks [except for sPLT]. Use of the PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS directive produces a library that is incapable of reading or writing ancillary chunks. If you are not using the progressive reading capability, you can turn that off with PNG_NO_PROGRESSIVE_READ (don’t confuse this with the INTERLACING capability, which you’ll still have).

All the reading and writing specific code are in separate files, so the linker should only grab the files it needs. However, if you want to make sure, or if you are building a stand alone library, all the reading files start with pngr and all the writing files start with pngw. The files that don’t match either (like png.c, pngtrans.c, etc.) are used for both reading and writing, and always need to be included. The progressive reader is in pngpread.c

If you are creating or distributing a dynamically linked library (a .so or DLL file), you should not remove or disable any parts of the library, as this will cause applications linked with different versions of the library to fail if they call functions not available in your library. The size of the library itself should not be an issue, because only those sections that are actually used will be loaded into memory.

Requesting debug printout:

The macro definition PNG_DEBUG can be used to request debugging printout. Set it to an integer value in the range 0 to 3. Higher numbers result in increasing amounts of debugging information. The information is printed to the "stderr" file, unless another file name is specified in the PNG_DEBUG_FILE macro definition.

When PNG_DEBUG > 0, the following functions (macros) become available:

png_debug(level, message) png_debug1(level, message, p1) png_debug2(level, message, p1, p2)

in which "level" is compared to PNG_DEBUG to decide whether to print the message, "message" is the formatted string to be printed, and p1 and p2 are parameters that are to be embedded in the string according to printf-style formatting directives. For example,

png_debug1(2, "foo=%d0, foo);

is expanded to

if(PNG_DEBUG > 2) fprintf(PNG_DEBUG_FILE, "foo=%d0, foo);

When PNG_DEBUG is defined but is zero, the macros aren’t defined, but you can still use PNG_DEBUG to control your own debugging:

#ifdef PNG_DEBUG fprintf(stderr, ... #endif

When PNG_DEBUG = 1, the macros are defined, but only png_debug statements having level = 0 will be printed. There aren’t any such statements in this version of libpng, but if you insert some they will be printed.

VI. 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 that do not use custom error functions to fail if the png_ptr was not initialized to zero. It is still possible to set the error callbacks AFTER png_read_init(), or to change them with png_set_error_fn(), which is essentially the same function, but with a new name to force compilation errors with applications that try to use the old method.

VII. Y2K Compliance in libpng

October 15, 1999

Since the PNG Development group is an ad-hoc body, we can’t make an official declaration.

This is your unofficial assurance that libpng from version 0.71 and upward through 1.0.5 are Y2K compliant. It is my belief that earlier versions were also Y2K compliant.

Libpng only has three year fields. One is a 2-byte unsigned integer that will hold years up to 65535. The other two hold the date in text format, and will hold years up to 9999.

The integer is "png_uint_16 year" in png_time_struct.

The strings are "png_charp time_buffer" in png_struct and "near_time_buffer", which is a local character string in png.c.

There are seven time-related functions:

png_convert_to_rfc_1123() in png.c (formerly png_convert_to_rfc_1152() in error) png_convert_from_struct_tm() in pngwrite.c, called in pngwrite.c png_convert_from_time_t() in pngwrite.c png_get_tIME() in pngget.c png_handle_tIME() in pngrutil.c, called in pngread.c png_set_tIME() in pngset.c png_write_tIME() in pngwutil.c, called in pngwrite.c

All appear to handle dates properly in a Y2K environment. The png_convert_from_time_t() function calls gmtime() to convert from system clock time, which returns (year - 1900), which we properly convert to the full 4-digit year. There is a possibility that applications using libpng are not passing 4-digit years into the png_convert_to_rfc_1123() function, or that they are incorrectly passing only a 2-digit year instead of "year - 1900" into the png_convert_from_struct_tm() function, but this is not under our control. The libpng documentation has always stated that it works with 4-digit years, and the APIs have been documented as such.

The tIME chunk itself is also Y2K compliant. It uses a 2-byte unsigned integer to hold the year, and can hold years as large as 65535.

zlib, upon which libpng depends, is also Y2K compliant. It contains no date-related code.

