1 libpng.txt - A description on how to use and modify libpng
3 libpng version 1.4.4 - September 23, 2010
4 Updated and distributed by Glenn Randers-Pehrson
5 <glennrp at users.sourceforge.net>
6 Copyright (c) 1998-2010 Glenn Randers-Pehrson
8 This document is released under the libpng license.
9 For conditions of distribution and use, see the disclaimer
14 libpng versions 0.97, January 1998, through 1.4.4 - September 23, 2010
15 Updated and distributed by Glenn Randers-Pehrson
16 Copyright (c) 1998-2010 Glenn Randers-Pehrson
18 libpng 1.0 beta 6 version 0.96 May 28, 1997
19 Updated and distributed by Andreas Dilger
20 Copyright (c) 1996, 1997 Andreas Dilger
22 libpng 1.0 beta 2 - version 0.88 January 26, 1996
23 For conditions of distribution and use, see copyright
24 notice in png.h. Copyright (c) 1995, 1996 Guy Eric
25 Schalnat, Group 42, Inc.
27 Updated/rewritten per request in the libpng FAQ
28 Copyright (c) 1995, 1996 Frank J. T. Wojcik
29 December 18, 1995 & January 20, 1996
33 This file describes how to use and modify the PNG reference library
34 (known as libpng) for your own use. There are five sections to this
35 file: introduction, structures, reading, writing, and modification and
36 configuration notes for various special platforms. In addition to this
37 file, example.c is a good starting point for using the library, as
38 it is heavily commented and should include everything most people
39 will need. We assume that libpng is already installed; see the
40 INSTALL file for instructions on how to install libpng.
42 For examples of libpng usage, see the files "example.c", "pngtest.c",
43 and the files in the "contrib" directory, all of which are included in
44 the libpng distribution.
46 Libpng was written as a companion to the PNG specification, as a way
47 of reducing the amount of time and effort it takes to support the PNG
48 file format in application programs.
50 The PNG specification (second edition), November 2003, is available as
51 a W3C Recommendation and as an ISO Standard (ISO/IEC 15948:2003 (E)) at
52 <http://www.w3.org/TR/2003/REC-PNG-20031110/
53 The W3C and ISO documents have identical technical content.
55 The PNG-1.2 specification is available at
56 <http://www.libpng.org/pub/png/documents/>. It is technically equivalent
57 to the PNG specification (second edition) but has some additional material.
59 The PNG-1.0 specification is available
60 as RFC 2083 <http://www.libpng.org/pub/png/documents/> and as a
61 W3C Recommendation <http://www.w3.org/TR/REC.png.html>.
63 Some additional chunks are described in the special-purpose public chunks
64 documents at <http://www.libpng.org/pub/png/documents/>.
67 about PNG, and the latest version of libpng, can be found at the PNG home
68 page, <http://www.libpng.org/pub/png/>.
70 Most users will not have to modify the library significantly; advanced
71 users may want to modify it more. All attempts were made to make it as
72 complete as possible, while keeping the code easy to understand.
73 Currently, this library only supports C. Support for other languages
76 Libpng has been designed to handle multiple sessions at one time,
77 to be easily modifiable, to be portable to the vast majority of
78 machines (ANSI, K&R, 16-, 32-, and 64-bit) available, and to be easy
79 to use. The ultimate goal of libpng is to promote the acceptance of
80 the PNG file format in whatever way possible. While there is still
81 work to be done (see the TODO file), libpng should cover the
82 majority of the needs of its users.
84 Libpng uses zlib for its compression and decompression of PNG files.
85 Further information about zlib, and the latest version of zlib, can
86 be found at the zlib home page, <http://www.info-zip.org/pub/infozip/zlib/>.
87 The zlib compression utility is a general purpose utility that is
88 useful for more than PNG files, and can be used without libpng.
89 See the documentation delivered with zlib for more details.
90 You can usually find the source files for the zlib utility wherever you
91 find the libpng source files.
93 Libpng is thread safe, provided the threads are using different
94 instances of the structures. Each thread should have its own
95 png_struct and png_info instances, and thus its own image.
96 Libpng does not protect itself against two threads using the
97 same instance of a structure.
101 There are two main structures that are important to libpng, png_struct
102 and png_info. The first, png_struct, is an internal structure that
103 will not, for the most part, be used by a user except as the first
104 variable passed to every libpng function call.
106 The png_info structure is designed to provide information about the
107 PNG file. At one time, the fields of png_info were intended to be
108 directly accessible to the user. However, this tended to cause problems
109 with applications using dynamically loaded libraries, and as a result
110 a set of interface functions for png_info (the png_get_*() and png_set_*()
111 functions) was developed. The fields of png_info are still available for
112 older applications, but it is suggested that applications use the new
113 interfaces if at all possible.
115 Applications that do make direct access to the members of png_struct (except
116 for png_ptr->jmpbuf) must be recompiled whenever the library is updated,
117 and applications that make direct access to the members of png_info must
118 be recompiled if they were compiled or loaded with libpng version 1.0.6,
119 in which the members were in a different order. In version 1.0.7, the
120 members of the png_info structure reverted to the old order, as they were
121 in versions 0.97c through 1.0.5. Starting with version 2.0.0, both
122 structures are going to be hidden, and the contents of the structures will
123 only be accessible through the png_get/png_set functions.
125 The png.h header file is an invaluable reference for programming with libpng.
126 And while I'm on the topic, make sure you include the libpng header file:
132 We'll now walk you through the possible functions to call when reading
133 in a PNG file sequentially, briefly explaining the syntax and purpose
134 of each one. See example.c and png.h for more detail. While
135 progressive reading is covered in the next section, you will still
136 need some of the functions discussed in this section to read a PNG
141 You will want to do the I/O initialization(*) before you get into libpng,
142 so if it doesn't work, you don't have much to undo. Of course, you
143 will also want to insure that you are, in fact, dealing with a PNG
144 file. Libpng provides a simple check to see if a file is a PNG file.
145 To use it, pass in the first 1 to 8 bytes of the file to the function
146 png_sig_cmp(), and it will return 0 (false) if the bytes match the
147 corresponding bytes of the PNG signature, or nonzero (true) otherwise.
148 Of course, the more bytes you pass in, the greater the accuracy of the
151 If you are intending to keep the file pointer open for use in libpng,
152 you must ensure you don't read more than 8 bytes from the beginning
153 of the file, and you also have to make a call to png_set_sig_bytes_read()
154 with the number of bytes you read from the beginning. Libpng will
155 then only check the bytes (if any) that your program didn't read.
157 (*): If you are not using the standard I/O functions, you will need
158 to replace them with custom functions. See the discussion under
162 FILE *fp = fopen(file_name, "rb");
167 fread(header, 1, number, fp);
168 is_png = !png_sig_cmp(header, 0, number);
175 Next, png_struct and png_info need to be allocated and initialized. In
176 order to ensure that the size of these structures is correct even with a
177 dynamically linked libpng, there are functions to initialize and
178 allocate the structures. We also pass the library version, optional
179 pointers to error handling functions, and a pointer to a data struct for
180 use by the error functions, if necessary (the pointer and functions can
181 be NULL if the default error handlers are to be used). See the section
182 on Changes to Libpng below regarding the old initialization functions.
183 The structure allocation functions quietly return NULL if they fail to
184 create the structure, so your application should check for that.
186 png_structp png_ptr = png_create_read_struct
187 (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
188 user_error_fn, user_warning_fn);
192 png_infop info_ptr = png_create_info_struct(png_ptr);
195 png_destroy_read_struct(&png_ptr,
196 (png_infopp)NULL, (png_infopp)NULL);
200 png_infop end_info = png_create_info_struct(png_ptr);
203 png_destroy_read_struct(&png_ptr, &info_ptr,
208 If you want to use your own memory allocation routines,
209 define PNG_USER_MEM_SUPPORTED and use
210 png_create_read_struct_2() instead of png_create_read_struct():
212 png_structp png_ptr = png_create_read_struct_2
213 (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
214 user_error_fn, user_warning_fn, (png_voidp)
215 user_mem_ptr, user_malloc_fn, user_free_fn);
217 The error handling routines passed to png_create_read_struct()
218 and the memory alloc/free routines passed to png_create_struct_2()
219 are only necessary if you are not using the libpng supplied error
220 handling and memory alloc/free functions.
222 When libpng encounters an error, it expects to longjmp back
223 to your routine. Therefore, you will need to call setjmp and pass
224 your png_jmpbuf(png_ptr). If you read the file from different
225 routines, you will need to update the jmpbuf field every time you enter
226 a new routine that will call a png_*() function.
228 See your documentation of setjmp/longjmp for your compiler for more
229 information on setjmp/longjmp. See the discussion on libpng error
230 handling in the Customizing Libpng section below for more information
231 on the libpng error handling. If an error occurs, and libpng longjmp's
232 back to your setjmp, you will want to call png_destroy_read_struct() to
235 if (setjmp(png_jmpbuf(png_ptr)))
237 png_destroy_read_struct(&png_ptr, &info_ptr,
243 If you would rather avoid the complexity of setjmp/longjmp issues,
244 you can compile libpng with PNG_NO_SETJMP, in which case
245 errors will result in a call to PNG_ABORT() which defaults to abort().
247 You can #define PNG_ABORT() to a function that does something
248 more useful than abort(), as long as your function does not
251 Now you need to set up the input code. The default for libpng is to
252 use the C function fread(). If you use this, you will need to pass a
253 valid FILE * in the function png_init_io(). Be sure that the file is
254 opened in binary mode. If you wish to handle reading data in another
255 way, you need not call the png_init_io() function, but you must then
256 implement the libpng I/O methods discussed in the Customizing Libpng
259 png_init_io(png_ptr, fp);
261 If you had previously opened the file and read any of the signature from
262 the beginning in order to see if this was a PNG file, you need to let
263 libpng know that there are some bytes missing from the start of the file.
265 png_set_sig_bytes(png_ptr, number);
267 You can change the zlib compression buffer size to be used while
268 reading compressed data with
270 png_set_compression_buffer_size(png_ptr, buffer_size);
272 where the default size is 8192 bytes. Note that the buffer size
273 is changed immediately and the buffer is reallocated immediately,
274 instead of setting a flag to be acted upon later.
276 Setting up callback code
278 You can set up a callback function to handle any unknown chunks in the
279 input stream. You must supply the function
281 read_chunk_callback(png_ptr ptr,
282 png_unknown_chunkp chunk);
284 /* The unknown chunk structure contains your
285 chunk data, along with similar data for any other
292 /* Note that libpng has already taken care of
295 /* put your code here. Search for your chunk in the
296 unknown chunk structure, process it, and return one
299 return (-n); /* chunk had an error */
300 return (0); /* did not recognize */
301 return (n); /* success */
304 (You can give your function another name that you like instead of
305 "read_chunk_callback")
307 To inform libpng about your function, use
309 png_set_read_user_chunk_fn(png_ptr, user_chunk_ptr,
310 read_chunk_callback);
312 This names not only the callback function, but also a user pointer that
313 you can retrieve with
315 png_get_user_chunk_ptr(png_ptr);
317 If you call the png_set_read_user_chunk_fn() function, then all unknown
318 chunks will be saved when read, in case your callback function will need
319 one or more of them. This behavior can be changed with the
320 png_set_keep_unknown_chunks() function, described below.
322 At this point, you can set up a callback function that will be
323 called after each row has been read, which you can use to control
324 a progress meter or the like. It's demonstrated in pngtest.c.
325 You must supply a function
327 void read_row_callback(png_ptr ptr, png_uint_32 row,
330 /* put your code here */
333 (You can give it another name that you like instead of "read_row_callback")
335 To inform libpng about your function, use
337 png_set_read_status_fn(png_ptr, read_row_callback);
339 Unknown-chunk handling
341 Now you get to set the way the library processes unknown chunks in the
342 input PNG stream. Both known and unknown chunks will be read. Normal
343 behavior is that known chunks will be parsed into information in
344 various info_ptr members while unknown chunks will be discarded. This
345 behavior can be wasteful if your application will never use some known
346 chunk types. To change this, you can call:
348 png_set_keep_unknown_chunks(png_ptr, keep,
349 chunk_list, num_chunks);
350 keep - 0: default unknown chunk handling
351 1: ignore; do not keep
352 2: keep only if safe-to-copy
353 3: keep even if unsafe-to-copy
354 You can use these definitions:
355 PNG_HANDLE_CHUNK_AS_DEFAULT 0
356 PNG_HANDLE_CHUNK_NEVER 1
357 PNG_HANDLE_CHUNK_IF_SAFE 2
358 PNG_HANDLE_CHUNK_ALWAYS 3
359 chunk_list - list of chunks affected (a byte string,
360 five bytes per chunk, NULL or '\0' if
362 num_chunks - number of chunks affected; if 0, all
363 unknown chunks are affected. If nonzero,
364 only the chunks in the list are affected
366 Unknown chunks declared in this way will be saved as raw data onto a
367 list of png_unknown_chunk structures. If a chunk that is normally
368 known to libpng is named in the list, it will be handled as unknown,
369 according to the "keep" directive. If a chunk is named in successive
370 instances of png_set_keep_unknown_chunks(), the final instance will
371 take precedence. The IHDR and IEND chunks should not be named in
372 chunk_list; if they are, libpng will process them normally anyway.
374 Here is an example of the usage of png_set_keep_unknown_chunks(),
375 where the private "vpAg" chunk will later be processed by a user chunk
378 png_byte vpAg[5]={118, 112, 65, 103, (png_byte) '\0'};
380 #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
381 png_byte unused_chunks[]=
383 104, 73, 83, 84, (png_byte) '\0', /* hIST */
384 105, 84, 88, 116, (png_byte) '\0', /* iTXt */
385 112, 67, 65, 76, (png_byte) '\0', /* pCAL */
386 115, 67, 65, 76, (png_byte) '\0', /* sCAL */
387 115, 80, 76, 84, (png_byte) '\0', /* sPLT */
388 116, 73, 77, 69, (png_byte) '\0', /* tIME */
394 #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
395 /* ignore all unknown chunks: */
396 png_set_keep_unknown_chunks(read_ptr, 1, NULL, 0);
397 /* except for vpAg: */
398 png_set_keep_unknown_chunks(read_ptr, 2, vpAg, 1);
399 /* also ignore unused known chunks: */
400 png_set_keep_unknown_chunks(read_ptr, 1, unused_chunks,
401 (int)sizeof(unused_chunks)/5);
406 The PNG specification allows the width and height of an image to be as
407 large as 2^31-1 (0x7fffffff), or about 2.147 billion rows and columns.
408 Since very few applications really need to process such large images,
409 we have imposed an arbitrary 1-million limit on rows and columns.
410 Larger images will be rejected immediately with a png_error() call. If
411 you wish to override this limit, you can use
413 png_set_user_limits(png_ptr, width_max, height_max);
415 to set your own limits, or use width_max = height_max = 0x7fffffffL
416 to allow all valid dimensions (libpng may reject some very large images
417 anyway because of potential buffer overflow conditions).
419 You should put this statement after you create the PNG structure and
420 before calling png_read_info(), png_read_png(), or png_process_data().
421 If you need to retrieve the limits that are being applied, use
423 width_max = png_get_user_width_max(png_ptr);
424 height_max = png_get_user_height_max(png_ptr);
426 The PNG specification sets no limit on the number of ancillary chunks
427 allowed in a PNG datastream. You can impose a limit on the total number
428 of sPLT, tEXt, iTXt, zTXt, and unknown chunks that will be stored, with
430 png_set_chunk_cache_max(png_ptr, user_chunk_cache_max);
432 where 0x7fffffffL means unlimited. You can retrieve this limit with
434 chunk_cache_max = png_get_chunk_cache_max(png_ptr);
436 This limit also applies to the number of buffers that can be allocated
437 by png_decompress_chunk() while decompressing iTXt, zTXt, and iCCP chunks.
