libpng-manual.txt - A description on how to use and modify libpng
- libpng version 1.6.21 - January 15, 2016
- Updated and distributed by Glenn Randers-Pehrson
- <glennrp at users.sourceforge.net>
- Copyright (c) 1998-2016 Glenn Randers-Pehrson
+ Copyright (c) 2018-2019 Cosmin Truta
+ Copyright (c) 1998-2018 Glenn Randers-Pehrson
This document is released under the libpng license.
For conditions of distribution and use, see the disclaimer
Based on:
- libpng versions 0.97, January 1998, through 1.6.21 - January 15, 2016
+ libpng version 1.6.36, December 2018, through 1.6.37 - April 2019
+ Updated and distributed by Cosmin Truta
+ Copyright (c) 2018-2019 Cosmin Truta
+
+ libpng versions 0.97, January 1998, through 1.6.35 - July 2018
Updated and distributed by Glenn Randers-Pehrson
- Copyright (c) 1998-2016 Glenn Randers-Pehrson
+ Copyright (c) 1998-2018 Glenn Randers-Pehrson
libpng 1.0 beta 6 - version 0.96 - May 28, 1997
Updated and distributed by Andreas Dilger
XIII. Detecting libpng
XIV. Source code repository
XV. Coding style
- XVI. Y2K Compliance in libpng
I. Introduction
The PNG specification (second edition), November 2003, is available as
a W3C Recommendation and as an ISO Standard (ISO/IEC 15948:2004 (E)) at
-<http://www.w3.org/TR/2003/REC-PNG-20031110/
+<https://www.w3.org/TR/2003/REC-PNG-20031110/>.
The W3C and ISO documents have identical technical content.
The PNG-1.2 specification is available at
-<http://png-mng.sourceforge.net/pub/png/spec/1.2/>.
+<https://png-mng.sourceforge.io/pub/png/spec/1.2/>.
It is technically equivalent
to the PNG specification (second edition) but has some additional material.
-The PNG-1.0 specification is available as RFC 2083
-<http://png-mng.sourceforge.net/pub/png/spec/1.0/> and as a
-W3C Recommendation <http://www.w3.org/TR/REC-png-961001>.
+The PNG-1.0 specification is available as RFC 2083 at
+<https://png-mng.sourceforge.io/pub/png/spec/1.0/> and as a
+W3C Recommendation at <https://www.w3.org/TR/REC-png-961001>.
Some additional chunks are described in the special-purpose public chunks
documents at <http://www.libpng.org/pub/png/spec/register/>
Libpng uses zlib for its compression and decompression of PNG files.
Further information about zlib, and the latest version of zlib, can
-be found at the zlib home page, <http://zlib.net/>.
+be found at the zlib home page, <https://zlib.net/>.
The zlib compression utility is a general purpose utility that is
useful for more than PNG files, and can be used without libpng.
See the documentation delivered with zlib for more details.
FILE *fp = fopen(file_name, "rb");
if (!fp)
{
- return (ERROR);
+ return ERROR;
}
if (fread(header, 1, number, fp) != number)
{
- return (ERROR);
+ return ERROR;
}
is_png = !png_sig_cmp(header, 0, number);
if (!is_png)
{
- return (NOT_PNG);
+ return NOT_PNG;
}
Next, png_struct and png_info need to be allocated and initialized. In
user_error_fn, user_warning_fn);
if (!png_ptr)
- return (ERROR);
+ return ERROR;
png_infop info_ptr = png_create_info_struct(png_ptr);
{
png_destroy_read_struct(&png_ptr,
(png_infopp)NULL, (png_infopp)NULL);
- return (ERROR);
+ return ERROR;
}
If you want to use your own memory allocation routines,
png_destroy_read_struct(&png_ptr, &info_ptr,
&end_info);
fclose(fp);
- return (ERROR);
+ return ERROR;
}
Pass (png_infopp)NULL instead of &end_info if you didn't create
The values for png_set_crc_action() say how libpng is to handle CRC errors in
ancillary and critical chunks, and whether to use the data contained
-therein. Note that it is impossible to "discard" data in a critical
-chunk.
