libpng-manual.txt - A description on how to use and modify libpng
- libpng version 1.6.9 - February 6, 2014
+ libpng version 1.6.19 - November 12, 2015
Updated and distributed by Glenn Randers-Pehrson
<glennrp at users.sourceforge.net>
- Copyright (c) 1998-2014 Glenn Randers-Pehrson
+ Copyright (c) 1998-2015 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 1.6.9 - February 6, 2014
+ libpng versions 0.97, January 1998, through 1.6.19 - November 12, 2015
Updated and distributed by Glenn Randers-Pehrson
- Copyright (c) 1998-2014 Glenn Randers-Pehrson
+ Copyright (c) 1998-2015 Glenn Randers-Pehrson
- libpng 1.0 beta 6 version 0.96 May 28, 1997
+ libpng 1.0 beta 6 - version 0.96 - May 28, 1997
Updated and distributed by Andreas Dilger
Copyright (c) 1996, 1997 Andreas Dilger
- libpng 1.0 beta 2 - version 0.88 January 26, 1996
+ libpng 1.0 beta 2 - version 0.88 - January 26, 1996
For conditions of distribution and use, see copyright
notice in png.h. Copyright (c) 1995, 1996 Guy Eric
Schalnat, Group 42, Inc.
file, example.c is a good starting point for using the library, as
it is heavily commented and should include everything most people
will need. We assume that libpng is already installed; see the
-INSTALL file for instructions on how to install libpng.
+INSTALL file for instructions on how to configure and install libpng.
For examples of libpng usage, see the files "example.c", "pngtest.c",
and the files in the "contrib" directory, all of which are included in
The W3C and ISO documents have identical technical content.
The PNG-1.2 specification is available at
-<http://www.libpng.org/pub/png/documents/>. It is technically equivalent
+<http://png-mng.sourceforge.net/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://www.libpng.org/pub/png/documents/> and as a
-W3C Recommendation <http://www.w3.org/TR/REC.png.html>.
+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>.
Some additional chunks are described in the special-purpose public chunks
-documents at <http://www.libpng.org/pub/png/documents/>.
+documents at <http://www.libpng.org/pub/png/spec/register/>
Other information
about PNG, and the latest version of libpng, can be found at the PNG home
Libpng uses zlib for its compression and decompression of PNG files.
Further information about zlib, and the latest version of zlib, can
-be found at the zlib home page, <http://www.info-zip.org/pub/infozip/zlib/>.
+be found at the zlib home page, <http://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.
contrib/pngminim/*. See the "$(PNGCONF):" target in the makefile and
pngusr.dfa in these directories.
-C. Configuration using PNG_USR_CONFIG
+C. Configuration using PNG_USER_CONFIG
-If -DPNG_USR_CONFIG is added to the CFLAGS when pnglibconf.h is built the file
-pngusr.h will automatically be included before the options in
+If -DPNG_USER_CONFIG is added to the CPPFLAGS when pnglibconf.h is built,
+the file pngusr.h will automatically be included before the options in
scripts/pnglibconf.dfa are processed. Your pngusr.h file should contain only
macro definitions turning features on or off or setting settings.
If you are intending to keep the file pointer open for use in libpng,
you must ensure you don't read more than 8 bytes from the beginning
-of the file, and you also have to make a call to png_set_sig_bytes_read()
+of the file, and you also have to make a call to png_set_sig_bytes()
with the number of bytes you read from the beginning. Libpng will
then only check the bytes (if any) that your program didn't read.
to replace them with custom functions. See the discussion under
Customizing libpng.