439 You can also set a limit on the amount of memory that a compressed chunk
440 other than IDAT can occupy, with
442 png_set_chunk_malloc_max(png_ptr, user_chunk_malloc_max);
444 and you can retrieve the limit with
446 chunk_malloc_max = png_get_chunk_malloc_max(png_ptr);
448 Any chunks that would cause either of these limits to be exceeded will
451 The high-level read interface
453 At this point there are two ways to proceed; through the high-level
454 read interface, or through a sequence of low-level read operations.
455 You can use the high-level interface if (a) you are willing to read
456 the entire image into memory, and (b) the input transformations
457 you want to do are limited to the following set:
459 PNG_TRANSFORM_IDENTITY No transformation
460 PNG_TRANSFORM_STRIP_16 Strip 16-bit samples to
462 PNG_TRANSFORM_STRIP_ALPHA Discard the alpha channel
463 PNG_TRANSFORM_PACKING Expand 1, 2 and 4-bit
465 PNG_TRANSFORM_PACKSWAP Change order of packed
467 PNG_TRANSFORM_EXPAND Perform set_expand()
468 PNG_TRANSFORM_INVERT_MONO Invert monochrome images
469 PNG_TRANSFORM_SHIFT Normalize pixels to the
471 PNG_TRANSFORM_BGR Flip RGB to BGR, RGBA
473 PNG_TRANSFORM_SWAP_ALPHA Flip RGBA to ARGB or GA
475 PNG_TRANSFORM_INVERT_ALPHA Change alpha from opacity
477 PNG_TRANSFORM_SWAP_ENDIAN Byte-swap 16-bit samples
478 PNG_TRANSFORM_GRAY_TO_RGB Expand grayscale samples
479 to RGB (or GA to RGBA)
481 (This excludes setting a background color, doing gamma transformation,
482 quantizing, and setting filler.) If this is the case, simply do this:
484 png_read_png(png_ptr, info_ptr, png_transforms, NULL)
486 where png_transforms is an integer containing the bitwise OR of some
487 set of transformation flags. This call is equivalent to png_read_info(),
488 followed the set of transformations indicated by the transform mask,
489 then png_read_image(), and finally png_read_end().
491 (The final parameter of this call is not yet used. Someday it might point
492 to transformation parameters required by some future input transform.)
494 You must use png_transforms and not call any png_set_transform() functions
495 when you use png_read_png().
497 After you have called png_read_png(), you can retrieve the image data
500 row_pointers = png_get_rows(png_ptr, info_ptr);
502 where row_pointers is an array of pointers to the pixel data for each row:
504 png_bytep row_pointers[height];
506 If you know your image size and pixel size ahead of time, you can allocate
507 row_pointers prior to calling png_read_png() with
509 if (height > PNG_UINT_32_MAX/png_sizeof(png_byte))
511 "Image is too tall to process in memory");
512 if (width > PNG_UINT_32_MAX/pixel_size)
514 "Image is too wide to process in memory");
515 row_pointers = png_malloc(png_ptr,
516 height*png_sizeof(png_bytep));
517 for (int i=0; i<height, i++)
518 row_pointers[i]=NULL; /* security precaution */
519 for (int i=0; i<height, i++)
520 row_pointers[i]=png_malloc(png_ptr,
522 png_set_rows(png_ptr, info_ptr, &row_pointers);
524 Alternatively you could allocate your image in one big block and define
525 row_pointers[i] to point into the proper places in your block.
527 If you use png_set_rows(), the application is responsible for freeing
528 row_pointers (and row_pointers[i], if they were separately allocated).
530 If you don't allocate row_pointers ahead of time, png_read_png() will
531 do it, and it'll be free'ed when you call png_destroy_*().
533 The low-level read interface
535 If you are going the low-level route, you are now ready to read all
536 the file information up to the actual image data. You do this with a
537 call to png_read_info().
539 png_read_info(png_ptr, info_ptr);
541 This will process all chunks up to but not including the image data.
543 Querying the info structure
545 Functions are used to get the information from the info_ptr once it
546 has been read. Note that these fields may not be completely filled
547 in until png_read_end() has read the chunk data following the image.
549 png_get_IHDR(png_ptr, info_ptr, &width, &height,
550 &bit_depth, &color_type, &interlace_type,
551 &compression_type, &filter_method);
553 width - holds the width of the image
554 in pixels (up to 2^31).
555 height - holds the height of the image
556 in pixels (up to 2^31).
557 bit_depth - holds the bit depth of one of the
558 image channels. (valid values are
559 1, 2, 4, 8, 16 and depend also on
560 the color_type. See also
561 significant bits (sBIT) below).
562 color_type - describes which color/alpha channels
565 (bit depths 1, 2, 4, 8, 16)
566 PNG_COLOR_TYPE_GRAY_ALPHA
568 PNG_COLOR_TYPE_PALETTE
569 (bit depths 1, 2, 4, 8)
572 PNG_COLOR_TYPE_RGB_ALPHA
575 PNG_COLOR_MASK_PALETTE
579 filter_method - (must be PNG_FILTER_TYPE_BASE
580 for PNG 1.0, and can also be
581 PNG_INTRAPIXEL_DIFFERENCING if
582 the PNG datastream is embedded in
583 a MNG-1.0 datastream)
584 compression_type - (must be PNG_COMPRESSION_TYPE_BASE
586 interlace_type - (PNG_INTERLACE_NONE or
589 Any or all of interlace_type, compression_type, or
590 filter_method can be NULL if you are
591 not interested in their values.
593 Note that png_get_IHDR() returns 32-bit data into
594 the application's width and height variables.
595 This is an unsafe situation if these are 16-bit
596 variables. In such situations, the
597 png_get_image_width() and png_get_image_height()
598 functions described below are safer.
600 width = png_get_image_width(png_ptr,
602 height = png_get_image_height(png_ptr,
604 bit_depth = png_get_bit_depth(png_ptr,
606 color_type = png_get_color_type(png_ptr,
608 filter_method = png_get_filter_type(png_ptr,
610 compression_type = png_get_compression_type(png_ptr,
612 interlace_type = png_get_interlace_type(png_ptr,
615 channels = png_get_channels(png_ptr, info_ptr);
616 channels - number of channels of info for the
617 color type (valid values are 1 (GRAY,
618 PALETTE), 2 (GRAY_ALPHA), 3 (RGB),
619 4 (RGB_ALPHA or RGB + filler byte))
620 rowbytes = png_get_rowbytes(png_ptr, info_ptr);
621 rowbytes - number of bytes needed to hold a row
623 signature = png_get_signature(png_ptr, info_ptr);
624 signature - holds the signature read from the
625 file (if any). The data is kept in
626 the same offset it would be if the
627 whole signature were read (i.e. if an
628 application had already read in 4
629 bytes of signature before starting
630 libpng, the remaining 4 bytes would
631 be in signature[4] through signature[7]
632 (see png_set_sig_bytes())).
634 These are also important, but their validity depends on whether the chunk
635 has been read. The png_get_valid(png_ptr, info_ptr, PNG_INFO_<chunk>) and
636 png_get_<chunk>(png_ptr, info_ptr, ...) functions return non-zero if the
637 data has been read, or zero if it is missing. The parameters to the
638 png_get_<chunk> are set directly if they are simple data types, or a
639 pointer into the info_ptr is returned for any complex types.
641 png_get_PLTE(png_ptr, info_ptr, &palette,
643 palette - the palette for the file
645 num_palette - number of entries in the palette
647 png_get_gAMA(png_ptr, info_ptr, &gamma);
648 gamma - the gamma the file is written
651 png_get_sRGB(png_ptr, info_ptr, &srgb_intent);
652 srgb_intent - the rendering intent (PNG_INFO_sRGB)
653 The presence of the sRGB chunk
654 means that the pixel data is in the
655 sRGB color space. This chunk also
656 implies specific values of gAMA and
659 png_get_iCCP(png_ptr, info_ptr, &name,
660 &compression_type, &profile, &proflen);
661 name - The profile name.
662 compression - The compression type; always
663 PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
664 You may give NULL to this argument to
666 profile - International Color Consortium color
667 profile data. May contain NULs.
668 proflen - length of profile data in bytes.
670 png_get_sBIT(png_ptr, info_ptr, &sig_bit);
671 sig_bit - the number of significant bits for
672 (PNG_INFO_sBIT) each of the gray,
673 red, green, and blue channels,
674 whichever are appropriate for the
675 given color type (png_color_16)
677 png_get_tRNS(png_ptr, info_ptr, &trans_alpha,
678 &num_trans, &trans_color);
679 trans_alpha - array of alpha (transparency)
680 entries for palette (PNG_INFO_tRNS)
681 trans_color - graylevel or color sample values of
682 the single transparent color for
683 non-paletted images (PNG_INFO_tRNS)
684 num_trans - number of transparent entries
687 png_get_hIST(png_ptr, info_ptr, &hist);
689 hist - histogram of palette (array of
692 png_get_tIME(png_ptr, info_ptr, &mod_time);
693 mod_time - time image was last modified
696 png_get_bKGD(png_ptr, info_ptr, &background);
697 background - background color (PNG_VALID_bKGD)
698 valid 16-bit red, green and blue
699 values, regardless of color_type
701 num_comments = png_get_text(png_ptr, info_ptr,
702 &text_ptr, &num_text);
703 num_comments - number of comments
704 text_ptr - array of png_text holding image
706 text_ptr[i].compression - type of compression used
707 on "text" PNG_TEXT_COMPRESSION_NONE
708 PNG_TEXT_COMPRESSION_zTXt
709 PNG_ITXT_COMPRESSION_NONE
710 PNG_ITXT_COMPRESSION_zTXt
711 text_ptr[i].key - keyword for comment. Must contain
713 text_ptr[i].text - text comments for current
714 keyword. Can be empty.
715 text_ptr[i].text_length - length of text string,
716 after decompression, 0 for iTXt
717 text_ptr[i].itxt_length - length of itxt string,
718 after decompression, 0 for tEXt/zTXt
719 text_ptr[i].lang - language of comment (empty
721 text_ptr[i].lang_key - keyword in UTF-8
722 (empty string for unknown).
723 Note that the itxt_length, lang, and lang_key
724 members of the text_ptr structure only exist
725 when the library is built with iTXt chunk support.
727 num_text - number of comments (same as
728 num_comments; you can put NULL here
729 to avoid the duplication)
730 Note while png_set_text() will accept text, language,
731 and translated keywords that can be NULL pointers, the
732 structure returned by png_get_text will always contain
733 regular zero-terminated C strings. They might be
734 empty strings but they will never be NULL pointers.
736 num_spalettes = png_get_sPLT(png_ptr, info_ptr,
738 palette_ptr - array of palette structures holding
739 contents of one or more sPLT chunks
741 num_spalettes - number of sPLT chunks read.
743 png_get_oFFs(png_ptr, info_ptr, &offset_x, &offset_y,
745 offset_x - positive offset from the left edge
747 offset_y - positive offset from the top edge
749 unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER
751 png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y,
753 res_x - pixels/unit physical resolution in
755 res_y - pixels/unit physical resolution in
757 unit_type - PNG_RESOLUTION_UNKNOWN,
760 png_get_sCAL(png_ptr, info_ptr, &unit, &width,
762 unit - physical scale units (an integer)
763 width - width of a pixel in physical scale units
764 height - height of a pixel in physical scale units
765 (width and height are doubles)
767 png_get_sCAL_s(png_ptr, info_ptr, &unit, &width,
769 unit - physical scale units (an integer)
770 width - width of a pixel in physical scale units
771 height - height of a pixel in physical scale units
772 (width and height are strings like "2.54")
774 num_unknown_chunks = png_get_unknown_chunks(png_ptr,
776 unknowns - array of png_unknown_chunk
777 structures holding unknown chunks
778 unknowns[i].name - name of unknown chunk
779 unknowns[i].data - data of unknown chunk
780 unknowns[i].size - size of unknown chunk's data
781 unknowns[i].location - position of chunk in file
783 The value of "i" corresponds to the order in which the
784 chunks were read from the PNG file or inserted with the
785 png_set_unknown_chunks() function.
787 The data from the pHYs chunk can be retrieved in several convenient
790 res_x = png_get_x_pixels_per_meter(png_ptr,
792 res_y = png_get_y_pixels_per_meter(png_ptr,
794 res_x_and_y = png_get_pixels_per_meter(png_ptr,
796 res_x = png_get_x_pixels_per_inch(png_ptr,
798 res_y = png_get_y_pixels_per_inch(png_ptr,
800 res_x_and_y = png_get_pixels_per_inch(png_ptr,
802 aspect_ratio = png_get_pixel_aspect_ratio(png_ptr,
805 (Each of these returns 0 [signifying "unknown"] if
806 the data is not present or if res_x is 0;
807 res_x_and_y is 0 if res_x != res_y)
809 The data from the oFFs chunk can be retrieved in several convenient
812 x_offset = png_get_x_offset_microns(png_ptr, info_ptr);
813 y_offset = png_get_y_offset_microns(png_ptr, info_ptr);
814 x_offset = png_get_x_offset_inches(png_ptr, info_ptr);
815 y_offset = png_get_y_offset_inches(png_ptr, info_ptr);
817 (Each of these returns 0 [signifying "unknown" if both
818 x and y are 0] if the data is not present or if the
819 chunk is present but the unit is the pixel)
821 For more information, see the png_info definition in png.h and the
822 PNG specification for chunk contents. Be careful with trusting
823 rowbytes, as some of the transformations could increase the space
824 needed to hold a row (expand, filler, gray_to_rgb, etc.).
825 See png_read_update_info(), below.
827 A quick word about text_ptr and num_text. PNG stores comments in
828 keyword/text pairs, one pair per chunk, with no limit on the number
829 of text chunks, and a 2^31 byte limit on their size. While there are
830 suggested keywords, there is no requirement to restrict the use to these
831 strings. It is strongly suggested that keywords and text be sensible
832 to humans (that's the point), so don't use abbreviations. Non-printing
833 symbols are not allowed. See the PNG specification for more details.
834 There is also no requirement to have text after the keyword.
836 Keywords should be limited to 79 Latin-1 characters without leading or
837 trailing spaces, but non-consecutive spaces are allowed within the
838 keyword. It is possible to have the same keyword any number of times.
839 The text_ptr is an array of png_text structures, each holding a
840 pointer to a language string, a pointer to a keyword and a pointer to
841 a text string. The text string, language code, and translated
842 keyword may be empty or NULL pointers. The keyword/text
843 pairs are put into the array in the order that they are received.
844 However, some or all of the text chunks may be after the image, so, to
845 make sure you have read all the text chunks, don't mess with these
846 until after you read the stuff after the image. This will be
847 mentioned again below in the discussion that goes with png_read_end().
849 Input transformations
851 After you've read the header information, you can set up the library
852 to handle any special transformations of the image data. The various
853 ways to transform the data will be described in the order that they
854 should occur. This is important, as some of these change the color
855 type and/or bit depth of the data, and some others only work on
856 certain color types and bit depths. Even though each transformation
857 checks to see if it has data that it can do something with, you should
858 make sure to only enable a transformation if it will be valid for the
859 data. For example, don't swap red and blue on grayscale data.
861 The colors used for the background and transparency values should be
862 supplied in the same format/depth as the current image data. They
863 are stored in the same format/depth as the image data in a bKGD or tRNS
864 chunk, so this is what libpng expects for this data. The colors are
865 transformed to keep in sync with the image data when an application
866 calls the png_read_update_info() routine (see below).
868 Data will be decoded into the supplied row buffers packed into bytes
869 unless the library has been told to transform it into another format.
870 For example, 4 bit/pixel paletted or grayscale data will be returned
871 2 pixels/byte with the leftmost pixel in the high-order bits of the
872 byte, unless png_set_packing() is called. 8-bit RGB data will be stored
873 in RGB RGB RGB format unless png_set_filler() or png_set_add_alpha()
874 is called to insert filler bytes, either before or after each RGB triplet.