+therein. Starting with libpng-1.6.26, this also governs how an ADLER32 error
+is handled while reading the IDAT chunk. Note that it is impossible to
+"discard" data in a critical chunk.
Choices for (int) crit_action are
PNG_CRC_DEFAULT 0 error/quit
PNG_CRC_QUIET_USE 4 quiet/use data
PNG_CRC_NO_CHANGE 5 use the current value
+When the setting for crit_action is PNG_CRC_QUIET_USE, the CRC and ADLER32
+checksums are not only ignored, but they are not evaluated.
+
Setting up callback code
You can set up a callback function to handle any unknown chunks in the
png_byte name[5];
png_byte *data;
- png_size_t size;
+ size_t size;
/* Note that libpng has already taken care of
the CRC handling */
unknown chunk structure, process it, and return one
of the following: */
- return (-n); /* chunk had an error */
- return (0); /* did not recognize */
- return (n); /* success */
+ return -n; /* chunk had an error */
+ return 0; /* did not recognize */
+ return n; /* success */
}
(You can give your function another name that you like instead of
passed in row number, and pass will always be 0. For the interlaced case the
same applies unless the row value is 0, in which case the row just handled was
the last one from one of the preceding passes. Because interlacing may skip a
-pass you cannot be sure that the preceding pass is just 'pass-1', if you really
+pass you cannot be sure that the preceding pass is just 'pass-1'; if you really
need to know what the last pass is record (row,pass) from the callback and use
the last recorded value each time.
chunk_cache_max = png_get_chunk_cache_max(png_ptr);
Libpng imposes a limit of 8 Megabytes (8,000,000 bytes) on the amount of
-memory that a compressed chunk other than IDAT can occupy, when decompressed.
-You can change this limit with
+memory that any chunk other than IDAT can occupy, originally or when
+decompressed (prior to libpng-1.6.32 the limit was only applied to compressed
+chunks after decompression). You can change this limit with
png_set_chunk_malloc_max(png_ptr, user_chunk_malloc_max);
png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB);
-This is the default libpng handling of the alpha channel - it is not
-pre-multiplied into the color components. In addition the call states
+Choices for the alpha_mode are
+
+ PNG_ALPHA_PNG 0 /* according to the PNG standard */
+ PNG_ALPHA_STANDARD 1 /* according to Porter/Duff */
+ PNG_ALPHA_ASSOCIATED 1 /* as above; this is the normal practice */
+ PNG_ALPHA_PREMULTIPLIED 1 /* as above */
+ PNG_ALPHA_OPTIMIZED 2 /* 'PNG' for opaque pixels, else 'STANDARD' */
+ PNG_ALPHA_BROKEN 3 /* the alpha channel is gamma encoded */
+
+PNG_ALPHA_PNG is the default libpng handling of the alpha channel. It is not
+pre-multiplied into the color components. In addition the call states
that the output is for a sRGB system and causes all PNG files without gAMA
chunks to be assumed to be encoded using sRGB.
png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC);
In this case the output is assumed to be something like an sRGB conformant
-display preceeded by a power-law lookup table of power 1.45. This is how
+display preceded by a power-law lookup table of power 1.45. This is how
early Mac systems behaved.
png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_GAMMA_LINEAR);
This is the classic Jim Blinn approach and will work in academic
environments where everything is done by the book. It has the shortcoming
of assuming that input PNG data with no gamma information is linear - this
-is unlikely to be correct unless the PNG files where generated locally.
+is unlikely to be correct unless the PNG files were generated locally.
Most of the time the output precision will be so low as to show
significant banding in dark areas of the image.
When the default gamma of PNG files doesn't match the output gamma.
If you have PNG files with no gamma information png_set_alpha_mode allows
-you to provide a default gamma, but it also sets the ouput gamma to the
+you to provide a default gamma, but it also sets the output gamma to the
matching value. If you know your PNG files have a gamma that doesn't
match the output you can take advantage of the fact that
png_set_alpha_mode always sets the output gamma but only sets the PNG
png_set_rows(png_ptr, info_ptr, &row_pointers);
Alternatively you could allocate your image in one big block and define
-row_pointers[i] to point into the proper places in your block.