-
FILE *fp = fopen(file_name, "rb");
if (!fp)
{
return (ERROR);
}
- fread(header, 1, number, fp);
- is_png = !png_sig_cmp(header, 0, number);
+ if (fread(header, 1, number, fp) != number)
+ {
+ return (ERROR);
+ }
+ is_png = !png_sig_cmp(header, 0, number);
if (!is_png)
{
return (NOT_PNG);
}
-
Next, png_struct and png_info need to be allocated and initialized. In
order to ensure that the size of these structures is correct even with a
dynamically linked libpng, there are functions to initialize and
The PNG specification allows the width and height of an image to be as
large as 2^31-1 (0x7fffffff), or about 2.147 billion rows and columns.
-Since very few applications really need to process such large images,
-we have imposed an arbitrary 1-million limit on rows and columns.
+For safety, libpng imposes a default limit of 1 million rows and columns.
Larger images will be rejected immediately with a png_error() call. If
-you wish to change this limit, you can use
+you wish to change these limits, you can use
png_set_user_limits(png_ptr, width_max, height_max);
-to set your own limits, or use width_max = height_max = 0x7fffffffL
-to allow all valid dimensions (libpng may reject some very large images
+to set your own limits (libpng may reject some very wide images
anyway because of potential buffer overflow conditions).
You should put this statement after you create the PNG structure and
height_max = png_get_user_height_max(png_ptr);
The PNG specification sets no limit on the number of ancillary chunks
-allowed in a PNG datastream. You can impose a limit on the total number
-of sPLT, tEXt, iTXt, zTXt, and unknown chunks that will be stored, with
+allowed in a PNG datastream. By default, libpng imposes a limit of
+a total of 1000 sPLT, tEXt, iTXt, zTXt, and unknown chunks to be stored.
+If you have set up both info_ptr and end_info_ptr, the limit applies
+separately to each. You can change the limit on the total number of such
+chunks that will be stored, with
png_set_chunk_cache_max(png_ptr, user_chunk_cache_max);
chunk_cache_max = png_get_chunk_cache_max(png_ptr);
-You can also set a limit on the amount of memory that a compressed chunk
-other than IDAT can occupy, with
+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
png_set_chunk_malloc_max(png_ptr, user_chunk_malloc_max);
case the required information is missing from the file. By default libpng
assumes that the PNG data matches your system, to keep this default call:
- png_set_gamma(png_ptr, screen_gamma, 1/screen_gamma/*file gamma*/);
+ png_set_gamma(png_ptr, screen_gamma, output_gamma);
or you can use the fixed point equivalent:
png_set_gamma_fixed(png_ptr, PNG_FP_1*screen_gamma,
- PNG_FP_1/screen_gamma);
+ PNG_FP_1*output_gamma);
If you don't know the gamma for your system it is probably 2.2 - a good
approximation to the IEC standard for display systems (sRGB). If images are
uses linear values for this reason, often with higher precision component values
to preserve overall accuracy.
+
+The output_gamma value expresses how to decode the output values, not how
+they are encoded. The values used correspond to the normal numbers used to
+describe the overall gamma of a computer display system; for example 2.2 for
+an sRGB conformant system. The values are scaled by 100000 in the _fixed
+version of the API (so 220000 for sRGB.)
+
+The inverse of the value is always used to provide a default for the PNG file
+encoding if it has no gAMA chunk and if png_set_gamma() has not been called
+to override the PNG gamma information.
+
+When the ALPHA_OPTIMIZED mode is selected the output gamma is used to encode
+opaque pixels however pixels with lower alpha values are not encoded,
+regardless of the output gamma setting.
+
+When the standard Porter Duff handling is requested with mode 1 the output
+encoding is set to be linear and the output_gamma value is only relevant
+as a default for input data that has no gamma information. The linear output
+encoding will be overridden if png_set_gamma() is called - the results may be
+highly unexpected!
+
+The following numbers are derived from the sRGB standard and the research
+behind it. sRGB is defined to be approximated by a PNG gAMA chunk value of
+0.45455 (1/2.2) for PNG. The value implicitly includes any viewing
+correction required to take account of any differences in the color
+environment of the original scene and the intended display environment; the
+value expresses how to *decode* the image for display, not how the original
+data was *encoded*.