875 16-bit RGB data will be returned RRGGBB RRGGBB, with the most significant
876 byte of the color value first, unless png_set_strip_16() is called to
877 transform it to regular RGB RGB triplets, or png_set_filler() or
878 png_set_add alpha() is called to insert filler bytes, either before or
879 after each RRGGBB triplet. Similarly, 8-bit or 16-bit grayscale data can
881 png_set_filler(), png_set_add_alpha(), or png_set_strip_16().
883 The following code transforms grayscale images of less than 8 to 8 bits,
884 changes paletted images to RGB, and adds a full alpha channel if there is
885 transparency information in a tRNS chunk. This is most useful on
886 grayscale images with bit depths of 2 or 4 or if there is a multiple-image
887 viewing application that wishes to treat all images in the same way.
889 if (color_type == PNG_COLOR_TYPE_PALETTE)
890 png_set_palette_to_rgb(png_ptr);
892 if (color_type == PNG_COLOR_TYPE_GRAY &&
893 bit_depth < 8) png_set_expand_gray_1_2_4_to_8(png_ptr);
895 if (png_get_valid(png_ptr, info_ptr,
896 PNG_INFO_tRNS)) png_set_tRNS_to_alpha(png_ptr);
898 These three functions are actually aliases for png_set_expand(), added
899 in libpng version 1.0.4, with the function names expanded to improve code
900 readability. In some future version they may actually do different
903 As of libpng version 1.2.9, png_set_expand_gray_1_2_4_to_8() was
904 added. It expands the sample depth without changing tRNS to alpha.
906 As of libpng version 1.4.4, not all possible expansions are supported.
908 In the following table, the 01 means grayscale with depth<8, 31 means
909 indexed with depth<8, other numerals represent the color type, "T" means
910 the tRNS chunk is present, A means an alpha channel is present, and O
911 means tRNS or alpha is present but all pixels in the image are opaque.
913 FROM 01 31 0 0T 0O 2 2T 2O 3 3T 3O 4A 4O 6A 6O
932 "-" means the transformation is not supported.
933 "X" means the transformation is obtained by png_set_expand().
934 "1" means the transformation is obtained by
935 png_set_expand_gray_1_2_4_to_8
936 "G" means the transformation is obtained by
937 png_set_gray_to_rgb().
938 "P" means the transformation is obtained by
939 png_set_expand_palette_to_rgb().
940 "T" means the transformation is obtained by
941 png_set_tRNS_to_alpha().
943 PNG can have files with 16 bits per channel. If you only can handle
944 8 bits per channel, this will strip the pixels down to 8 bit.
947 png_set_strip_16(png_ptr);
949 If, for some reason, you don't need the alpha channel on an image,
950 and you want to remove it rather than combining it with the background
951 (but the image author certainly had in mind that you *would* combine
952 it with the background, so that's what you should probably do):
954 if (color_type & PNG_COLOR_MASK_ALPHA)
955 png_set_strip_alpha(png_ptr);
957 In PNG files, the alpha channel in an image
958 is the level of opacity. If you need the alpha channel in an image to
959 be the level of transparency instead of opacity, you can invert the
960 alpha channel (or the tRNS chunk data) after it's read, so that 0 is
961 fully opaque and 255 (in 8-bit or paletted images) or 65535 (in 16-bit
962 images) is fully transparent, with
964 png_set_invert_alpha(png_ptr);
966 PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
967 they can, resulting in, for example, 8 pixels per byte for 1 bit
968 files. This code expands to 1 pixel per byte without changing the
969 values of the pixels:
972 png_set_packing(png_ptr);
974 PNG files have possible bit depths of 1, 2, 4, 8, and 16. All pixels
975 stored in a PNG image have been "scaled" or "shifted" up to the next
976 higher possible bit depth (e.g. from 5 bits/sample in the range [0,31]
977 to 8 bits/sample in the range [0, 255]). However, it is also possible
978 to convert the PNG pixel data back to the original bit depth of the
979 image. This call reduces the pixels back down to the original bit depth:
981 png_color_8p sig_bit;
983 if (png_get_sBIT(png_ptr, info_ptr, &sig_bit))
984 png_set_shift(png_ptr, sig_bit);
986 PNG files store 3-color pixels in red, green, blue order. This code
987 changes the storage of the pixels to blue, green, red:
989 if (color_type == PNG_COLOR_TYPE_RGB ||
990 color_type == PNG_COLOR_TYPE_RGB_ALPHA)
991 png_set_bgr(png_ptr);
993 PNG files store RGB pixels packed into 3 or 6 bytes. This code expands them
994 into 4 or 8 bytes for windowing systems that need them in this format:
996 if (color_type == PNG_COLOR_TYPE_RGB)
997 png_set_filler(png_ptr, filler, PNG_FILLER_BEFORE);
999 where "filler" is the 8 or 16-bit number to fill with, and the location is
1000 either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether
1001 you want the filler before the RGB or after. This transformation
1002 does not affect images that already have full alpha channels. To add an
1003 opaque alpha channel, use filler=0xff or 0xffff and PNG_FILLER_AFTER which
1004 will generate RGBA pixels.
1006 Note that png_set_filler() does not change the color type. If you want
1007 to do that, you can add a true alpha channel with
1009 if (color_type == PNG_COLOR_TYPE_RGB ||
1010 color_type == PNG_COLOR_TYPE_GRAY)
1011 png_set_add_alpha(png_ptr, filler, PNG_FILLER_AFTER);
1013 where "filler" contains the alpha value to assign to each pixel.
1014 This function was added in libpng-1.2.7.
1016 If you are reading an image with an alpha channel, and you need the
1017 data as ARGB instead of the normal PNG format RGBA:
1019 if (color_type == PNG_COLOR_TYPE_RGB_ALPHA)
1020 png_set_swap_alpha(png_ptr);
1022 For some uses, you may want a grayscale image to be represented as
1023 RGB. This code will do that conversion:
1025 if (color_type == PNG_COLOR_TYPE_GRAY ||
1026 color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
1027 png_set_gray_to_rgb(png_ptr);
1029 Conversely, you can convert an RGB or RGBA image to grayscale or grayscale
1032 if (color_type == PNG_COLOR_TYPE_RGB ||
1033 color_type == PNG_COLOR_TYPE_RGB_ALPHA)
1034 png_set_rgb_to_gray_fixed(png_ptr, error_action,
1035 int red_weight, int green_weight);
1037 error_action = 1: silently do the conversion
1038 error_action = 2: issue a warning if the original
1039 image has any pixel where
1040 red != green or red != blue
1041 error_action = 3: issue an error and abort the
1042 conversion if the original
1043 image has any pixel where
1044 red != green or red != blue
1046 red_weight: weight of red component times 100000
1047 green_weight: weight of green component times 100000
1048 If either weight is negative, default
1049 weights (21268, 71514) are used.
1051 If you have set error_action = 1 or 2, you can
1052 later check whether the image really was gray, after processing
1053 the image rows, with the png_get_rgb_to_gray_status(png_ptr) function.
1054 It will return a png_byte that is zero if the image was gray or
1055 1 if there were any non-gray pixels. bKGD and sBIT data
1056 will be silently converted to grayscale, using the green channel
1057 data, regardless of the error_action setting.
1059 With red_weight+green_weight<=100000,
1060 the normalized graylevel is computed:
1062 int rw = red_weight * 65536;
1063 int gw = green_weight * 65536;
1064 int bw = 65536 - (rw + gw);
1065 gray = (rw*red + gw*green + bw*blue)/65536;
1067 The default values approximate those recommended in the Charles
1068 Poynton's Color FAQ, <http://www.inforamp.net/~poynton/>
1069 Copyright (c) 1998-01-04 Charles Poynton <poynton at inforamp.net>
1071 Y = 0.212671 * R + 0.715160 * G + 0.072169 * B
1073 Libpng approximates this with
1075 Y = 0.21268 * R + 0.7151 * G + 0.07217 * B
1077 which can be expressed with integers as
1079 Y = (6969 * R + 23434 * G + 2365 * B)/32768
1081 The calculation is done in a linear colorspace, if the image gamma
1084 If you have a grayscale and you are using png_set_expand_depth(),
1085 png_set_expand(), or png_set_gray_to_rgb to change to truecolor or to
1086 a higher bit-depth, you must either supply the background color as a gray
1087 value at the original file bit-depth (need_expand = 1) or else supply the
1088 background color as an RGB triplet at the final, expanded bit depth
1089 (need_expand = 0). Similarly, if you are reading a paletted image, you
1090 must either supply the background color as a palette index (need_expand = 1)
1091 or as an RGB triplet that may or may not be in the palette (need_expand = 0).
1093 png_color_16 my_background;
1094 png_color_16p image_background;
1096 if (png_get_bKGD(png_ptr, info_ptr, &image_background))
1097 png_set_background(png_ptr, image_background,
1098 PNG_BACKGROUND_GAMMA_FILE, 1, 1.0);
1100 png_set_background(png_ptr, &my_background,
1101 PNG_BACKGROUND_GAMMA_SCREEN, 0, 1.0);
1103 The png_set_background() function tells libpng to composite images
1104 with alpha or simple transparency against the supplied background
1105 color. If the PNG file contains a bKGD chunk (PNG_INFO_bKGD valid),
1106 you may use this color, or supply another color more suitable for
1107 the current display (e.g., the background color from a web page). You
1108 need to tell libpng whether the color is in the gamma space of the
1109 display (PNG_BACKGROUND_GAMMA_SCREEN for colors you supply), the file
1110 (PNG_BACKGROUND_GAMMA_FILE for colors from the bKGD chunk), or one
1111 that is neither of these gammas (PNG_BACKGROUND_GAMMA_UNIQUE - I don't
1112 know why anyone would use this, but it's here).
1114 To properly display PNG images on any kind of system, the application needs
1115 to know what the display gamma is. Ideally, the user will know this, and
1116 the application will allow them to set it. One method of allowing the user
1117 to set the display gamma separately for each system is to check for a
1118 SCREEN_GAMMA or DISPLAY_GAMMA environment variable, which will hopefully be
1121 Note that display_gamma is the overall gamma correction required to produce
1122 pleasing results, which depends on the lighting conditions in the surrounding
1123 environment. In a dim or brightly lit room, no compensation other than
1124 the physical gamma exponent of the monitor is needed, while in a dark room
1125 a slightly smaller exponent is better.
1127 double gamma, screen_gamma;
1129 if (/* We have a user-defined screen
1132 screen_gamma = user_defined_screen_gamma;
1134 /* One way that applications can share the same
1135 screen gamma value */
1136 else if ((gamma_str = getenv("SCREEN_GAMMA"))
1139 screen_gamma = (double)atof(gamma_str);
1141 /* If we don't have another value */
1144 screen_gamma = 2.2; /* A good guess for a
1145 PC monitor in a bright office or a dim room */
1146 screen_gamma = 2.0; /* A good guess for a
1147 PC monitor in a dark room */
1148 screen_gamma = 1.7 or 1.0; /* A good
1149 guess for Mac systems */
1152 The png_set_gamma() function handles gamma transformations of the data.
1153 Pass both the file gamma and the current screen_gamma. If the file does
1154 not have a gamma value, you can pass one anyway if you have an idea what
1155 it is (usually 0.45455 is a good guess for GIF images on PCs). Note
1156 that file gammas are inverted from screen gammas. See the discussions
1157 on gamma in the PNG specification for an excellent description of what
1158 gamma is, and why all applications should support it. It is strongly
1159 recommended that PNG viewers support gamma correction.
1161 if (png_get_gAMA(png_ptr, info_ptr, &gamma))
1162 png_set_gamma(png_ptr, screen_gamma, gamma);
1164 png_set_gamma(png_ptr, screen_gamma, 0.45455);
1166 If you need to reduce an RGB file to a paletted file, or if a paletted
1167 file has more entries then will fit on your screen, png_set_quantize()
1168 will do that. Note that this is a simple match dither that merely
1169 finds the closest color available. This should work fairly well with
1170 optimized palettes, and fairly badly with linear color cubes. If you
1171 pass a palette that is larger then maximum_colors, the file will
1172 reduce the number of colors in the palette so it will fit into
1173 maximum_colors. If there is a histogram, it will use it to make
1174 more intelligent choices when reducing the palette. If there is no
1175 histogram, it may not do as good a job.
1177 if (color_type & PNG_COLOR_MASK_COLOR)
1179 if (png_get_valid(png_ptr, info_ptr,
1182 png_uint_16p histogram = NULL;
1184 png_get_hIST(png_ptr, info_ptr,
1186 png_set_quantize(png_ptr, palette, num_palette,
1187 max_screen_colors, histogram, 1);
1191 png_color std_color_cube[MAX_SCREEN_COLORS] =
1194 png_set_quantize(png_ptr, std_color_cube,
1195 MAX_SCREEN_COLORS, MAX_SCREEN_COLORS,
1200 PNG files describe monochrome as black being zero and white being one.
1201 The following code will reverse this (make black be one and white be
1204 if (bit_depth == 1 && color_type == PNG_COLOR_TYPE_GRAY)
1205 png_set_invert_mono(png_ptr);
1207 This function can also be used to invert grayscale and gray-alpha images:
1209 if (color_type == PNG_COLOR_TYPE_GRAY ||
1210 color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
1211 png_set_invert_mono(png_ptr);
1213 PNG files store 16 bit pixels in network byte order (big-endian,
1214 ie. most significant bits first). This code changes the storage to the
1215 other way (little-endian, i.e. least significant bits first, the
1216 way PCs store them):
1218 if (bit_depth == 16)
1219 png_set_swap(png_ptr);
1221 If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you
1222 need to change the order the pixels are packed into bytes, you can use:
1225 png_set_packswap(png_ptr);
1227 Finally, you can write your own transformation function if none of
1228 the existing ones meets your needs. This is done by setting a callback
1231 png_set_read_user_transform_fn(png_ptr,
1234 You must supply the function
1236 void read_transform_fn(png_ptr ptr, row_info_ptr
1237 row_info, png_bytep data)
1239 See pngtest.c for a working example. Your function will be called
1240 after all of the other transformations have been processed.
1242 You can also set up a pointer to a user structure for use by your
1243 callback function, and you can inform libpng that your transform
1244 function will change the number of channels or bit depth with the
1247 png_set_user_transform_info(png_ptr, user_ptr,
1248 user_depth, user_channels);
1250 The user's application, not libpng, is responsible for allocating and
1251 freeing any memory required for the user structure.
1253 You can retrieve the pointer via the function
1254 png_get_user_transform_ptr(). For example:
1256 voidp read_user_transform_ptr =
1257 png_get_user_transform_ptr(png_ptr);
1259 The last thing to handle is interlacing; this is covered in detail below,
1260 but you must call the function here if you want libpng to handle expansion
1261 of the interlaced image.
1263 number_of_passes = png_set_interlace_handling(png_ptr);
1265 After setting the transformations, libpng can update your png_info
1266 structure to reflect any transformations you've requested with this
1267 call. This is most useful to update the info structure's rowbytes
1268 field so you can use it to allocate your image memory. This function
1269 will also update your palette with the correct screen_gamma and
1270 background if these have been given with the calls above.
1272 png_read_update_info(png_ptr, info_ptr);
1274 After you call png_read_update_info(), you can allocate any
1275 memory you need to hold the image. The row data is simply
1276 raw byte data for all forms of images. As the actual allocation
1277 varies among applications, no example will be given. If you
1278 are allocating one large chunk, you will need to build an
1279 array of pointers to each row, as it will be needed for some
1280 of the functions below.
1284 After you've allocated memory, you can read the image data.
1285 The simplest way to do this is in one function call. If you are
1286 allocating enough memory to hold the whole image, you can just
1287 call png_read_image() and libpng will read in all the image data
1288 and put it in the memory area supplied. You will need to pass in
1289 an array of pointers to each row.
1291 This function automatically handles interlacing, so you don't need
1292 to call png_set_interlace_handling() or call this function multiple
1293 times, or any of that other stuff necessary with png_read_rows().