+row_pointers[i] to point into the proper places in your block, but first
+be sure that your platform is able to allocate such a large buffer:
+
+ /* Guard against integer overflow */
+ if (height > PNG_SIZE_MAX/(width*pixel_size)) {
+ png_error(png_ptr,"image_data buffer would be too large");
+ }
+
+ png_bytep buffer=png_malloc(png_ptr,height*width*pixel_size);
+
+ for (int i=0; i<height, i++)
+ row_pointers[i]=buffer+i*width*pixel_size;
+
+ png_set_rows(png_ptr, info_ptr, &row_pointers);
If you use png_set_rows(), the application is responsible for freeing
row_pointers (and row_pointers[i], if they were separately allocated).
rowbytes = png_get_rowbytes(png_ptr, info_ptr);
rowbytes - number of bytes needed to hold a row
+ This value, the bit_depth, color_type,
+ and the number of channels can change
+ if you use transforms such as
+ png_set_expand(). See
+ png_read_update_info(), below.
signature = png_get_signature(png_ptr, info_ptr);
the single transparent color for
non-paletted images (PNG_INFO_tRNS)
+ png_get_eXIf_1(png_ptr, info_ptr, &num_exif, &exif);
+ (PNG_INFO_eXIf)
+
+ exif - Exif profile (array of png_byte)
+
png_get_hIST(png_ptr, info_ptr, &hist);
(PNG_INFO_hIST)
array of pointers to each row, as it will be needed for some
of the functions below.
+Be sure that your platform can allocate the buffer that you'll need.
+libpng internally checks for oversize width, but you'll need to
+do your own check for number_of_rows*width*pixel_size if you are using
+a multiple-row buffer:
+
+ /* Guard against integer overflow */
+ if (number_of_rows > PNG_SIZE_MAX/(width*pixel_size)) {
+ png_error(png_ptr,"image_data buffer would be too large");
+ }
+
Remember: Before you call png_read_update_info(), the png_get_*()
functions return the values corresponding to the original PNG image.
After you call png_read_update_info the values refer to the image
after each pass, the "rectangle" effect is generally considered the
better looking one.
-If you only want the "sparkle" effect, just call png_read_rows() as
+If you only want the "sparkle" effect, just call png_read_row() or
+png_read_rows() as
normal, with the third parameter NULL. Make sure you make pass over
the image number_of_passes times, and you don't change the data in the
rows between calls. You can change the locations of the data, just
png_read_rows(png_ptr, row_pointers, NULL,
number_of_rows);
+ or
+ png_read_row(png_ptr, row_pointers, NULL);
If you only want the first effect (the rectangles), do the same as
before except pass the row buffer in the third parameter, and leave
png_read_rows(png_ptr, NULL, row_pointers,
number_of_rows);
+ or
+ png_read_row(png_ptr, NULL, row_pointers);
If you don't want libpng to handle the interlacing details, just call
png_read_rows() PNG_INTERLACE_ADAM7_PASSES times to read in all the images.
{
png_destroy_read_struct(&png_ptr, &info_ptr,
(png_infopp)NULL);
- return (ERROR);
+ return ERROR;
}
png_read_end(png_ptr, end_info);
PNG_INFO_gAMA, PNG_INFO_sBIT,
PNG_INFO_cHRM, PNG_INFO_PLTE,
PNG_INFO_tRNS, PNG_INFO_bKGD,
+ PNG_INFO_eXIf,
PNG_INFO_hIST, PNG_INFO_pHYs,
PNG_INFO_oFFs, PNG_INFO_tIME,
PNG_INFO_pCAL, PNG_INFO_sRGB,
user_error_fn, user_warning_fn);
if (!png_ptr)
- return (ERROR);
+ return ERROR;
info_ptr = png_create_info_struct(png_ptr);
{
png_destroy_read_struct(&png_ptr,
(png_infopp)NULL, (png_infopp)NULL);
- return (ERROR);
+ return ERROR;
}
if (setjmp(png_jmpbuf(png_ptr)))
{
png_destroy_read_struct(&png_ptr, &info_ptr,
(png_infopp)NULL);
- return (ERROR);
+ return ERROR;
}
/* This one's new. You can provide functions
{
png_destroy_read_struct(&png_ptr, &info_ptr,
(png_infopp)NULL);
- return (ERROR);
+ return ERROR;
}
/* This one's new also. Simply give it a chunk
FILE *fp = fopen(file_name, "wb");
if (!fp)
- return (ERROR);
+ return ERROR;
Next, png_struct and png_info need to be allocated and initialized.