+
+sRGB provides a peg for the PNG standard by defining a viewing environment.
+sRGB itself, and earlier TV standards, actually use a more complex transform
+(a linear portion then a gamma 2.4 power law) than PNG can express. (PNG is
+limited to simple power laws.) By saying that an image for direct display on
+an sRGB conformant system should be stored with a gAMA chunk value of 45455
+(11.3.3.2 and 11.3.3.5 of the ISO PNG specification) the PNG specification
+makes it possible to derive values for other display systems and
+environments.
+
+The Mac value is deduced from the sRGB based on an assumption that the actual
+extra viewing correction used in early Mac display systems was implemented as
+a power 1.45 lookup table.
+
+Any system where a programmable lookup table is used or where the behavior of
+the final display device characteristics can be changed requires system
+specific code to obtain the current characteristic. However this can be
+difficult and most PNG gamma correction only requires an approximate value.
+
+By default, if png_set_alpha_mode() is not called, libpng assumes that all
+values are unencoded, linear, values and that the output device also has a
+linear characteristic. This is only very rarely correct - it is invariably
+better to call png_set_alpha_mode() with PNG_DEFAULT_sRGB than rely on the
+default if you don't know what the right answer is!
+
+The special value PNG_GAMMA_MAC_18 indicates an older Mac system (pre Mac OS
+10.6) which used a correction table to implement a somewhat lower gamma on an
+otherwise sRGB system.
+
+Both these values are reserved (not simple gamma values) in order to allow
+more precise correction internally in the future.
+
+NOTE: the values can be passed to either the fixed or floating
+point APIs, but the floating point API will also accept floating point
+values.
+
The second thing you may need to tell libpng about is how your system handles
alpha channel information. Some, but not all, PNG files contain an alpha
channel. To display these files correctly you need to compose the data onto a
The mode is as follows:
- PNG_ALPHA_PNG: The data is encoded according to the PNG specification. Red,
-green and blue, or gray, components are gamma encoded color
-values and are not premultiplied by the alpha value. The
-alpha value is a linear measure of the contribution of the
-pixel to the corresponding final output pixel.
+ PNG_ALPHA_PNG: The data is encoded according to the PNG
+specification. Red, green and blue, or gray, components are
+gamma encoded color values and are not premultiplied by the
+alpha value. The alpha value is a linear measure of the
+contribution of the pixel to the corresponding final output pixel.
You should normally use this format if you intend to perform
color correction on the color values; most, maybe all, color
The remaining modes assume you don't need to do any further color correction or
that if you do, your color correction software knows all about alpha (it
-probably doesn't!)
-
- PNG_ALPHA_STANDARD: The data libpng produces
-is encoded in the standard way
-assumed by most correctly written graphics software.
+probably doesn't!). They 'associate' the alpha with the color information by
+storing color channel values that have been scaled by the alpha. The
+advantage is that the color channels can be resampled (the image can be
+scaled) in this form. The disadvantage is that normal practice is to store
+linear, not (gamma) encoded, values and this requires 16-bit channels for
+still images rather than the 8-bit channels that are just about sufficient if
+gamma encoding is used. In addition all non-transparent pixel values,
+including completely opaque ones, must be gamma encoded to produce the final
+image. These are the 'STANDARD', 'ASSOCIATED' or 'PREMULTIPLIED' modes
+described below (the latter being the two common names for associated alpha
+color channels). Note that PNG files always contain non-associated color
+channels; png_set_alpha_mode() with one of the modes causes the decoder to
+convert the pixels to an associated form before returning them to your
+application.