1295 png_read_image(png_ptr, row_pointers);
1297 where row_pointers is:
1299 png_bytep row_pointers[height];
1301 You can point to void or char or whatever you use for pixels.
1303 If you don't want to read in the whole image at once, you can
1304 use png_read_rows() instead. If there is no interlacing (check
1305 interlace_type == PNG_INTERLACE_NONE), this is simple:
1307 png_read_rows(png_ptr, row_pointers, NULL,
1310 where row_pointers is the same as in the png_read_image() call.
1312 If you are doing this just one row at a time, you can do this with
1313 a single row_pointer instead of an array of row_pointers:
1315 png_bytep row_pointer = row;
1316 png_read_row(png_ptr, row_pointer, NULL);
1318 If the file is interlaced (interlace_type != 0 in the IHDR chunk), things
1319 get somewhat harder. The only current (PNG Specification version 1.2)
1320 interlacing type for PNG is (interlace_type == PNG_INTERLACE_ADAM7)
1321 is a somewhat complicated 2D interlace scheme, known as Adam7, that
1322 breaks down an image into seven smaller images of varying size, based
1325 libpng can fill out those images or it can give them to you "as is".
1326 If you want them filled out, there are two ways to do that. The one
1327 mentioned in the PNG specification is to expand each pixel to cover
1328 those pixels that have not been read yet (the "rectangle" method).
1329 This results in a blocky image for the first pass, which gradually
1330 smooths out as more pixels are read. The other method is the "sparkle"
1331 method, where pixels are drawn only in their final locations, with the
1332 rest of the image remaining whatever colors they were initialized to
1333 before the start of the read. The first method usually looks better,
1334 but tends to be slower, as there are more pixels to put in the rows.
1336 If you don't want libpng to handle the interlacing details, just call
1337 png_read_rows() seven times to read in all seven images. Each of the
1338 images is a valid image by itself, or they can all be combined on an
1339 8x8 grid to form a single image (although if you intend to combine them
1340 you would be far better off using the libpng interlace handling).
1342 The first pass will return an image 1/8 as wide as the entire image
1343 (every 8th column starting in column 0) and 1/8 as high as the original
1344 (every 8th row starting in row 0), the second will be 1/8 as wide
1345 (starting in column 4) and 1/8 as high (also starting in row 0). The
1346 third pass will be 1/4 as wide (every 4th pixel starting in column 0) and
1347 1/8 as high (every 8th row starting in row 4), and the fourth pass will
1348 be 1/4 as wide and 1/4 as high (every 4th column starting in column 2,
1349 and every 4th row starting in row 0). The fifth pass will return an
1350 image 1/2 as wide, and 1/4 as high (starting at column 0 and row 2),
1351 while the sixth pass will be 1/2 as wide and 1/2 as high as the original
1352 (starting in column 1 and row 0). The seventh and final pass will be as
1353 wide as the original, and 1/2 as high, containing all of the odd
1354 numbered scanlines. Phew!
1356 If you want libpng to expand the images, call this before calling
1357 png_start_read_image() or png_read_update_info():
1359 if (interlace_type == PNG_INTERLACE_ADAM7)
1361 = png_set_interlace_handling(png_ptr);
1363 This will return the number of passes needed. Currently, this
1364 is seven, but may change if another interlace type is added.
1365 This function can be called even if the file is not interlaced,
1366 where it will return one pass.
1368 If you are not going to display the image after each pass, but are
1369 going to wait until the entire image is read in, use the sparkle
1370 effect. This effect is faster and the end result of either method
1371 is exactly the same. If you are planning on displaying the image
1372 after each pass, the "rectangle" effect is generally considered the
1375 If you only want the "sparkle" effect, just call png_read_rows() as
1376 normal, with the third parameter NULL. Make sure you make pass over
1377 the image number_of_passes times, and you don't change the data in the
1378 rows between calls. You can change the locations of the data, just
1379 not the data. Each pass only writes the pixels appropriate for that
1380 pass, and assumes the data from previous passes is still valid.
1382 png_read_rows(png_ptr, row_pointers, NULL,
1385 If you only want the first effect (the rectangles), do the same as
1386 before except pass the row buffer in the third parameter, and leave
1387 the second parameter NULL.
1389 png_read_rows(png_ptr, NULL, row_pointers,
1392 Finishing a sequential read
1394 After you are finished reading the image through the
1395 low-level interface, you can finish reading the file. If you are
1396 interested in comments or time, which may be stored either before or
1397 after the image data, you should pass the separate png_info struct if
1398 you want to keep the comments from before and after the image
1399 separate. If you are not interested, you can pass NULL.
1401 png_read_end(png_ptr, end_info);
1403 When you are done, you can free all memory allocated by libpng like this:
1405 png_destroy_read_struct(&png_ptr, &info_ptr,
1408 It is also possible to individually free the info_ptr members that
1409 point to libpng-allocated storage with the following function:
1411 png_free_data(png_ptr, info_ptr, mask, seq)
1412 mask - identifies data to be freed, a mask
1413 containing the bitwise OR of one or
1415 PNG_FREE_PLTE, PNG_FREE_TRNS,
1416 PNG_FREE_HIST, PNG_FREE_ICCP,
1417 PNG_FREE_PCAL, PNG_FREE_ROWS,
1418 PNG_FREE_SCAL, PNG_FREE_SPLT,
1419 PNG_FREE_TEXT, PNG_FREE_UNKN,
1420 or simply PNG_FREE_ALL
1421 seq - sequence number of item to be freed
1424 This function may be safely called when the relevant storage has
1425 already been freed, or has not yet been allocated, or was allocated
1426 by the user and not by libpng, and will in those cases do nothing.
1427 The "seq" parameter is ignored if only one item of the selected data
1428 type, such as PLTE, is allowed. If "seq" is not -1, and multiple items
1429 are allowed for the data type identified in the mask, such as text or
1430 sPLT, only the n'th item in the structure is freed, where n is "seq".
1432 The default behavior is only to free data that was allocated internally
1433 by libpng. This can be changed, so that libpng will not free the data,
1434 or so that it will free data that was allocated by the user with png_malloc()
1435 or png_zalloc() and passed in via a png_set_*() function, with
1437 png_data_freer(png_ptr, info_ptr, freer, mask)
1438 mask - which data elements are affected
1439 same choices as in png_free_data()
1441 PNG_DESTROY_WILL_FREE_DATA
1442 PNG_SET_WILL_FREE_DATA
1443 PNG_USER_WILL_FREE_DATA
1445 This function only affects data that has already been allocated.
1446 You can call this function after reading the PNG data but before calling
1447 any png_set_*() functions, to control whether the user or the png_set_*()
1448 function is responsible for freeing any existing data that might be present,
1449 and again after the png_set_*() functions to control whether the user
1450 or png_destroy_*() is supposed to free the data. When the user assumes
1451 responsibility for libpng-allocated data, the application must use
1452 png_free() to free it, and when the user transfers responsibility to libpng
1453 for data that the user has allocated, the user must have used png_malloc()
1454 or png_zalloc() to allocate it.
1456 If you allocated your row_pointers in a single block, as suggested above in
1457 the description of the high level read interface, you must not transfer
1458 responsibility for freeing it to the png_set_rows or png_read_destroy function,
1459 because they would also try to free the individual row_pointers[i].
1461 If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword
1462 separately, do not transfer responsibility for freeing text_ptr to libpng,
1463 because when libpng fills a png_text structure it combines these members with
1464 the key member, and png_free_data() will free only text_ptr.key. Similarly,
1465 if you transfer responsibility for free'ing text_ptr from libpng to your
1466 application, your application must not separately free those members.
1468 The png_free_data() function will turn off the "valid" flag for anything
1469 it frees. If you need to turn the flag off for a chunk that was freed by
1470 your application instead of by libpng, you can use
1472 png_set_invalid(png_ptr, info_ptr, mask);
1473 mask - identifies the chunks to be made invalid,
1474 containing the bitwise OR of one or
1476 PNG_INFO_gAMA, PNG_INFO_sBIT,
1477 PNG_INFO_cHRM, PNG_INFO_PLTE,
1478 PNG_INFO_tRNS, PNG_INFO_bKGD,
1479 PNG_INFO_hIST, PNG_INFO_pHYs,
1480 PNG_INFO_oFFs, PNG_INFO_tIME,
1481 PNG_INFO_pCAL, PNG_INFO_sRGB,
1482 PNG_INFO_iCCP, PNG_INFO_sPLT,
1483 PNG_INFO_sCAL, PNG_INFO_IDAT
1485 For a more compact example of reading a PNG image, see the file example.c.
1487 Reading PNG files progressively
1489 The progressive reader is slightly different then the non-progressive
1490 reader. Instead of calling png_read_info(), png_read_rows(), and
1491 png_read_end(), you make one call to png_process_data(), which calls
1492 callbacks when it has the info, a row, or the end of the image. You
1493 set up these callbacks with png_set_progressive_read_fn(). You don't
1494 have to worry about the input/output functions of libpng, as you are
1495 giving the library the data directly in png_process_data(). I will
1496 assume that you have read the section on reading PNG files above,
1497 so I will only highlight the differences (although I will show
1500 png_structp png_ptr;
1503 /* An example code fragment of how you would
1504 initialize the progressive reader in your
1507 initialize_png_reader()
1509 png_ptr = png_create_read_struct
1510 (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
1511 user_error_fn, user_warning_fn);
1514 info_ptr = png_create_info_struct(png_ptr);
1517 png_destroy_read_struct(&png_ptr, (png_infopp)NULL,
1522 if (setjmp(png_jmpbuf(png_ptr)))
1524 png_destroy_read_struct(&png_ptr, &info_ptr,
1529 /* This one's new. You can provide functions
1530 to be called when the header info is valid,
1531 when each row is completed, and when the image
1532 is finished. If you aren't using all functions,
1533 you can specify NULL parameters. Even when all
1534 three functions are NULL, you need to call
1535 png_set_progressive_read_fn(). You can use
1536 any struct as the user_ptr (cast to a void pointer
1537 for the function call), and retrieve the pointer
1538 from inside the callbacks using the function
1540 png_get_progressive_ptr(png_ptr);
1542 which will return a void pointer, which you have
1543 to cast appropriately.
1545 png_set_progressive_read_fn(png_ptr, (void *)user_ptr,
1546 info_callback, row_callback, end_callback);
1551 /* A code fragment that you call as you receive blocks
1554 process_data(png_bytep buffer, png_uint_32 length)
1556 if (setjmp(png_jmpbuf(png_ptr)))
1558 png_destroy_read_struct(&png_ptr, &info_ptr,
1563 /* This one's new also. Simply give it a chunk
1564 of data from the file stream (in order, of
1565 course). On machines with segmented memory
1566 models machines, don't give it any more than
1567 64K. The library seems to run fine with sizes
1568 of 4K. Although you can give it much less if
1569 necessary (I assume you can give it chunks of
1570 1 byte, I haven't tried less then 256 bytes
1571 yet). When this function returns, you may
1572 want to display any rows that were generated
1573 in the row callback if you don't already do
1576 png_process_data(png_ptr, info_ptr, buffer, length);
1580 /* This function is called (as set by
1581 png_set_progressive_read_fn() above) when enough data
1582 has been supplied so all of the header has been
1586 info_callback(png_structp png_ptr, png_infop info)
1588 /* Do any setup here, including setting any of
1589 the transformations mentioned in the Reading
1590 PNG files section. For now, you _must_ call
1591 either png_start_read_image() or
1592 png_read_update_info() after all the
1593 transformations are set (even if you don't set
1594 any). You may start getting rows before
1595 png_process_data() returns, so this is your
1596 last chance to prepare for that.
1600 /* This function is called when each row of image
1603 row_callback(png_structp png_ptr, png_bytep new_row,
1604 png_uint_32 row_num, int pass)
1606 /* If the image is interlaced, and you turned
1607 on the interlace handler, this function will
1608 be called for every row in every pass. Some
1609 of these rows will not be changed from the
1610 previous pass. When the row is not changed,
1611 the new_row variable will be NULL. The rows
1612 and passes are called in order, so you don't
1613 really need the row_num and pass, but I'm
1614 supplying them because it may make your life
1617 For the non-NULL rows of interlaced images,
1618 you must call png_progressive_combine_row()
1619 passing in the row and the old row. You can
1620 call this function for NULL rows (it will just
1621 return) and for non-interlaced images (it just
1622 does the memcpy for you) if it will make the
1623 code easier. Thus, you can just do this for
1627 png_progressive_combine_row(png_ptr, old_row,
1630 /* where old_row is what was displayed for
1631 previously for the row. Note that the first
1632 pass (pass == 0, really) will completely cover
1633 the old row, so the rows do not have to be
1634 initialized. After the first pass (and only
1635 for interlaced images), you will have to pass
1636 the current row, and the function will combine
1637 the old row and the new row.
1642 end_callback(png_structp png_ptr, png_infop info)
1644 /* This function is called after the whole image
1645 has been read, including any chunks after the
1646 image (up to and including the IEND). You
1647 will usually have the same info chunk as you
1648 had in the header, although some data may have
1649 been added to the comments and time fields.
1651 Most people won't do much here, perhaps setting
1652 a flag that marks the image as finished.
1660 Much of this is very similar to reading. However, everything of
1661 importance is repeated here, so you won't have to constantly look
1662 back up in the reading section to understand writing.
1666 You will want to do the I/O initialization before you get into libpng,
1667 so if it doesn't work, you don't have anything to undo. If you are not
1668 using the standard I/O functions, you will need to replace them with
1669 custom writing functions. See the discussion under Customizing libpng.
1671 FILE *fp = fopen(file_name, "wb");
1677 Next, png_struct and png_info need to be allocated and initialized.
1678 As these can be both relatively large, you may not want to store these
1679 on the stack, unless you have stack space to spare. Of course, you
1680 will want to check if they return NULL. If you are also reading,
1681 you won't want to name your read structure and your write structure
1682 both "png_ptr"; you can call them anything you like, such as
1683 "read_ptr" and "write_ptr". Look at pngtest.c, for example.
1685 png_structp png_ptr = png_create_write_struct
1686 (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
1687 user_error_fn, user_warning_fn);
1691 png_infop info_ptr = png_create_info_struct(png_ptr);
1694 png_destroy_write_struct(&png_ptr,
1699 If you want to use your own memory allocation routines,
1700 define PNG_USER_MEM_SUPPORTED and use
1701 png_create_write_struct_2() instead of png_create_write_struct():
1703 png_structp png_ptr = png_create_write_struct_2
1704 (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
1705 user_error_fn, user_warning_fn, (png_voidp)
1706 user_mem_ptr, user_malloc_fn, user_free_fn);
1708 After you have these structures, you will need to set up the
1709 error handling. When libpng encounters an error, it expects to
1710 longjmp() back to your routine. Therefore, you will need to call
1711 setjmp() and pass the png_jmpbuf(png_ptr). If you
1712 write the file from different routines, you will need to update
1713 the png_jmpbuf(png_ptr) every time you enter a new routine that will
1714 call a png_*() function. See your documentation of setjmp/longjmp
1715 for your compiler for more information on setjmp/longjmp. See
1716 the discussion on libpng error handling in the Customizing Libpng
1717 section below for more information on the libpng error handling.
1719 if (setjmp(png_jmpbuf(png_ptr)))
1721 png_destroy_write_struct(&png_ptr, &info_ptr);
1728 If you would rather avoid the complexity of setjmp/longjmp issues,
1729 you can compile libpng with PNG_NO_SETJMP, in which case
1730 errors will result in a call to PNG_ABORT() which defaults to abort().
1732 You can #define PNG_ABORT() to a function that does something
1733 more useful than abort(), as long as your function does not
1736 Now you need to set up the output code. The default for libpng is to
1737 use the C function fwrite(). If you use this, you will need to pass a
1738 valid FILE * in the function png_init_io(). Be sure that the file is
1739 opened in binary mode. Again, if you wish to handle writing data in
1740 another way, see the discussion on libpng I/O handling in the Customizing
1741 Libpng section below.
1743 png_init_io(png_ptr, fp);
1745 If you are embedding your PNG into a datastream such as MNG, and don't
1746 want libpng to write the 8-byte signature, or if you have already
1747 written the signature in your application, use
1749 png_set_sig_bytes(png_ptr, 8);
1751 to inform libpng that it should not write a signature.