Glenn Randers-Pehrson libpng maintainer PNG Development Group

NOTE

Note about libpng version numbers:

Due to various miscommunications, unforeseen code incompatibilities and occasional factors outside the authors’ control, version numbering on the library has not always been consistent and straightforward. The following table summarizes matters since version 0.89c, which was the first widely used release:

source png.h png.h shared-lib version string int version ------- ------ ------ ---------- 0.89c 0.89 89 1.0.89 0.90 0.90 90 0.90 [should be 2.0.90] 0.95 0.95 95 0.95 [should be 2.0.95] 0.96 0.96 96 0.96 [should be 2.0.96] 0.97b 1.00.97 97 1.0.1 [should be 2.0.97] 0.97c 0.97 97 2.0.97 0.98 0.98 98 2.0.98 0.99 0.99 98 2.0.99 0.99a-m 0.99 99 2.0.99 1.00 1.00 100 2.1.0 [int should be 10000] 1.0.0 1.0.0 100 2.1.0 [int should be 10000] 1.0.1 1.0.1 10001 2.1.0

Henceforth the source version will match the shared-library minor and patch numbers; the shared-library major version number will be used for changes in backward compatibility, as it is intended. The PNG_PNGLIB_VER macro, which is not used within libpng but is available for applications, is an unsigned integer of the form xyyzz corresponding to the source version x.y.z (leading zeros in y and z).

SEE ALSO

libpngpf(3), png(5)

libpng:

ftp://ftp.uu.net/graphics/png http://www.cdrom.com/pub/png

zlib:

(generally) at the same location as libpng or at
ftp://ftp.uu.net/pub/archiving/zip/zlib
http://www.cdrom.com/pub/infozip/zlib

PNGspecification:RFC2083

(generally) at the same location as libpng or at
ftp://ds.internic.net/rfc/rfc2083.txt
or (as a W3C Recommendation) at
http://www.w3.org/TR/REC-png.html

In the case of any inconsistency between the PNG specification and this library, the specification takes precedence.

AUTHORS

This man page: Glenn Randers-Pehrson <randeg@alum.rpi.edu>

The contributing authors would like to thank all those who helped with testing, bug fixes, and patience. This wouldn’t have been possible without all of you.

Thanks to Frank J. T. Wojcik for helping with the documentation.

Libpng version 1.0.5 - October 15, 1999: Initially created in 1995 by Guy Eric Schalnat, then of Group 42, Inc. Currently maintained by Glenn Randers-Pehrson (randeg@alum.rpi.edu).

Supported by the PNG development group
(png-implement@ccrc.wustl.edu).

COPYRIGHT NOTICE:

Copyright (c) 1995, 1996 Guy Eric Schalnat, Group 42, Inc. (libpng versions 0.5, May 1995, through 0.89c, May 1996) Copyright (c) 1996, 1997 Andreas Dilger (libpng versions 0.90, December 1996, through 0.96, May 1997) Copyright (c) 1998, 1999 Glenn Randers-Pehrson (libpng versions 0.97, January 1998, through 1.0.5, October 15, 1999)

For the purposes of this copyright and license, "Contributing Authors" is defined as the following set of individuals:

John Bowler Kevin Bracey Sam Bushell Andreas Dilger Magnus Holmgren Tom Lane Dave Martindale Glenn Randers-Pehrson Greg Roelofs Guy Eric Schalnat Paul Schmidt Tom Tanner Willem van Schaik Tim Wegner

The PNG Reference Library (libpng) is supplied "AS IS". The Contributing Authors and Group 42, Inc. disclaim all warranties, expressed or implied, including, without limitation, the warranties of merchantability and of fitness for any purpose. The Contributing Authors and Group 42, Inc. assume no liability for direct, indirect, incidental, special, exemplary, or consequential damages, which may result from the use of the PNG Reference Library, even if advised of the possibility of such damage.

Permission is hereby granted to use, copy, modify, and distribute this source code, or portions hereof, for any purpose, without fee, subject to the following restrictions:

1. The origin of this source code must not be misrepresented.

2. Altered versions must be plainly marked as such and must not be misrepresented as being the original source.

3. This Copyright notice may not be removed or altered from any source or altered source distribution.

The Contributing Authors and Group 42, Inc. specifically permit, without fee, and encourage the use of this source code as a component to supporting the PNG file format in commercial products. If you use this source code in a product, acknowledgment is not required but would be appreciated.

A "png_get_copyright" function is available, for convenient use in "about" boxes and the like:

printf("%s",png_get_copyright(NULL));

Also, the PNG logo (in PNG format, of course) is supplied in the file "pngnow.png".

Libpng is OSI Certified Open Source Software. OSI Certified is a certification mark of the Open Source Initiative.