+
+Since it is not necessary to perform arithmetic on opaque color values so
+long as they are not to be resampled and are in the final color space it is
+possible to optimize the handling of alpha by storing the opaque pixels in
+the PNG format (adjusted for the output color space) while storing partially
+opaque pixels in the standard, linear, format. The accuracy required for
+standard alpha composition is relatively low, because the pixels are
+isolated, therefore typically the accuracy loss in storing 8-bit linear
+values is acceptable. (This is not true if the alpha channel is used to
+simulate transparency over large areas - use 16 bits or the PNG mode in
+this case!) This is the 'OPTIMIZED' mode. For this mode a pixel is
+treated as opaque only if the alpha value is equal to the maximum value.
+
+ PNG_ALPHA_STANDARD: The data libpng produces is encoded in the
+standard way assumed by most correctly written graphics software.
The gamma encoding will be removed by libpng and the
linear component values will be pre-multiplied by the
alpha channel.
supports it, use png_set_expand_16() to force all
components to 16 bits.
- PNG_ALPHA_OPTIMIZED: This mode is the same
-as PNG_ALPHA_STANDARD except that
-completely opaque pixels are gamma encoded according to
+ PNG_ALPHA_OPTIMIZED: This mode is the same as PNG_ALPHA_STANDARD
+except that completely opaque pixels are gamma encoded according to
the screen_gamma value. Pixels with alpha less than 1.0
will still have linear components.
You can also try this format if your software is broken;
it might look better.
- PNG_ALPHA_BROKEN: This is PNG_ALPHA_STANDARD;
-however, all component values,
-including the alpha channel are gamma encoded. This is
-an appropriate format to try if your software, or more
-likely hardware, is totally broken, i.e., if it performs
-linear arithmetic directly on gamma encoded values.
-
-In most cases of broken software or hardware the bug in the final display
-manifests as a subtle halo around composited parts of the image. You may not
-even perceive this as a halo; the composited part of the image may simply appear
-separate from the background, as though it had been cut out of paper and pasted
-on afterward.
+ PNG_ALPHA_BROKEN: This is PNG_ALPHA_STANDARD; however, all component
+values, including the alpha channel are gamma encoded. This is
+broken because, in practice, no implementation that uses this choice
+correctly undoes the encoding before handling alpha composition. Use this
+choice only if other serious errors in the software or hardware you use
+mandate it. In most cases of broken software or hardware the bug in the
+final display manifests as a subtle halo around composited parts of the
+image. You may not even perceive this as a halo; the composited part of
+the image may simply appear separate from the background, as though it had
+been cut out of paper and pasted on afterward.
If you don't have to deal with bugs in software or hardware, or if you can fix
them, there are three recommended ways of using png_set_alpha_mode():
mode is libpng-specific you also need to write your own composition
software.
+The following are examples of calls to png_set_alpha_mode to achieve the
+required overall gamma correction and, where necessary, alpha
+premultiplication.
+
+ 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
+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
+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.
+Most of the time the output precision will be so low as to show
+significant banding in dark areas of the image.
+
+ png_set_expand_16(pp);
+ png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_DEFAULT_sRGB);
+
+This is a somewhat more realistic Jim Blinn inspired approach. PNG files
+are assumed to have the sRGB encoding if not marked with a gamma value and
+the output is always 16 bits per component. This permits accurate scaling
+and processing of the data. If you know that your input PNG files were
+generated locally you might need to replace PNG_DEFAULT_sRGB with the
+correct value for your system.
+
+ png_set_alpha_mode(pp, PNG_ALPHA_OPTIMIZED, PNG_DEFAULT_sRGB);
+
+If you just need to composite the PNG image onto an existing background
+and if you control the code that does this you can use the optimization
+setting. In this case you just copy completely opaque pixels to the
+output. For pixels that are not completely transparent (you just skip
+those) you do the composition math using png_composite or png_composite_16
+below then encode the resultant 8-bit or 16-bit values to match the output
+encoding.