1755 At this point, you can set up a callback function that will be
1756 called after each row has been written, which you can use to control
1757 a progress meter or the like. It's demonstrated in pngtest.c.
1758 You must supply a function
1760 void write_row_callback(png_ptr, png_uint_32 row,
1763 /* put your code here */
1766 (You can give it another name that you like instead of "write_row_callback")
1768 To inform libpng about your function, use
1770 png_set_write_status_fn(png_ptr, write_row_callback);
1772 You now have the option of modifying how the compression library will
1773 run. The following functions are mainly for testing, but may be useful
1774 in some cases, like if you need to write PNG files extremely fast and
1775 are willing to give up some compression, or if you want to get the
1776 maximum possible compression at the expense of slower writing. If you
1777 have no special needs in this area, let the library do what it wants by
1778 not calling this function at all, as it has been tuned to deliver a good
1779 speed/compression ratio. The second parameter to png_set_filter() is
1780 the filter method, for which the only valid values are 0 (as of the
1781 July 1999 PNG specification, version 1.2) or 64 (if you are writing
1782 a PNG datastream that is to be embedded in a MNG datastream). The third
1783 parameter is a flag that indicates which filter type(s) are to be tested
1784 for each scanline. See the PNG specification for details on the specific
1788 /* turn on or off filtering, and/or choose
1789 specific filters. You can use either a single
1790 PNG_FILTER_VALUE_NAME or the bitwise OR of one
1791 or more PNG_FILTER_NAME masks. */
1792 png_set_filter(png_ptr, 0,
1793 PNG_FILTER_NONE | PNG_FILTER_VALUE_NONE |
1794 PNG_FILTER_SUB | PNG_FILTER_VALUE_SUB |
1795 PNG_FILTER_UP | PNG_FILTER_VALUE_UP |
1796 PNG_FILTER_AVG | PNG_FILTER_VALUE_AVG |
1797 PNG_FILTER_PAETH | PNG_FILTER_VALUE_PAETH|
1801 wants to start and stop using particular filters during compression,
1802 it should start out with all of the filters (to ensure that the previous
1803 row of pixels will be stored in case it's needed later), and then add
1804 and remove them after the start of compression.
1806 If you are writing a PNG datastream that is to be embedded in a MNG
1807 datastream, the second parameter can be either 0 or 64.
1809 The png_set_compression_*() functions interface to the zlib compression
1810 library, and should mostly be ignored unless you really know what you are
1811 doing. The only generally useful call is png_set_compression_level()
1812 which changes how much time zlib spends on trying to compress the image
1813 data. See the Compression Library (zlib.h and algorithm.txt, distributed
1814 with zlib) for details on the compression levels.
1816 /* set the zlib compression level */
1817 png_set_compression_level(png_ptr,
1818 Z_BEST_COMPRESSION);
1820 /* set other zlib parameters */
1821 png_set_compression_mem_level(png_ptr, 8);
1822 png_set_compression_strategy(png_ptr,
1823 Z_DEFAULT_STRATEGY);
1824 png_set_compression_window_bits(png_ptr, 15);
1825 png_set_compression_method(png_ptr, 8);
1826 png_set_compression_buffer_size(png_ptr, 8192)
1828 extern PNG_EXPORT(void,png_set_zbuf_size)
1830 Setting the contents of info for output
1832 You now need to fill in the png_info structure with all the data you
1833 wish to write before the actual image. Note that the only thing you
1834 are allowed to write after the image is the text chunks and the time
1835 chunk (as of PNG Specification 1.2, anyway). See png_write_end() and
1836 the latest PNG specification for more information on that. If you
1837 wish to write them before the image, fill them in now, and flag that
1838 data as being valid. If you want to wait until after the data, don't
1839 fill them until png_write_end(). For all the fields in png_info and
1840 their data types, see png.h. For explanations of what the fields
1841 contain, see the PNG specification.
1843 Some of the more important parts of the png_info are:
1845 png_set_IHDR(png_ptr, info_ptr, width, height,
1846 bit_depth, color_type, interlace_type,
1847 compression_type, filter_method)
1848 width - holds the width of the image
1849 in pixels (up to 2^31).
1850 height - holds the height of the image
1851 in pixels (up to 2^31).
1852 bit_depth - holds the bit depth of one of the
1854 (valid values are 1, 2, 4, 8, 16
1855 and depend also on the
1856 color_type. See also significant
1858 color_type - describes which color/alpha
1859 channels are present.
1861 (bit depths 1, 2, 4, 8, 16)
1862 PNG_COLOR_TYPE_GRAY_ALPHA
1864 PNG_COLOR_TYPE_PALETTE
1865 (bit depths 1, 2, 4, 8)
1868 PNG_COLOR_TYPE_RGB_ALPHA
1871 PNG_COLOR_MASK_PALETTE
1872 PNG_COLOR_MASK_COLOR
1873 PNG_COLOR_MASK_ALPHA
1875 interlace_type - PNG_INTERLACE_NONE or
1877 compression_type - (must be
1878 PNG_COMPRESSION_TYPE_DEFAULT)
1879 filter_method - (must be PNG_FILTER_TYPE_DEFAULT
1880 or, if you are writing a PNG to
1881 be embedded in a MNG datastream,
1883 PNG_INTRAPIXEL_DIFFERENCING)
1885 If you call png_set_IHDR(), the call must appear before any of the
1886 other png_set_*() functions, because they might require access to some of
1887 the IHDR settings. The remaining png_set_*() functions can be called
1890 If you wish, you can reset the compression_type, interlace_type, or
1891 filter_method later by calling png_set_IHDR() again; if you do this, the
1892 width, height, bit_depth, and color_type must be the same in each call.
1894 png_set_PLTE(png_ptr, info_ptr, palette,
1896 palette - the palette for the file
1897 (array of png_color)
1898 num_palette - number of entries in the palette
1900 png_set_gAMA(png_ptr, info_ptr, gamma);
1901 gamma - the gamma the image was created
1904 png_set_sRGB(png_ptr, info_ptr, srgb_intent);
1905 srgb_intent - the rendering intent
1906 (PNG_INFO_sRGB) The presence of
1907 the sRGB chunk means that the pixel
1908 data is in the sRGB color space.
1909 This chunk also implies specific
1910 values of gAMA and cHRM. Rendering
1911 intent is the CSS-1 property that
1912 has been defined by the International
1914 (http://www.color.org).
1916 PNG_sRGB_INTENT_SATURATION,
1917 PNG_sRGB_INTENT_PERCEPTUAL,
1918 PNG_sRGB_INTENT_ABSOLUTE, or
1919 PNG_sRGB_INTENT_RELATIVE.
1922 png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr,
1924 srgb_intent - the rendering intent
1925 (PNG_INFO_sRGB) The presence of the
1926 sRGB chunk means that the pixel
1927 data is in the sRGB color space.
1928 This function also causes gAMA and
1929 cHRM chunks with the specific values
1930 that are consistent with sRGB to be
1933 png_set_iCCP(png_ptr, info_ptr, name, compression_type,
1935 name - The profile name.
1936 compression - The compression type; always
1937 PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
1938 You may give NULL to this argument to
1940 profile - International Color Consortium color
1941 profile data. May contain NULs.
1942 proflen - length of profile data in bytes.
1944 png_set_sBIT(png_ptr, info_ptr, sig_bit);
1945 sig_bit - the number of significant bits for
1946 (PNG_INFO_sBIT) each of the gray, red,
1947 green, and blue channels, whichever are
1948 appropriate for the given color type
1951 png_set_tRNS(png_ptr, info_ptr, trans_alpha,
1952 num_trans, trans_color);
1953 trans_alpha - array of alpha (transparency)
1954 entries for palette (PNG_INFO_tRNS)
1955 trans_color - graylevel or color sample values
1956 (in order red, green, blue) of the
1957 single transparent color for
1958 non-paletted images (PNG_INFO_tRNS)
1959 num_trans - number of transparent entries
1962 png_set_hIST(png_ptr, info_ptr, hist);
1964 hist - histogram of palette (array of
1967 png_set_tIME(png_ptr, info_ptr, mod_time);
1968 mod_time - time image was last modified
1971 png_set_bKGD(png_ptr, info_ptr, background);
1972 background - background color (PNG_VALID_bKGD)
1974 png_set_text(png_ptr, info_ptr, text_ptr, num_text);
1975 text_ptr - array of png_text holding image
1977 text_ptr[i].compression - type of compression used
1978 on "text" PNG_TEXT_COMPRESSION_NONE
1979 PNG_TEXT_COMPRESSION_zTXt
1980 PNG_ITXT_COMPRESSION_NONE
1981 PNG_ITXT_COMPRESSION_zTXt
1982 text_ptr[i].key - keyword for comment. Must contain
1984 text_ptr[i].text - text comments for current
1985 keyword. Can be NULL or empty.
1986 text_ptr[i].text_length - length of text string,
1987 after decompression, 0 for iTXt
1988 text_ptr[i].itxt_length - length of itxt string,
1989 after decompression, 0 for tEXt/zTXt
1990 text_ptr[i].lang - language of comment (NULL or
1992 text_ptr[i].translated_keyword - keyword in UTF-8 (NULL
1993 or empty for unknown).
1994 Note that the itxt_length, lang, and lang_key
1995 members of the text_ptr structure only exist
1996 when the library is built with iTXt chunk support.
1998 num_text - number of comments
2000 png_set_sPLT(png_ptr, info_ptr, &palette_ptr,
2002 palette_ptr - array of png_sPLT_struct structures
2003 to be added to the list of palettes
2004 in the info structure.
2005 num_spalettes - number of palette structures to be
2008 png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y,
2010 offset_x - positive offset from the left
2012 offset_y - positive offset from the top
2014 unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER
2016 png_set_pHYs(png_ptr, info_ptr, res_x, res_y,
2018 res_x - pixels/unit physical resolution
2020 res_y - pixels/unit physical resolution
2022 unit_type - PNG_RESOLUTION_UNKNOWN,
2023 PNG_RESOLUTION_METER
2025 png_set_sCAL(png_ptr, info_ptr, unit, width, height)
2026 unit - physical scale units (an integer)
2027 width - width of a pixel in physical scale units
2028 height - height of a pixel in physical scale units
2029 (width and height are doubles)
2031 png_set_sCAL_s(png_ptr, info_ptr, unit, width, height)
2032 unit - physical scale units (an integer)
2033 width - width of a pixel in physical scale units
2034 height - height of a pixel in physical scale units
2035 (width and height are strings like "2.54")
2037 png_set_unknown_chunks(png_ptr, info_ptr, &unknowns,
2039 unknowns - array of png_unknown_chunk
2040 structures holding unknown chunks
2041 unknowns[i].name - name of unknown chunk
2042 unknowns[i].data - data of unknown chunk
2043 unknowns[i].size - size of unknown chunk's data
2044 unknowns[i].location - position to write chunk in file
2045 0: do not write chunk
2046 PNG_HAVE_IHDR: before PLTE
2047 PNG_HAVE_PLTE: before IDAT
2048 PNG_AFTER_IDAT: after IDAT
2050 The "location" member is set automatically according to
2051 what part of the output file has already been written.
2052 You can change its value after calling png_set_unknown_chunks()
2053 as demonstrated in pngtest.c. Within each of the "locations",
2054 the chunks are sequenced according to their position in the
2055 structure (that is, the value of "i", which is the order in which
2056 the chunk was either read from the input file or defined with
2057 png_set_unknown_chunks).
2059 A quick word about text and num_text. text is an array of png_text
2060 structures. num_text is the number of valid structures in the array.
2061 Each png_text structure holds a language code, a keyword, a text value,
2062 and a compression type.
2064 The compression types have the same valid numbers as the compression
2065 types of the image data. Currently, the only valid number is zero.
2066 However, you can store text either compressed or uncompressed, unlike
2067 images, which always have to be compressed. So if you don't want the
2068 text compressed, set the compression type to PNG_TEXT_COMPRESSION_NONE.
2069 Because tEXt and zTXt chunks don't have a language field, if you
2070 specify PNG_TEXT_COMPRESSION_NONE or PNG_TEXT_COMPRESSION_zTXt
2071 any language code or translated keyword will not be written out.
2073 Until text gets around 1000 bytes, it is not worth compressing it.
2074 After the text has been written out to the file, the compression type
2075 is set to PNG_TEXT_COMPRESSION_NONE_WR or PNG_TEXT_COMPRESSION_zTXt_WR,
2076 so that it isn't written out again at the end (in case you are calling
2077 png_write_end() with the same struct.
2079 The keywords that are given in the PNG Specification are:
2081 Title Short (one line) title or
2083 Author Name of image's creator
2084 Description Description of image (possibly long)
2085 Copyright Copyright notice
2086 Creation Time Time of original image creation
2087 (usually RFC 1123 format, see below)
2088 Software Software used to create the image
2089 Disclaimer Legal disclaimer
2090 Warning Warning of nature of content
2091 Source Device used to create the image
2092 Comment Miscellaneous comment; conversion
2093 from other image format
2095 The keyword-text pairs work like this. Keywords should be short
2096 simple descriptions of what the comment is about. Some typical
2097 keywords are found in the PNG specification, as is some recommendations
2098 on keywords. You can repeat keywords in a file. You can even write
2099 some text before the image and some after. For example, you may want
2100 to put a description of the image before the image, but leave the
2101 disclaimer until after, so viewers working over modem connections
2102 don't have to wait for the disclaimer to go over the modem before
2103 they start seeing the image. Finally, keywords should be full
2104 words, not abbreviations. Keywords and text are in the ISO 8859-1
2105 (Latin-1) character set (a superset of regular ASCII) and can not
2106 contain NUL characters, and should not contain control or other
2107 unprintable characters. To make the comments widely readable, stick
2108 with basic ASCII, and avoid machine specific character set extensions
2109 like the IBM-PC character set. The keyword must be present, but
2110 you can leave off the text string on non-compressed pairs.
2111 Compressed pairs must have a text string, as only the text string
2112 is compressed anyway, so the compression would be meaningless.
2114 PNG supports modification time via the png_time structure. Two
2115 conversion routines are provided, png_convert_from_time_t() for
2116 time_t and png_convert_from_struct_tm() for struct tm. The
2117 time_t routine uses gmtime(). You don't have to use either of
2118 these, but if you wish to fill in the png_time structure directly,
2119 you should provide the time in universal time (GMT) if possible
2120 instead of your local time. Note that the year number is the full
2121 year (e.g. 1998, rather than 98 - PNG is year 2000 compliant!), and
2122 that months start with 1.
2124 If you want to store the time of the original image creation, you should
2125 use a plain tEXt chunk with the "Creation Time" keyword. This is
2126 necessary because the "creation time" of a PNG image is somewhat vague,
2127 depending on whether you mean the PNG file, the time the image was
2128 created in a non-PNG format, a still photo from which the image was
2129 scanned, or possibly the subject matter itself. In order to facilitate
2130 machine-readable dates, it is recommended that the "Creation Time"
2131 tEXt chunk use RFC 1123 format dates (e.g. "22 May 1997 18:07:10 GMT"),
2132 although this isn't a requirement. Unlike the tIME chunk, the
2133 "Creation Time" tEXt chunk is not expected to be automatically changed
2134 by the software. To facilitate the use of RFC 1123 dates, a function
2135 png_convert_to_rfc1123(png_timep) is provided to convert from PNG
2136 time to an RFC 1123 format string.
2138 Writing unknown chunks
2140 You can use the png_set_unknown_chunks function to queue up chunks
2141 for writing. You give it a chunk name, raw data, and a size; that's
2142 all there is to it. The chunks will be written by the next following
2143 png_write_info_before_PLTE, png_write_info, or png_write_end function.