As these can be both relatively large, you may not want to store these
user_error_fn, user_warning_fn);
if (!png_ptr)
- return (ERROR);
+ return ERROR;
png_infop info_ptr = png_create_info_struct(png_ptr);
if (!info_ptr)
{
png_destroy_write_struct(&png_ptr,
(png_infopp)NULL);
- return (ERROR);
+ return ERROR;
}
If you want to use your own memory allocation routines,
{
png_destroy_write_struct(&png_ptr, &info_ptr);
fclose(fp);
- return (ERROR);
+ return ERROR;
}
...
return;
PNG_FILTER_UP | PNG_FILTER_VALUE_UP |
PNG_FILTER_AVG | PNG_FILTER_VALUE_AVG |
PNG_FILTER_PAETH | PNG_FILTER_VALUE_PAETH|
- PNG_ALL_FILTERS);
+ PNG_ALL_FILTERS | PNG_FAST_FILTERS);
If an application wants to start and stop using particular filters during
compression, it should start out with all of the filters (to ensure that
single transparent color for
non-paletted images (PNG_INFO_tRNS)
+ png_set_eXIf_1(png_ptr, info_ptr, num_exif, exif);
+
+ exif - Exif profile (array of
+ png_byte) (PNG_INFO_eXIf)
+
png_set_hIST(png_ptr, info_ptr, hist);
hist - histogram of palette (array of
formats do not accommodate your needs then you can, and should, use the more
sophisticated APIs above - these support a wide variety of in-memory formats
and a wide variety of sophisticated transformations to those formats as well
-as a wide variety of APIs to manipulate ancilliary information.
+as a wide variety of APIs to manipulate ancillary information.
To read a PNG file using the simplified API:
When the simplified API needs to convert between sRGB and linear colorspaces,
the actual sRGB transfer curve defined in the sRGB specification (see the
-article at http://en.wikipedia.org/wiki/SRGB) is used, not the gamma=1/2.2
+article at https://en.wikipedia.org/wiki/SRGB) is used, not the gamma=1/2.2
approximation used elsewhere in libpng.
When an alpha channel is present it is expected to denote pixel coverage
the 'flags' field of png_image.
PNG_IMAGE_FLAG_COLORSPACE_NOT_sRGB == 0x01
- This indicates the the RGB values of the in-memory bitmap do not
+ This indicates that the RGB values of the in-memory bitmap do not
correspond to the red, green and blue end-points defined by sRGB.
PNG_IMAGE_FLAG_FAST == 0x02
The PNG header is read from the stdio FILE object.
int png_image_begin_read_from_memory(png_imagep image,
- png_const_voidp memory, png_size_t size)
+ png_const_voidp memory, size_t size)
The PNG header is read from the given memory buffer.
When the simplified API needs to convert between sRGB and linear colorspaces,
the actual sRGB transfer curve defined in the sRGB specification (see the
-article at http://en.wikipedia.org/wiki/SRGB) is used, not the gamma=1/2.2
+article at https://en.wikipedia.org/wiki/SRGB) is used, not the gamma=1/2.2
approximation used elsewhere in libpng.
WRITE APIS
Write the image to the named file.
+ int png_image_write_to_memory (png_imagep image, void *memory,
+ png_alloc_size_t * PNG_RESTRICT memory_bytes,
+ int convert_to_8_bit, const void *buffer, ptrdiff_t row_stride,
+ const void *colormap));
+
+ Write the image to memory.