+
+ Other cases
+
+If neither the PNG nor the standard linear encoding work for you because
+of the software or hardware you use then you have a big problem. The PNG
+case will probably result in halos around the image. The linear encoding
+will probably result in a washed out, too bright, image (it's actually too
+contrasty.) Try the ALPHA_OPTIMIZED mode above - this will probably
+substantially reduce the halos. Alternatively try:
+
+ png_set_alpha_mode(pp, PNG_ALPHA_BROKEN, PNG_DEFAULT_sRGB);
+
+This option will also reduce the halos, but there will be slight dark
+halos round the opaque parts of the image where the background is light.
+In the OPTIMIZED mode the halos will be light halos where the background
+is dark. Take your pick - the halos are unavoidable unless you can get
+your hardware/software fixed! (The OPTIMIZED approach is slightly
+faster.)
+
+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
+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
+default if it is not already set:
+
+ png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB);
+ png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC);
+
+The first call sets both the default and the output gamma values, the
+second call overrides the output gamma without changing the default. This
+is easier than achieving the same effect with png_set_gamma. You must use
+PNG_ALPHA_PNG for the first call - internal checking in png_set_alpha will
+fire if more than one call to png_set_alpha_mode and png_set_background is
+made in the same read operation, however multiple calls with PNG_ALPHA_PNG
+are ignored.
+
If you don't need, or can't handle, the alpha channel you can call
png_set_background() to remove it by compositing against a fixed color. Don't
call png_set_strip_alpha() to do this - it will leave spurious pixel values in
the PNG datastream is embedded in
a MNG-1.0 datastream)
- Any or all of interlace_type, compression_type, or
- filter_method can be NULL if you are
- not interested in their values.
+ Any of width, height, color_type, bit_depth,
+ interlace_type, compression_type, or filter_method can
+ be NULL if you are not interested in their values.
Note that png_get_IHDR() returns 32-bit data into
the application's width and height variables.
- This is an unsafe situation if these are 16-bit
+ This is an unsafe situation if these are not png_uint_32
variables. In such situations, the
png_get_image_width() and png_get_image_height()
functions described below are safer.
png_get_sRGB(png_ptr, info_ptr, &srgb_intent);
- file_srgb_intent - the rendering intent (PNG_INFO_sRGB)
+ srgb_intent - the rendering intent (PNG_INFO_sRGB)
The presence of the sRGB chunk
means that the pixel data is in the
sRGB color space. This chunk also
Data will be decoded into the supplied row buffers packed into bytes
unless the library has been told to transform it into another format.
For example, 4 bit/pixel paletted or grayscale data will be returned
-2 pixels/byte with the leftmost pixel in the high-order bits of the
-byte, unless png_set_packing() is called. 8-bit RGB data will be stored
+2 pixels/byte with the leftmost pixel in the high-order bits of the byte,
+unless png_set_packing() is called. 8-bit RGB data will be stored
in RGB RGB RGB format unless png_set_filler() or png_set_add_alpha()
is called to insert filler bytes, either before or after each RGB triplet.
+
16-bit RGB data will be returned RRGGBB RRGGBB, with the most significant
byte of the color value first, unless png_set_scale_16() is called to
transform it to regular RGB RGB triplets, or png_set_filler() or
-png_set_add alpha() is called to insert filler bytes, either before or
-after each RRGGBB triplet. Similarly, 8-bit or 16-bit grayscale data can
+png_set_add alpha() is called to insert two filler bytes, either before
+or after each RRGGBB triplet. Similarly, 8-bit or 16-bit grayscale data can
be modified with png_set_filler(), png_set_add_alpha(), png_set_strip_16(),
or png_set_scale_16().
if (color_type == PNG_COLOR_TYPE_RGB)
png_set_filler(png_ptr, filler, PNG_FILLER_BEFORE);
-where "filler" is the 8 or 16-bit number to fill with, and the location is
-either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether
-you want the filler before the RGB or after. This transformation
-does not affect images that already have full alpha channels. To add an
-opaque alpha channel, use filler=0xff or 0xffff and PNG_FILLER_AFTER which
-will generate RGBA pixels.