2144 Any chunks previously read into the info structure's unknown-chunk
2145 list will also be written out in a sequence that satisfies the PNG
2146 specification's ordering rules.
2148 The high-level write interface
2150 At this point there are two ways to proceed; through the high-level
2151 write interface, or through a sequence of low-level write operations.
2152 You can use the high-level interface if your image data is present
2153 in the info structure. All defined output
2154 transformations are permitted, enabled by the following masks.
2156 PNG_TRANSFORM_IDENTITY No transformation
2157 PNG_TRANSFORM_PACKING Pack 1, 2 and 4-bit samples
2158 PNG_TRANSFORM_PACKSWAP Change order of packed
2160 PNG_TRANSFORM_INVERT_MONO Invert monochrome images
2161 PNG_TRANSFORM_SHIFT Normalize pixels to the
2163 PNG_TRANSFORM_BGR Flip RGB to BGR, RGBA
2165 PNG_TRANSFORM_SWAP_ALPHA Flip RGBA to ARGB or GA
2167 PNG_TRANSFORM_INVERT_ALPHA Change alpha from opacity
2169 PNG_TRANSFORM_SWAP_ENDIAN Byte-swap 16-bit samples
2170 PNG_TRANSFORM_STRIP_FILLER Strip out filler
2172 PNG_TRANSFORM_STRIP_FILLER_BEFORE Strip out leading
2174 PNG_TRANSFORM_STRIP_FILLER_AFTER Strip out trailing
2177 If you have valid image data in the info structure (you can use
2178 png_set_rows() to put image data in the info structure), simply do this:
2180 png_write_png(png_ptr, info_ptr, png_transforms, NULL)
2182 where png_transforms is an integer containing the bitwise OR of some set of
2183 transformation flags. This call is equivalent to png_write_info(),
2184 followed the set of transformations indicated by the transform mask,
2185 then png_write_image(), and finally png_write_end().
2187 (The final parameter of this call is not yet used. Someday it might point
2188 to transformation parameters required by some future output transform.)
2190 You must use png_transforms and not call any png_set_transform() functions
2191 when you use png_write_png().
2193 The low-level write interface
2195 If you are going the low-level route instead, you are now ready to
2196 write all the file information up to the actual image data. You do
2197 this with a call to png_write_info().
2199 png_write_info(png_ptr, info_ptr);
2201 Note that there is one transformation you may need to do before
2202 png_write_info(). In PNG files, the alpha channel in an image is the
2203 level of opacity. If your data is supplied as a level of transparency,
2204 you can invert the alpha channel before you write it, so that 0 is
2205 fully transparent and 255 (in 8-bit or paletted images) or 65535
2206 (in 16-bit images) is fully opaque, with
2208 png_set_invert_alpha(png_ptr);
2210 This must appear before png_write_info() instead of later with the
2211 other transformations because in the case of paletted images the tRNS
2212 chunk data has to be inverted before the tRNS chunk is written. If
2213 your image is not a paletted image, the tRNS data (which in such cases
2214 represents a single color to be rendered as transparent) won't need to
2215 be changed, and you can safely do this transformation after your
2216 png_write_info() call.
2218 If you need to write a private chunk that you want to appear before
2219 the PLTE chunk when PLTE is present, you can write the PNG info in
2220 two steps, and insert code to write your own chunk between them:
2222 png_write_info_before_PLTE(png_ptr, info_ptr);
2223 png_set_unknown_chunks(png_ptr, info_ptr, ...);
2224 png_write_info(png_ptr, info_ptr);
2226 After you've written the file information, you can set up the library
2227 to handle any special transformations of the image data. The various
2228 ways to transform the data will be described in the order that they
2229 should occur. This is important, as some of these change the color
2230 type and/or bit depth of the data, and some others only work on
2231 certain color types and bit depths. Even though each transformation
2232 checks to see if it has data that it can do something with, you should
2233 make sure to only enable a transformation if it will be valid for the
2234 data. For example, don't swap red and blue on grayscale data.
2236 PNG files store RGB pixels packed into 3 or 6 bytes. This code tells
2237 the library to strip input data that has 4 or 8 bytes per pixel down
2238 to 3 or 6 bytes (or strip 2 or 4-byte grayscale+filler data to 1 or 2
2241 png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);
2243 where the 0 is unused, and the location is either PNG_FILLER_BEFORE or
2244 PNG_FILLER_AFTER, depending upon whether the filler byte in the pixel
2245 is stored XRGB or RGBX.
2247 PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
2248 they can, resulting in, for example, 8 pixels per byte for 1 bit files.
2249 If the data is supplied at 1 pixel per byte, use this code, which will
2250 correctly pack the pixels into a single byte:
2252 png_set_packing(png_ptr);
2254 PNG files reduce possible bit depths to 1, 2, 4, 8, and 16. If your
2255 data is of another bit depth, you can write an sBIT chunk into the
2256 file so that decoders can recover the original data if desired.
2258 /* Set the true bit depth of the image data */
2259 if (color_type & PNG_COLOR_MASK_COLOR)
2261 sig_bit.red = true_bit_depth;
2262 sig_bit.green = true_bit_depth;
2263 sig_bit.blue = true_bit_depth;
2267 sig_bit.gray = true_bit_depth;
2269 if (color_type & PNG_COLOR_MASK_ALPHA)
2271 sig_bit.alpha = true_bit_depth;
2274 png_set_sBIT(png_ptr, info_ptr, &sig_bit);
2276 If the data is stored in the row buffer in a bit depth other than
2277 one supported by PNG (e.g. 3 bit data in the range 0-7 for a 4-bit PNG),
2278 this will scale the values to appear to be the correct bit depth as
2281 png_set_shift(png_ptr, &sig_bit);
2283 PNG files store 16 bit pixels in network byte order (big-endian,
2284 ie. most significant bits first). This code would be used if they are
2285 supplied the other way (little-endian, i.e. least significant bits
2286 first, the way PCs store them):
2289 png_set_swap(png_ptr);
2291 If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you
2292 need to change the order the pixels are packed into bytes, you can use:
2295 png_set_packswap(png_ptr);
2297 PNG files store 3 color pixels in red, green, blue order. This code
2298 would be used if they are supplied as blue, green, red:
2300 png_set_bgr(png_ptr);
2302 PNG files describe monochrome as black being zero and white being
2303 one. This code would be used if the pixels are supplied with this reversed
2304 (black being one and white being zero):
2306 png_set_invert_mono(png_ptr);
2308 Finally, you can write your own transformation function if none of
2309 the existing ones meets your needs. This is done by setting a callback
2312 png_set_write_user_transform_fn(png_ptr,
2313 write_transform_fn);
2315 You must supply the function
2317 void write_transform_fn(png_ptr ptr, row_info_ptr
2318 row_info, png_bytep data)
2320 See pngtest.c for a working example. Your function will be called
2321 before any of the other transformations are processed.
2323 You can also set up a pointer to a user structure for use by your
2326 png_set_user_transform_info(png_ptr, user_ptr, 0, 0);
2328 The user_channels and user_depth parameters of this function are ignored
2329 when writing; you can set them to zero as shown.
2331 You can retrieve the pointer via the function png_get_user_transform_ptr().
2334 voidp write_user_transform_ptr =
2335 png_get_user_transform_ptr(png_ptr);
2337 It is possible to have libpng flush any pending output, either manually,
2338 or automatically after a certain number of lines have been written. To
2339 flush the output stream a single time call:
2341 png_write_flush(png_ptr);
2343 and to have libpng flush the output stream periodically after a certain
2344 number of scanlines have been written, call:
2346 png_set_flush(png_ptr, nrows);
2348 Note that the distance between rows is from the last time png_write_flush()
2349 was called, or the first row of the image if it has never been called.
2350 So if you write 50 lines, and then png_set_flush 25, it will flush the
2351 output on the next scanline, and every 25 lines thereafter, unless
2352 png_write_flush() is called before 25 more lines have been written.
2353 If nrows is too small (less than about 10 lines for a 640 pixel wide
2354 RGB image) the image compression may decrease noticeably (although this
2355 may be acceptable for real-time applications). Infrequent flushing will
2356 only degrade the compression performance by a few percent over images
2357 that do not use flushing.
2359 Writing the image data
2361 That's it for the transformations. Now you can write the image data.
2362 The simplest way to do this is in one function call. If you have the
2363 whole image in memory, you can just call png_write_image() and libpng
2364 will write the image. You will need to pass in an array of pointers to
2365 each row. This function automatically handles interlacing, so you don't
2366 need to call png_set_interlace_handling() or call this function multiple
2367 times, or any of that other stuff necessary with png_write_rows().
2369 png_write_image(png_ptr, row_pointers);
2371 where row_pointers is:
2373 png_byte *row_pointers[height];
2375 You can point to void or char or whatever you use for pixels.
2377 If you don't want to write the whole image at once, you can
2378 use png_write_rows() instead. If the file is not interlaced,
2381 png_write_rows(png_ptr, row_pointers,
2384 row_pointers is the same as in the png_write_image() call.
2386 If you are just writing one row at a time, you can do this with
2387 a single row_pointer instead of an array of row_pointers:
2389 png_bytep row_pointer = row;
2391 png_write_row(png_ptr, row_pointer);
2393 When the file is interlaced, things can get a good deal more complicated.
2394 The only currently (as of the PNG Specification version 1.2, dated July
2395 1999) defined interlacing scheme for PNG files is the "Adam7" interlace
2396 scheme, that breaks down an image into seven smaller images of varying
2397 size. libpng will build these images for you, or you can do them
2398 yourself. If you want to build them yourself, see the PNG specification
2399 for details of which pixels to write when.
2401 If you don't want libpng to handle the interlacing details, just
2402 use png_set_interlace_handling() and call png_write_rows() the
2403 correct number of times to write all seven sub-images.
2405 If you want libpng to build the sub-images, call this before you start
2409 png_set_interlace_handling(png_ptr);
2411 This will return the number of passes needed. Currently, this is seven,
2412 but may change if another interlace type is added.
2414 Then write the complete image number_of_passes times.
2416 png_write_rows(png_ptr, row_pointers,
2419 As some of these rows are not used, and thus return immediately, you may
2420 want to read about interlacing in the PNG specification, and only update
2421 the rows that are actually used.
2423 Finishing a sequential write
2425 After you are finished writing the image, you should finish writing
2426 the file. If you are interested in writing comments or time, you should
2427 pass an appropriately filled png_info pointer. If you are not interested,
2430 png_write_end(png_ptr, info_ptr);
2432 When you are done, you can free all memory used by libpng like this:
2434 png_destroy_write_struct(&png_ptr, &info_ptr);
2436 It is also possible to individually free the info_ptr members that
2437 point to libpng-allocated storage with the following function:
2439 png_free_data(png_ptr, info_ptr, mask, seq)
2440 mask - identifies data to be freed, a mask
2441 containing the bitwise OR of one or
2443 PNG_FREE_PLTE, PNG_FREE_TRNS,
2444 PNG_FREE_HIST, PNG_FREE_ICCP,
2445 PNG_FREE_PCAL, PNG_FREE_ROWS,
2446 PNG_FREE_SCAL, PNG_FREE_SPLT,
2447 PNG_FREE_TEXT, PNG_FREE_UNKN,
2448 or simply PNG_FREE_ALL
2449 seq - sequence number of item to be freed
2452 This function may be safely called when the relevant storage has
2453 already been freed, or has not yet been allocated, or was allocated
2454 by the user and not by libpng, and will in those cases do nothing.
2455 The "seq" parameter is ignored if only one item of the selected data
2456 type, such as PLTE, is allowed. If "seq" is not -1, and multiple items
2457 are allowed for the data type identified in the mask, such as text or
2458 sPLT, only the n'th item in the structure is freed, where n is "seq".
2460 If you allocated data such as a palette that you passed in to libpng
2461 with png_set_*, you must not free it until just before the call to
2462 png_destroy_write_struct().
2464 The default behavior is only to free data that was allocated internally
2465 by libpng. This can be changed, so that libpng will not free the data,
2466 or so that it will free data that was allocated by the user with png_malloc()
2467 or png_zalloc() and passed in via a png_set_*() function, with
2469 png_data_freer(png_ptr, info_ptr, freer, mask)
2470 mask - which data elements are affected
2471 same choices as in png_free_data()
2473 PNG_DESTROY_WILL_FREE_DATA
2474 PNG_SET_WILL_FREE_DATA
2475 PNG_USER_WILL_FREE_DATA
2477 For example, to transfer responsibility for some data from a read structure
2478 to a write structure, you could use
2480 png_data_freer(read_ptr, read_info_ptr,
2481 PNG_USER_WILL_FREE_DATA,
2482 PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)
2483 png_data_freer(write_ptr, write_info_ptr,
2484 PNG_DESTROY_WILL_FREE_DATA,
2485 PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)
2487 thereby briefly reassigning responsibility for freeing to the user but
2488 immediately afterwards reassigning it once more to the write_destroy
2489 function. Having done this, it would then be safe to destroy the read
2490 structure and continue to use the PLTE, tRNS, and hIST data in the write
2493 This function only affects data that has already been allocated.
2494 You can call this function before calling after the png_set_*() functions
2495 to control whether the user or png_destroy_*() is supposed to free the data.
2496 When the user assumes responsibility for libpng-allocated data, the
2497 application must use
2498 png_free() to free it, and when the user transfers responsibility to libpng
2499 for data that the user has allocated, the user must have used png_malloc()
2500 or png_zalloc() to allocate it.
2502 If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword
2503 separately, do not transfer responsibility for freeing text_ptr to libpng,
2504 because when libpng fills a png_text structure it combines these members with
2505 the key member, and png_free_data() will free only text_ptr.key. Similarly,
2506 if you transfer responsibility for free'ing text_ptr from libpng to your
2507 application, your application must not separately free those members.
2508 For a more compact example of writing a PNG image, see the file example.c.
2510 V. Modifying/Customizing libpng:
2512 There are two issues here. The first is changing how libpng does
2513 standard things like memory allocation, input/output, and error handling.
2514 The second deals with more complicated things like adding new chunks,
2515 adding new transformations, and generally changing how libpng works.
2516 Both of those are compile-time issues; that is, they are generally
2517 determined at the time the code is written, and there is rarely a need
2518 to provide the user with a means of changing them.
2520 Memory allocation, input/output, and error handling
2522 All of the memory allocation, input/output, and error handling in libpng
2523 goes through callbacks that are user-settable. The default routines are
2524 in pngmem.c, pngrio.c, pngwio.c, and pngerror.c, respectively. To change
2525 these functions, call the appropriate png_set_*_fn() function.
2527 Memory allocation is done through the functions png_malloc(), png_calloc(),
2528 and png_free(). These currently just call the standard C functions.
2529 png_calloc() calls png_malloc() and then png_memset() to clear the newly
2530 allocated memory to zero. If your pointers can't access more then 64K
2531 at a time, you will want to set MAXSEG_64K in zlib.h. Since it is
2532 unlikely that the method of handling memory allocation on a platform
2533 will change between applications, these functions must be modified in
2534 the library at compile time. If you prefer to use a different method
2535 of allocating and freeing data, you can use png_create_read_struct_2() or
2536 png_create_write_struct_2() to register your own functions as described
2537 above. These functions also provide a void pointer that can be retrieved
2540 mem_ptr=png_get_mem_ptr(png_ptr);
2542 Your replacement memory functions must have prototypes as follows:
2544 png_voidp malloc_fn(png_structp png_ptr,
2545 png_alloc_size_t size);
2546 void free_fn(png_structp png_ptr, png_voidp ptr);
2548 Your malloc_fn() must return NULL in case of failure. The png_malloc()
2549 function will normally call png_error() if it receives a NULL from the
2550 system memory allocator or from your replacement malloc_fn().
2552 Your free_fn() will never be called with a NULL ptr, since libpng's
2553 png_free() checks for NULL before calling free_fn().