+
int png_image_write_to_stdio(png_imagep image, FILE *file,
int convert_to_8_bit, const void *buffer,
png_int_32 row_stride, const void *colormap)
The replacement I/O functions must have prototypes as follows:
void user_read_data(png_structp png_ptr,
- png_bytep data, png_size_t length);
+ png_bytep data, size_t length);
void user_write_data(png_structp png_ptr,
- png_bytep data, png_size_t length);
+ png_bytep data, size_t length);
void user_flush_data(png_structp png_ptr);
png_voidp error_ptr, png_error_ptr error_fn,
png_error_ptr warning_fn);
- png_voidp error_ptr = png_get_error_ptr(png_ptr);
-
If NULL is supplied for either error_fn or warning_fn, then the libpng
default function will be used, calling fprintf() and/or longjmp() if a
problem is encountered. The replacement error functions should have
void user_warning_fn(png_structp png_ptr,
png_const_charp warning_msg);
+Then, within your user_error_fn or user_warning_fn, you can retrieve
+the error_ptr if you need it, by calling
+
+ png_voidp error_ptr = png_get_error_ptr(png_ptr);
+
The motivation behind using setjmp() and longjmp() is the C++ throw and
catch exception handling methods. This makes the code much easier to write,
as there is no need to check every return code of every function call.
after a longjmp, so the user may want to be careful about doing anything
after setjmp returns non-zero besides returning itself. Consult your
compiler documentation for more details. For an alternative approach, you
-may wish to use the "cexcept" facility (see http://cexcept.sourceforge.net),
+may wish to use the "cexcept" facility (see https://cexcept.sourceforge.io/),
which is illustrated in pngvalid.c and in contrib/visupng.
Beginning in libpng-1.4.0, the png_set_benign_errors() API became available.
The 'method' parameter sets the main filtering method, which is
currently only '0' in the PNG 1.2 specification. The 'filters'
parameter sets which filter(s), if any, should be used for each
-scanline. Possible values are PNG_ALL_FILTERS and PNG_NO_FILTERS
-to turn filtering on and off, respectively.
+scanline. Possible values are PNG_ALL_FILTERS, PNG_NO_FILTERS,
+or PNG_FAST_FILTERS to turn filtering on and off, or to turn on
+just the fast-decoding subset of filters, respectively.
Individual filter types are PNG_FILTER_NONE, PNG_FILTER_SUB,
PNG_FILTER_UP, PNG_FILTER_AVG, PNG_FILTER_PAETH, which can be bitwise
currently does not allocate the filter buffers until png_write_row()
is called for the first time.)
- filters = PNG_FILTER_NONE | PNG_FILTER_SUB
+ filters = PNG_NO_FILTERS;
+ filters = PNG_ALL_FILTERS;
+ filters = PNG_FAST_FILTERS;
+
+ or
+
+ filters = PNG_FILTER_NONE | PNG_FILTER_SUB |
PNG_FILTER_UP | PNG_FILTER_AVG |
- PNG_FILTER_PAETH | PNG_ALL_FILTERS;
+ PNG_FILTER_PAETH;
png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE,
filters);
+
The second parameter can also be
PNG_INTRAPIXEL_DIFFERENCING if you are
writing a PNG to be embedded in a MNG
having level = 0 will be printed. There aren't any such statements in
this version of libpng, but if you insert some they will be printed.
-VII. MNG support
+VII. MNG support
The MNG specification (available at http://www.libpng.org/pub/mng) allows
certain extensions to PNG for PNG images that are embedded in MNG datastreams.
and the MHDR and MEND chunks. Libpng does not provide support for these
or any other MNG chunks; your application must provide its own support for
them. You may wish to consider using libmng (available at
-http://www.libmng.com) instead.
+https://www.libmng.com/) instead.
-VIII. Changes to Libpng from version 0.88
+VIII. Changes to Libpng from version 0.88
It should be noted that versions of libpng later than 0.96 are not
distributed by the original libpng author, Guy Schalnat, nor by
png_uint_32 application_vn = PNG_LIBPNG_VER;
-IX. Changes to Libpng from version 1.0.x to 1.2.x
+IX. Changes to Libpng from version 1.0.x to 1.2.x
Support for user memory management was enabled by default. To
accomplish this, the functions png_create_read_struct_2(),
png_set_expand_gray_1_2_4_to_8()
which does not. It has been deprecated since libpng-1.0.18 and 1.2.9.
-X. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x
+X. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x
Private libpng prototypes and macro definitions were moved from
png.h and pngconf.h into a new pngpriv.h header file.
the process.