2555 Input/Output in libpng is done through png_read() and png_write(),
2556 which currently just call fread() and fwrite(). The FILE * is stored in
2557 png_struct and is initialized via png_init_io(). If you wish to change
2558 the method of I/O, the library supplies callbacks that you can set
2559 through the function png_set_read_fn() and png_set_write_fn() at run
2560 time, instead of calling the png_init_io() function. These functions
2561 also provide a void pointer that can be retrieved via the function
2562 png_get_io_ptr(). For example:
2564 png_set_read_fn(png_structp read_ptr,
2565 voidp read_io_ptr, png_rw_ptr read_data_fn)
2567 png_set_write_fn(png_structp write_ptr,
2568 voidp write_io_ptr, png_rw_ptr write_data_fn,
2569 png_flush_ptr output_flush_fn);
2571 voidp read_io_ptr = png_get_io_ptr(read_ptr);
2572 voidp write_io_ptr = png_get_io_ptr(write_ptr);
2574 The replacement I/O functions must have prototypes as follows:
2576 void user_read_data(png_structp png_ptr,
2577 png_bytep data, png_size_t length);
2578 void user_write_data(png_structp png_ptr,
2579 png_bytep data, png_size_t length);
2580 void user_flush_data(png_structp png_ptr);
2582 The user_read_data() function is responsible for detecting and
2583 handling end-of-data errors.
2585 Supplying NULL for the read, write, or flush functions sets them back
2586 to using the default C stream functions, which expect the io_ptr to
2587 point to a standard *FILE structure. It is probably a mistake
2588 to use NULL for one of write_data_fn and output_flush_fn but not both
2589 of them, unless you have built libpng with PNG_NO_WRITE_FLUSH defined.
2590 It is an error to read from a write stream, and vice versa.
2592 Error handling in libpng is done through png_error() and png_warning().
2593 Errors handled through png_error() are fatal, meaning that png_error()
2594 should never return to its caller. Currently, this is handled via
2595 setjmp() and longjmp() (unless you have compiled libpng with
2596 PNG_NO_SETJMP, in which case it is handled via PNG_ABORT()),
2597 but you could change this to do things like exit() if you should wish,
2598 as long as your function does not return.
2600 On non-fatal errors, png_warning() is called
2601 to print a warning message, and then control returns to the calling code.
2602 By default png_error() and png_warning() print a message on stderr via
2603 fprintf() unless the library is compiled with PNG_NO_CONSOLE_IO defined
2604 (because you don't want the messages) or PNG_NO_STDIO defined (because
2605 fprintf() isn't available). If you wish to change the behavior of the error
2606 functions, you will need to set up your own message callbacks. These
2607 functions are normally supplied at the time that the png_struct is created.
2608 It is also possible to redirect errors and warnings to your own replacement
2609 functions after png_create_*_struct() has been called by calling:
2611 png_set_error_fn(png_structp png_ptr,
2612 png_voidp error_ptr, png_error_ptr error_fn,
2613 png_error_ptr warning_fn);
2615 png_voidp error_ptr = png_get_error_ptr(png_ptr);
2617 If NULL is supplied for either error_fn or warning_fn, then the libpng
2618 default function will be used, calling fprintf() and/or longjmp() if a
2619 problem is encountered. The replacement error functions should have
2620 parameters as follows:
2622 void user_error_fn(png_structp png_ptr,
2623 png_const_charp error_msg);
2624 void user_warning_fn(png_structp png_ptr,
2625 png_const_charp warning_msg);
2627 The motivation behind using setjmp() and longjmp() is the C++ throw and
2628 catch exception handling methods. This makes the code much easier to write,
2629 as there is no need to check every return code of every function call.
2630 However, there are some uncertainties about the status of local variables
2631 after a longjmp, so the user may want to be careful about doing anything
2632 after setjmp returns non-zero besides returning itself. Consult your
2633 compiler documentation for more details. For an alternative approach, you
2634 may wish to use the "cexcept" facility (see http://cexcept.sourceforge.net).
2638 If you need to read or write custom chunks, you may need to get deeper
2639 into the libpng code. The library now has mechanisms for storing
2640 and writing chunks of unknown type; you can even declare callbacks
2641 for custom chunks. However, this may not be good enough if the
2642 library code itself needs to know about interactions between your
2643 chunk and existing `intrinsic' chunks.
2645 If you need to write a new intrinsic chunk, first read the PNG
2646 specification. Acquire a first level of understanding of how it works.
2647 Pay particular attention to the sections that describe chunk names,
2648 and look at how other chunks were designed, so you can do things
2649 similarly. Second, check out the sections of libpng that read and
2650 write chunks. Try to find a chunk that is similar to yours and use
2651 it as a template. More details can be found in the comments inside
2652 the code. It is best to handle unknown chunks in a generic method,
2653 via callback functions, instead of by modifying libpng functions.
2655 If you wish to write your own transformation for the data, look through
2656 the part of the code that does the transformations, and check out some of
2657 the simpler ones to get an idea of how they work. Try to find a similar
2658 transformation to the one you want to add and copy off of it. More details
2659 can be found in the comments inside the code itself.
2661 Configuring for 16 bit platforms
2663 You will want to look into zconf.h to tell zlib (and thus libpng) that
2664 it cannot allocate more then 64K at a time. Even if you can, the memory
2665 won't be accessible. So limit zlib and libpng to 64K by defining MAXSEG_64K.
2669 For DOS users who only have access to the lower 640K, you will
2670 have to limit zlib's memory usage via a png_set_compression_mem_level()
2671 call. See zlib.h or zconf.h in the zlib library for more information.
2673 Configuring for Medium Model
2675 Libpng's support for medium model has been tested on most of the popular
2676 compilers. Make sure MAXSEG_64K gets defined, USE_FAR_KEYWORD gets
2677 defined, and FAR gets defined to far in pngconf.h, and you should be
2678 all set. Everything in the library (except for zlib's structure) is
2679 expecting far data. You must use the typedefs with the p or pp on
2680 the end for pointers (or at least look at them and be careful). Make
2681 note that the rows of data are defined as png_bytepp, which is an
2682 unsigned char far * far *.
2684 Configuring for gui/windowing platforms:
2686 You will need to write new error and warning functions that use the GUI
2687 interface, as described previously, and set them to be the error and
2688 warning functions at the time that png_create_*_struct() is called,
2689 in order to have them available during the structure initialization.
2690 They can be changed later via png_set_error_fn(). On some compilers,
2691 you may also have to change the memory allocators (png_malloc, etc.).
2693 Configuring for compiler xxx:
2695 All includes for libpng are in pngconf.h. If you need to add, change
2696 or delete an include, this is the place to do it.
2697 The includes that are not needed outside libpng are placed in pngpriv.h,
2698 which is only used by the routines inside libpng itself.
2699 The files in libpng proper only include pngpriv.h and png.h, which
2700 in turn includes pngconf.h.
2704 There are special functions to configure the compression. Perhaps the
2705 most useful one changes the compression level, which currently uses
2706 input compression values in the range 0 - 9. The library normally
2707 uses the default compression level (Z_DEFAULT_COMPRESSION = 6). Tests
2708 have shown that for a large majority of images, compression values in
2709 the range 3-6 compress nearly as well as higher levels, and do so much
2710 faster. For online applications it may be desirable to have maximum speed
2711 (Z_BEST_SPEED = 1). With versions of zlib after v0.99, you can also
2712 specify no compression (Z_NO_COMPRESSION = 0), but this would create
2713 files larger than just storing the raw bitmap. You can specify the
2714 compression level by calling:
2716 png_set_compression_level(png_ptr, level);
2718 Another useful one is to reduce the memory level used by the library.
2719 The memory level defaults to 8, but it can be lowered if you are
2720 short on memory (running DOS, for example, where you only have 640K).
2721 Note that the memory level does have an effect on compression; among
2722 other things, lower levels will result in sections of incompressible
2723 data being emitted in smaller stored blocks, with a correspondingly
2724 larger relative overhead of up to 15% in the worst case.
2726 png_set_compression_mem_level(png_ptr, level);
2728 The other functions are for configuring zlib. They are not recommended
2729 for normal use and may result in writing an invalid PNG file. See
2730 zlib.h for more information on what these mean.
2732 png_set_compression_strategy(png_ptr,
2734 png_set_compression_window_bits(png_ptr,
2736 png_set_compression_method(png_ptr, method);
2737 png_set_compression_buffer_size(png_ptr, size);
2739 Controlling row filtering
2741 If you want to control whether libpng uses filtering or not, which
2742 filters are used, and how it goes about picking row filters, you
2743 can call one of these functions. The selection and configuration
2744 of row filters can have a significant impact on the size and
2745 encoding speed and a somewhat lesser impact on the decoding speed
2746 of an image. Filtering is enabled by default for RGB and grayscale
2747 images (with and without alpha), but not for paletted images nor
2748 for any images with bit depths less than 8 bits/pixel.
2750 The 'method' parameter sets the main filtering method, which is
2751 currently only '0' in the PNG 1.2 specification. The 'filters'
2752 parameter sets which filter(s), if any, should be used for each
2753 scanline. Possible values are PNG_ALL_FILTERS and PNG_NO_FILTERS
2754 to turn filtering on and off, respectively.
2756 Individual filter types are PNG_FILTER_NONE, PNG_FILTER_SUB,
2757 PNG_FILTER_UP, PNG_FILTER_AVG, PNG_FILTER_PAETH, which can be bitwise
2758 ORed together with '|' to specify one or more filters to use.
2759 These filters are described in more detail in the PNG specification.
2760 If you intend to change the filter type during the course of writing
2761 the image, you should start with flags set for all of the filters
2762 you intend to use so that libpng can initialize its internal
2763 structures appropriately for all of the filter types. (Note that this
2764 means the first row must always be adaptively filtered, because libpng
2765 currently does not allocate the filter buffers until png_write_row()
2766 is called for the first time.)
2768 filters = PNG_FILTER_NONE | PNG_FILTER_SUB
2769 PNG_FILTER_UP | PNG_FILTER_AVG |
2770 PNG_FILTER_PAETH | PNG_ALL_FILTERS;
2772 png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE,
2774 The second parameter can also be
2775 PNG_INTRAPIXEL_DIFFERENCING if you are
2776 writing a PNG to be embedded in a MNG
2777 datastream. This parameter must be the
2778 same as the value of filter_method used
2781 It is also possible to influence how libpng chooses from among the
2782 available filters. This is done in one or both of two ways - by
2783 telling it how important it is to keep the same filter for successive
2784 rows, and by telling it the relative computational costs of the filters.
2786 double weights[3] = {1.5, 1.3, 1.1},
2787 costs[PNG_FILTER_VALUE_LAST] =
2788 {1.0, 1.3, 1.3, 1.5, 1.7};
2790 png_set_filter_heuristics(png_ptr,
2791 PNG_FILTER_HEURISTIC_WEIGHTED, 3,
2794 The weights are multiplying factors that indicate to libpng that the
2795 row filter should be the same for successive rows unless another row filter
2796 is that many times better than the previous filter. In the above example,
2797 if the previous 3 filters were SUB, SUB, NONE, the SUB filter could have a
2798 "sum of absolute differences" 1.5 x 1.3 times higher than other filters
2799 and still be chosen, while the NONE filter could have a sum 1.1 times
2800 higher than other filters and still be chosen. Unspecified weights are
2801 taken to be 1.0, and the specified weights should probably be declining
2802 like those above in order to emphasize recent filters over older filters.
2804 The filter costs specify for each filter type a relative decoding cost
2805 to be considered when selecting row filters. This means that filters
2806 with higher costs are less likely to be chosen over filters with lower
2807 costs, unless their "sum of absolute differences" is that much smaller.
2808 The costs do not necessarily reflect the exact computational speeds of
2809 the various filters, since this would unduly influence the final image
2812 Note that the numbers above were invented purely for this example and
2813 are given only to help explain the function usage. Little testing has
2814 been done to find optimum values for either the costs or the weights.
2816 Removing unwanted object code
2818 There are a bunch of #define's in pngconf.h that control what parts of
2819 libpng are compiled. All the defines end in _SUPPORTED. If you are
2820 never going to use a capability, you can change the #define to #undef
2821 before recompiling libpng and save yourself code and data space, or
2822 you can turn off individual capabilities with defines that begin with
2825 You can also turn all of the transforms and ancillary chunk capabilities
2826 off en masse with compiler directives that define
2827 PNG_NO_READ[or WRITE]_TRANSFORMS, or PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS,
2829 along with directives to turn on any of the capabilities that you do
2830 want. The PNG_NO_READ[or WRITE]_TRANSFORMS directives disable the extra
2831 transformations but still leave the library fully capable of reading
2832 and writing PNG files with all known public chunks. Use of the
2833 PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS directive produces a library
2834 that is incapable of reading or writing ancillary chunks. If you are
2835 not using the progressive reading capability, you can turn that off
2836 with PNG_NO_PROGRESSIVE_READ (don't confuse this with the INTERLACING
2837 capability, which you'll still have).
2839 All the reading and writing specific code are in separate files, so the
2840 linker should only grab the files it needs. However, if you want to
2841 make sure, or if you are building a stand alone library, all the
2842 reading files start with pngr and all the writing files start with
2843 pngw. The files that don't match either (like png.c, pngtrans.c, etc.)
2844 are used for both reading and writing, and always need to be included.
2845 The progressive reader is in pngpread.c
2847 If you are creating or distributing a dynamically linked library (a .so
2848 or DLL file), you should not remove or disable any parts of the library,
2849 as this will cause applications linked with different versions of the
2850 library to fail if they call functions not available in your library.
2851 The size of the library itself should not be an issue, because only
2852 those sections that are actually used will be loaded into memory.
2854 Requesting debug printout
2856 The macro definition PNG_DEBUG can be used to request debugging
2857 printout. Set it to an integer value in the range 0 to 3. Higher
2858 numbers result in increasing amounts of debugging information. The
2859 information is printed to the "stderr" file, unless another file
2860 name is specified in the PNG_DEBUG_FILE macro definition.
2862 When PNG_DEBUG > 0, the following functions (macros) become available:
2864 png_debug(level, message)
2865 png_debug1(level, message, p1)
2866 png_debug2(level, message, p1, p2)
2868 in which "level" is compared to PNG_DEBUG to decide whether to print
2869 the message, "message" is the formatted string to be printed,
2870 and p1 and p2 are parameters that are to be embedded in the string
2871 according to printf-style formatting directives. For example,
2873 png_debug1(2, "foo=%d\n", foo);
2878 fprintf(PNG_DEBUG_FILE, "foo=%d\n", foo);
2880 When PNG_DEBUG is defined but is zero, the macros aren't defined, but you
2881 can still use PNG_DEBUG to control your own debugging:
2887 When PNG_DEBUG = 1, the macros are defined, but only png_debug statements
2888 having level = 0 will be printed. There aren't any such statements in
2889 this version of libpng, but if you insert some they will be printed.
2893 The MNG specification (available at http://www.libpng.org/pub/mng) allows
2894 certain extensions to PNG for PNG images that are embedded in MNG datastreams.
2895 Libpng can support some of these extensions. To enable them, use the
2896 png_permit_mng_features() function:
2898 feature_set = png_permit_mng_features(png_ptr, mask)
2899 mask is a png_uint_32 containing the bitwise OR of the
2900 features you want to enable. These include
2901 PNG_FLAG_MNG_EMPTY_PLTE
2902 PNG_FLAG_MNG_FILTER_64
2903 PNG_ALL_MNG_FEATURES
2904 feature_set is a png_uint_32 that is the bitwise AND of
2905 your mask with the set of MNG features that is
2906 supported by the version of libpng that you are using.
2908 It is an error to use this function when reading or writing a standalone
2909 PNG file with the PNG 8-byte signature. The PNG datastream must be wrapped
2910 in a MNG datastream. As a minimum, it must have the MNG 8-byte signature
2911 and the MHDR and MEND chunks. Libpng does not provide support for these
2912 or any other MNG chunks; your application must provide its own support for
2913 them. You may wish to consider using libmng (available at
2914 http://www.libmng.com) instead.
2916 VII. Changes to Libpng from version 0.88
2918 It should be noted that versions of libpng later than 0.96 are not
2919 distributed by the original libpng author, Guy Schalnat, nor by
2920 Andreas Dilger, who had taken over from Guy during 1996 and 1997, and
2921 distributed versions 0.89 through 0.96, but rather by another member
2922 of the original PNG Group, Glenn Randers-Pehrson. Guy and Andreas are
2923 still alive and well, but they have moved on to other things.
2925 The old libpng functions png_read_init(), png_write_init(),
2926 png_info_init(), png_read_destroy(), and png_write_destroy() have been
2927 moved to PNG_INTERNAL in version 0.95 to discourage their use. These
2928 functions will be removed from libpng version 2.0.0.
2930 The preferred method of creating and initializing the libpng structures is
2931 via the png_create_read_struct(), png_create_write_struct(), and
2932 png_create_info_struct() because they isolate the size of the structures
2933 from the application, allow version error checking, and also allow the
2934 use of custom error handling routines during the initialization, which
2935 the old functions do not. The functions png_read_destroy() and
2936 png_write_destroy() do not actually free the memory that libpng
2937 allocated for these structs, but just reset the data structures, so they
2938 can be used instead of png_destroy_read_struct() and
2939 png_destroy_write_struct() if you feel there is too much system overhead
2940 allocating and freeing the png_struct for each image read.
2942 Setting the error callbacks via png_set_message_fn() before
2943 png_read_init() as was suggested in libpng-0.88 is no longer supported
2944 because this caused applications that do not use custom error functions
2945 to fail if the png_ptr was not initialized to zero. It is still possible
2946 to set the error callbacks AFTER png_read_init(), or to change them with
2947 png_set_error_fn(), which is essentially the same function, but with a new
2948 name to force compilation errors with applications that try to use the old
2951 Starting with version 1.0.7, you can find out which version of the library
2952 you are using at run-time:
2954 png_uint_32 libpng_vn = png_access_version_number();
2956 The number libpng_vn is constructed from the major version, minor
2957 version with leading zero, and release number with leading zero,
2958 (e.g., libpng_vn for version 1.0.7 is 10007).
2960 You can also check which version of png.h you used when compiling your
2963 png_uint_32 application_vn = PNG_LIBPNG_VER;
2965 VIII. Changes to Libpng from version 1.0.x to 1.2.x
2967 Support for user memory management was enabled by default. To
2968 accomplish this, the functions png_create_read_struct_2(),
2969 png_create_write_struct_2(), png_set_mem_fn(), png_get_mem_ptr(),
2970 png_malloc_default(), and png_free_default() were added.
2972 Support for the iTXt chunk has been enabled by default as of
2975 Support for certain MNG features was enabled.
2977 Support for numbered error messages was added. However, we never got
2978 around to actually numbering the error messages. The function
2979 png_set_strip_error_numbers() was added (Note: the prototype for this
2980 function was inadvertently removed from png.h in PNG_NO_ASSEMBLER_CODE
2981 builds of libpng-1.2.15. It was restored in libpng-1.2.36).
2983 The png_malloc_warn() function was added at libpng-1.2.3. This issues
2984 a png_warning and returns NULL instead of aborting when it fails to
2985 acquire the requested memory allocation.
2987 Support for setting user limits on image width and height was enabled
2988 by default. The functions png_set_user_limits(), png_get_user_width_max(),
2989 and png_get_user_height_max() were added at libpng-1.2.6.
2991 The png_set_add_alpha() function was added at libpng-1.2.7.
2993 The function png_set_expand_gray_1_2_4_to_8() was added at libpng-1.2.9.
2994 Unlike png_set_gray_1_2_4_to_8(), the new function does not expand the
2995 tRNS chunk to alpha. The png_set_gray_1_2_4_to_8() function is
2998 A number of macro definitions in support of runtime selection of
2999 assembler code features (especially Intel MMX code support) were
3000 added at libpng-1.2.0:
3002 PNG_ASM_FLAG_MMX_SUPPORT_COMPILED
3003 PNG_ASM_FLAG_MMX_SUPPORT_IN_CPU
3004 PNG_ASM_FLAG_MMX_READ_COMBINE_ROW
3005 PNG_ASM_FLAG_MMX_READ_INTERLACE
3006 PNG_ASM_FLAG_MMX_READ_FILTER_SUB
3007 PNG_ASM_FLAG_MMX_READ_FILTER_UP
3008 PNG_ASM_FLAG_MMX_READ_FILTER_AVG
3009 PNG_ASM_FLAG_MMX_READ_FILTER_PAETH
3010 PNG_ASM_FLAGS_INITIALIZED
3016 We added the following functions in support of runtime
3017 selection of assembler code features:
3019 png_get_mmx_flagmask()
3020 png_set_mmx_thresholds()
3022 png_get_mmx_bitdepth_threshold()
3023 png_get_mmx_rowbytes_threshold()
3026 We replaced all of these functions with simple stubs in libpng-1.2.20,
3027 when the Intel assembler code was removed due to a licensing issue.
3029 These macros are deprecated:
3031 PNG_READ_TRANSFORMS_NOT_SUPPORTED
3032 PNG_PROGRESSIVE_READ_NOT_SUPPORTED
3033 PNG_NO_SEQUENTIAL_READ_SUPPORTED
3034 PNG_WRITE_TRANSFORMS_NOT_SUPPORTED
3035 PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED
3036 PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED
3038 They have been replaced, respectively, by:
3040 PNG_NO_READ_TRANSFORMS
3041 PNG_NO_PROGRESSIVE_READ
3042 PNG_NO_SEQUENTIAL_READ
3043 PNG_NO_WRITE_TRANSFORMS
3044 PNG_NO_READ_ANCILLARY_CHUNKS
3045 PNG_NO_WRITE_ANCILLARY_CHUNKS
3047 PNG_MAX_UINT was replaced with PNG_UINT_31_MAX. It has been
3048 deprecated since libpng-1.0.16 and libpng-1.2.6.
3051 png_check_sig(sig, num)
3053 !png_sig_cmp(sig, 0, num)
3054 It has been deprecated since libpng-0.90.
3057 png_set_gray_1_2_4_to_8()
3058 which also expands tRNS to alpha was replaced with
3059 png_set_expand_gray_1_2_4_to_8()
3060 which does not. It has been deprecated since libpng-1.0.18 and 1.2.9.
3062 IX. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x
3064 Private libpng prototypes and macro definitions were moved from
3065 png.h and pngconf.h into a new pngpriv.h header file.
3067 Functions png_set_benign_errors(), png_benign_error(), and
3068 png_chunk_benign_error() were added.
3070 Support for setting the maximum amount of memory that the application
3071 will allocate for reading chunks was added, as a security measure.
3072 The functions png_set_chunk_cache_max() and png_get_chunk_cache_max()
3073 were added to the library.
3075 We implemented support for I/O states by adding png_ptr member io_state
3076 and functions png_get_io_chunk_name() and png_get_io_state() in pngget.c
3078 We added PNG_TRANSFORM_GRAY_TO_RGB to the available high-level
3081 Checking for and reporting of errors in the IHDR chunk is more thorough.
3083 Support for global arrays was removed, to improve thread safety.
3085 Some obsolete/deprecated macros and functions have been removed.
3087 Typecasted NULL definitions such as
3088 #define png_voidp_NULL (png_voidp)NULL
3089 were eliminated. If you used these in your application, just use
3092 The png_struct and info_struct members "trans" and "trans_values" were
3093 changed to "trans_alpha" and "trans_color", respectively.
3095 The obsolete, unused pnggccrd.c and pngvcrd.c files and related makefiles
3098 The PNG_1_0_X and PNG_1_2_X macros were eliminated.
3100 The PNG_LEGACY_SUPPORTED macro was eliminated.
3102 Many WIN32_WCE #ifdefs were removed.
3104 The functions png_read_init(info_ptr), png_write_init(info_ptr),
3105 png_info_init(info_ptr), png_read_destroy(), and png_write_destroy()
3106 have been removed. They have been deprecated since libpng-0.95.
3108 The png_permit_empty_plte() was removed. It has been deprecated
3109 since libpng-1.0.9. Use png_permit_mng_features() instead.
3111 We removed the obsolete stub functions png_get_mmx_flagmask(),
3112 png_set_mmx_thresholds(), png_get_asm_flags(),
3113 png_get_mmx_bitdepth_threshold(), png_get_mmx_rowbytes_threshold(),
3114 png_set_asm_flags(), and png_mmx_supported()
3116 We removed the obsolete png_check_sig(), png_memcpy_check(), and
3117 png_memset_check() functions. Instead use !png_sig_cmp(), png_memcpy(),
3118 and png_memset(), respectively.
3120 The function png_set_gray_1_2_4_to_8() was removed. It has been
3121 deprecated since libpng-1.0.18 and 1.2.9, when it was replaced with
3122 png_set_expand_gray_1_2_4_to_8() because the former function also
3123 expanded palette images.
3125 We changed the prototype for png_malloc() from
3126 png_malloc(png_structp png_ptr, png_uint_32 size)
3128 png_malloc(png_structp png_ptr, png_alloc_size_t size)
3130 This also applies to the prototype for the user replacement malloc_fn().
3132 The png_calloc() function was added and is used in place of
3133 of "png_malloc(); png_memset();" except in the case in png_read_png()
3134 where the array consists of pointers; in this case a "for" loop is used
3135 after the png_malloc() to set the pointers to NULL, to give robust.
3136 behavior in case the application runs out of memory part-way through
3139 We changed the prototypes of png_get_compression_buffer_size() and
3140 png_set_compression_buffer_size() to work with png_size_t instead of
3143 Support for numbered error messages was removed by default, since we
3144 never got around to actually numbering the error messages. The function
3145 png_set_strip_error_numbers() was removed from the library by default.
3147 The png_zalloc() and png_zfree() functions are no longer exported.
3148 The png_zalloc() function no longer zeroes out the memory that it
3151 Support for dithering was disabled by default in libpng-1.4.0, because
3152 been well tested and doesn't actually "dither". The code was not
3153 removed, however, and could be enabled by building libpng with
3154 PNG_READ_DITHER_SUPPORTED defined. In libpng-1.4.2, this support
3155 was reenabled, but the function was renamed png_set_quantize() to
3156 reflect more accurately what it actually does. At the same time,
3157 the PNG_DITHER_[RED,GREEN_BLUE]_BITS macros were also renamed to
3158 PNG_QUANTIZE_[RED,GREEN,BLUE]_BITS.
3160 We removed the trailing '.' from the warning and error messages.
3164 The png_get_io_ptr() function has been present since libpng-0.88, has never
3165 changed, and is unaffected by conditional compilation macros. It is the
3166 best choice for use in configure scripts for detecting the presence of any
3167 libpng version since 0.88. In an autoconf "configure.in" you could use
3169 AC_CHECK_LIB(png, png_get_io_ptr, ...
3171 XI. Source code repository
3173 Since about February 2009, version 1.2.34, libpng has been under "git" source
3174 control. The git repository was built from old libpng-x.y.z.tar.gz files
3175 going back to version 0.70. You can access the git repository (read only)
3178 git://libpng.git.sourceforge.net/gitroot/libpng
3180 or you can browse it via "gitweb" at
3182 http://libpng.git.sourceforge.net/git/gitweb.cgi?p=libpng
3184 Patches can be sent to glennrp at users.sourceforge.net or to
3185 png-mng-implement at lists.sourceforge.net or you can upload them to
3186 the libpng bug tracker at
3188 http://libpng.sourceforge.net
3192 Our coding style is similar to the "Allman" style, with curly
3193 braces on separate lines:
3200 else if (another condition)
3205 The braces can be omitted from simple one-line actions:
3210 We use 3-space indentation, except for continued statements which
3211 are usually indented the same as the first line of the statement
3212 plus four more spaces.
3214 For macro definitions we use 2-space indentation, always leaving the "#"
3215 in the first column.
3217 #ifndef PNG_NO_FEATURE
3218 # ifndef PNG_FEATURE_SUPPORTED
3219 # define PNG_FEATURE_SUPPORTED
3223 Comments appear with the leading "/*" at the same indentation as
3224 the statement that follows the comment:
3226 /* Single-line comment */
3229 /* This is a multiple-line
3234 Very short comments can be placed after the end of the statement
3235 to which they pertain:
3237 statement; /* comment */
3239 We don't use C++ style ("//") comments. We have, however,
3240 used them in the past in some now-abandoned MMX assembler
3243 Functions and their curly braces are not indented, and
3244 exported functions are marked with PNGAPI:
3246 /* This is a public function that is visible to
3247 * application programers. It does thus-and-so.
3250 png_exported_function(png_ptr, png_info, foo)
3255 The prototypes for all exported functions appear in png.h,
3256 above the comment that says
3258 /* Maintainer: Put new public prototypes here ... */
3260 We mark all non-exported functions with "/* PRIVATE */"":
3263 png_non_exported_function(png_ptr, png_info, foo)
3268 The prototypes for non-exported functions (except for those in
3271 above the comment that says
3273 /* Maintainer: Put new private prototypes here ^ and in libpngpf.3 */
3275 The names of all exported functions and variables begin
3276 with "png_", and all publicly visible C preprocessor
3277 macros begin with "PNG_".
3279 We put a space after each comma and after each semicolon
3280 in "for" statments, and we put spaces before and after each
3281 C binary operator and after "for" or "while", and before
3282 "?". We don't put a space between a typecast and the expression
3283 being cast, nor do we put one between a function name and the
3284 left parenthesis that follows it:
3286 for (i = 2; i > 0; --i)
3287 y[i] = a(x) + (int)b;
3289 We prefer #ifdef and #ifndef to #if defined() and if !defined()
3290 when there is only one macro being tested.
3292 We do not use the TAB character for indentation in the C sources.
3294 Lines do not exceed 80 characters.
3296 Other rules can be inferred by inspecting the libpng source.
3298 XIII. Y2K Compliance in libpng
3302 Since the PNG Development group is an ad-hoc body, we can't make
3303 an official declaration.
3305 This is your unofficial assurance that libpng from version 0.71 and
3306 upward through 1.4.4 are Y2K compliant. It is my belief that earlier
3307 versions were also Y2K compliant.
3309 Libpng only has three year fields. One is a 2-byte unsigned integer that
3310 will hold years up to 65535. The other two hold the date in text
3311 format, and will hold years up to 9999.
3314 "png_uint_16 year" in png_time_struct.
3317 "png_charp time_buffer" in png_struct and
3318 "near_time_buffer", which is a local character string in png.c.
3320 There are seven time-related functions:
3322 png_convert_to_rfc_1123() in png.c
3323 (formerly png_convert_to_rfc_1152() in error)
3324 png_convert_from_struct_tm() in pngwrite.c, called
3326 png_convert_from_time_t() in pngwrite.c
3327 png_get_tIME() in pngget.c
3328 png_handle_tIME() in pngrutil.c, called in pngread.c
3329 png_set_tIME() in pngset.c
3330 png_write_tIME() in pngwutil.c, called in pngwrite.c
3332 All appear to handle dates properly in a Y2K environment. The
3333 png_convert_from_time_t() function calls gmtime() to convert from system
3334 clock time, which returns (year - 1900), which we properly convert to
3335 the full 4-digit year. There is a possibility that applications using
3336 libpng are not passing 4-digit years into the png_convert_to_rfc_1123()
3337 function, or that they are incorrectly passing only a 2-digit year
3338 instead of "year - 1900" into the png_convert_from_struct_tm() function,
3339 but this is not under our control. The libpng documentation has always
3340 stated that it works with 4-digit years, and the APIs have been
3343 The tIME chunk itself is also Y2K compliant. It uses a 2-byte unsigned
3344 integer to hold the year, and can hold years as large as 65535.
3346 zlib, upon which libpng depends, is also Y2K compliant. It contains
3347 no date-related code.
3350 Glenn Randers-Pehrson
3352 PNG Development Group