We changed the prototypes of png_get_compression_buffer_size() and
-png_set_compression_buffer_size() to work with png_size_t instead of
+png_set_compression_buffer_size() to work with size_t instead of
png_uint_32.
Support for numbered error messages was removed by default, since we
We removed the trailing '.' from the warning and error messages.
-XI. Changes to Libpng from version 1.4.x to 1.5.x
+XI. Changes to Libpng from version 1.4.x to 1.5.x
From libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro (but not the
function) incorrectly returned a value of type png_uint_32.
the 1.4.5 API and the 1.5.0 API; however, the ability to directly access
members of the main libpng control structures, png_struct and png_info,
deprecated in earlier versions of libpng, has been completely removed from
-libpng 1.5.
+libpng 1.5, and new private "pngstruct.h", "pnginfo.h", and "pngdebug.h"
+header files were created.
We no longer include zlib.h in png.h. The include statement has been moved
to pngstruct.h, where it is not accessible by applications. Applications that
There are changes of form in png.h, including new and changed macros to
declare parts of the API. Some API functions with arguments that are
pointers to data not modified within the function have been corrected to
-declare these arguments with PNG_CONST.
+declare these arguments with const.
Much of the internal use of C macros to control the library build has also
changed and some of this is visible in the exported header files, in
that it could be used to override them. Now this function will reduce or
increase the limits.
-Starting in libpng-1.5.10, the user limits can be set en masse with the
-configuration option PNG_SAFE_LIMITS_SUPPORTED. If this option is enabled,
-a set of "safe" limits is applied in pngpriv.h. These can be overridden by
-application calls to png_set_user_limits(), png_set_user_chunk_cache_max(),
-and/or png_set_user_malloc_max() that increase or decrease the limits. Also,
-in libpng-1.5.10 the default width and height limits were increased
-from 1,000,000 to 0x7fffffff (i.e., made unlimited). Therefore, the
-limits are now
- default safe
+Starting in libpng-1.5.22, default user limits were established. These
+can be overridden by application calls to png_set_user_limits(),
+png_set_user_chunk_cache_max(), and/or png_set_user_malloc_max().
+The limits are now
+ max possible default
png_user_width_max 0x7fffffff 1,000,000
png_user_height_max 0x7fffffff 1,000,000
- png_user_chunk_cache_max 0 (unlimited) 128
+ png_user_chunk_cache_max 0 (unlimited) 1000
png_user_chunk_malloc_max 0 (unlimited) 8,000,000
The png_set_option() function (and the "options" member of the png struct) was
to choose at app buildtime whether or not to use macros (previously
impossible because the functions weren't in the default build.)
-XII. Changes to Libpng from version 1.5.x to 1.6.x
+XII. Changes to Libpng from version 1.5.x to 1.6.x
A "simplified API" has been added (see documentation in png.h and a simple
example in contrib/examples/pngtopng.c). The new publicly visible API
png_image_free()
write functions
png_image_write_to_file()
+ png_image_write_to_memory()
png_image_write_to_stdio()
Starting with libpng-1.6.0, you can configure libpng to prefix all exported
enforced, and the palette was always limited to 256 entries. An over-length
PLTE chunk found in an input PNG is silently truncated.
-XIII. Detecting libpng
+Starting with libpng-1.6.31, the eXIf chunk is supported. Libpng does not
+attempt to decode the Exif profile; it simply returns a byte array
+containing the profile to the calling application which must do its own
+decoding.
+
+XIII. Detecting libpng
The png_get_io_ptr() function has been present since libpng-0.88, has never
changed, and is unaffected by conditional compilation macros. It is the
going back to version 0.70. You can access the git repository (read only)
at
- git://git.code.sf.net/p/libpng/code
+ https://github.com/glennrp/libpng or
+ https://git.code.sf.net/p/libpng/code.git
+
+or you can browse it with a web browser at
-or you can browse it with a web browser by selecting the "code" button at
+ https://github.com/glennrp/libpng or
+ https://sourceforge.net/p/libpng/code/ci/libpng16/tree/
- https://sourceforge.net/projects/libpng
+Patches can be sent to png-mng-implement at lists.sourceforge.net or
+uploaded to the libpng bug tracker at
-Patches can be sent to glennrp at users.sourceforge.net or to
-png-mng-implement at lists.sourceforge.net or you can upload them to
-the libpng bug tracker at
+ https://libpng.sourceforge.io/
- http://libpng.sourceforge.net
+or as a "pull request" to
+
+ https://github.com/glennrp/libpng/pulls
We also accept patches built from the tar or zip distributions, and
-simple verbal discriptions of bug fixes, reported either to the
+simple verbal descriptions of bug fixes, reported either to the
SourceForge bug tracker, to the png-mng-implement at lists.sf.net
-mailing list, or directly to glennrp.
+mailing list, as github issues.
XV. Coding style
Our coding style is similar to the "Allman" style
-(See http://en.wikipedia.org/wiki/Indent_style#Allman_style), with curly
+(See https://en.wikipedia.org/wiki/Indent_style#Allman_style), with curly
braces on separate lines:
if (condition)
The braces can be omitted from simple one-line actions:
if (condition)
- return (0);
+ return 0;
We use 3-space indentation, except for continued statements which
are usually indented the same as the first line of the statement
though it were a function.
Control keywords if, for, while, and switch are always followed by a space
-to distinguish them from function calls, which have no trailing space.
+to distinguish them from function calls, which have no trailing space.
We put a space after each comma and after each semicolon
in "for" statements, and we put spaces before and after each
We prefer to use underscores rather than camelCase in names, except
for a few type names that we inherit from zlib.h.
-We prefer "if (something != 0)" and "if (something == 0)"
-over "if (something)" and if "(!something)", respectively.
+We prefer "if (something != 0)" and "if (something == 0)" over
+"if (something)" and if "(!something)", respectively, and for pointers
+we prefer "if (some_pointer != NULL)" or "if (some_pointer == NULL)".
We do not use the TAB character for indentation in the C sources.
Lines do not exceed 80 characters.
Other rules can be inferred by inspecting the libpng source.
-
-XVI. Y2K Compliance in libpng
-
-Since the PNG Development group is an ad-hoc body, we can't make
-an official declaration.
-
-This is your unofficial assurance that libpng from version 0.71 and
-upward through 1.6.21 are Y2K compliant. It is my belief that earlier
-versions were also Y2K compliant.
-
-Libpng only has two year fields. One is a 2-byte unsigned integer
-that will hold years up to 65535. The other, which is deprecated,
-holds the date in text format, and will hold years up to 9999.
-
-The integer is
- "png_uint_16 year" in png_time_struct.
-
-The string is
- "char time_buffer[29]" in png_struct. This is no longer used
-in libpng-1.6.x and will be removed from libpng-1.7.0.
-
-There are seven time-related functions:
-
- png_convert_to_rfc_1123_buffer() in png.c
- (formerly png_convert_to_rfc_1152() in error, and
- also formerly png_convert_to_rfc_1123())
- png_convert_from_struct_tm() in pngwrite.c, called
- in pngwrite.c
- png_convert_from_time_t() in pngwrite.c
- png_get_tIME() in pngget.c
- png_handle_tIME() in pngrutil.c, called in pngread.c
- png_set_tIME() in pngset.c
- png_write_tIME() in pngwutil.c, called in pngwrite.c
-
-All appear to handle dates properly in a Y2K environment. The
-png_convert_from_time_t() function calls gmtime() to convert from system
-clock time, which returns (year - 1900), which we properly convert to
-the full 4-digit year. There is a possibility that applications using
-libpng are not passing 4-digit years into the png_convert_to_rfc_1123()
-function, or that they are incorrectly passing only a 2-digit year
-instead of "year - 1900" into the png_convert_from_struct_tm() function,
-but this is not under our control. The libpng documentation has always
-stated that it works with 4-digit years, and the APIs have been
-documented as such.
-
-The tIME chunk itself is also Y2K compliant. It uses a 2-byte unsigned
-integer to hold the year, and can hold years as large as 65535.
-
-zlib, upon which libpng depends, is also Y2K compliant. It contains
-no date-related code.
-
-
- Glenn Randers-Pehrson
- libpng maintainer
- PNG Development Group
projects / platform / upstream / libpng.git / blobdiff