+where "filler" is the 8-bit or 16-bit number to fill with, and the location
+is either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether
+you want the filler before the RGB or after. When filling an 8-bit pixel,
+the least significant 8 bits of the number are used, if a 16-bit number is
+supplied. This transformation does not affect images that already have full
+alpha channels. To add an opaque alpha channel, use filler=0xffff and
+PNG_FILLER_AFTER which will generate RGBA pixels.
Note that png_set_filler() does not change the color type. If you want
to do that, you can add a true alpha channel with
png_set_add_alpha(png_ptr, filler, PNG_FILLER_AFTER);
where "filler" contains the alpha value to assign to each pixel.
-This function was added in libpng-1.2.7.
+The png_set_add_alpha() function was added in libpng-1.2.7.
If you are reading an image with an alpha channel, and you need the
data as ARGB instead of the normal PNG format RGBA:
The default values come from the PNG file cHRM chunk if present; otherwise, the
defaults correspond to the ITU-R recommendation 709, and also the sRGB color
space, as recommended in the Charles Poynton's Colour FAQ,
-<http://www.poynton.com/>, in section 9:
+Copyright (c) 2006-11-28 Charles Poynton, in section 9:
- <http://www.poynton.com/notes/colour_and_gamma/ColorFAQ.html#RTFToC9>
+<http://www.poynton.com/notes/colour_and_gamma/ColorFAQ.html#RTFToC9>
Y = 0.2126 * R + 0.7152 * G + 0.0722 * B
png_set_gamma(png_ptr, screen_gamma, 0.45455);
If you need to reduce an RGB file to a paletted file, or if a paletted
-file has more entries then will fit on your screen, png_set_quantize()
+file has more entries than will fit on your screen, png_set_quantize()
will do that. Note that this is a simple match quantization that merely
finds the closest color available. This should work fairly well with
optimized palettes, but fairly badly with linear color cubes. If you
Finishing a sequential read
After you are finished reading the image through the
-low-level interface, you can finish reading the file. If you are
-interested in comments or time, which may be stored either before or
-after the image data, you should pass the separate png_info struct if
-you want to keep the comments from before and after the image
+low-level interface, you can finish reading the file.
+
+If you want to use a different crc action for handling CRC errors in
+chunks after the image data, you can call png_set_crc_action()
+again at this point.
+
+If you are interested in comments or time, which may be stored either
+before or after the image data, you should pass the separate png_info
+struct if you want to keep the comments from before and after the image
separate.
png_infop end_info = png_create_info_struct(png_ptr);
If you are not interested, you should still call png_read_end()
but you can pass NULL, avoiding the need to create an end_info structure.
+If you do this, libpng will not process any chunks after IDAT other than
+skipping over them and perhaps (depending on whether you have called
+png_set_crc_action) checking their CRCs while looking for the IEND chunk.
png_read_end(png_ptr, (png_infop)NULL);
64K. The library seems to run fine with sizes
of 4K. Although you can give it much less if
necessary (I assume you can give it chunks of
- 1 byte, I haven't tried less then 256 bytes
+ 1 byte, I haven't tried less than 256 bytes
yet). When this function returns, you may
want to display any rows that were generated
in the row callback if you don't already do
png_progressive_combine_row(png_ptr, old_row,
new_row);
- /* where old_row is what was displayed for
+ /* where old_row is what was displayed
previously for the row. Note that the first
pass (pass == 0, really) will completely cover
the old row, so the rows do not have to be
although this isn't a requirement. Unlike the tIME chunk, the
"Creation Time" tEXt chunk is not expected to be automatically changed
by the software. To facilitate the use of RFC 1123 dates, a function
-png_convert_to_rfc1123_buffer(png_ptr, buffer, png_timep) is provided to
+png_convert_to_rfc1123_buffer(buffer, png_timep) is provided to
convert from PNG time to an RFC 1123 format string. The caller must provide
a writeable buffer of at least 29 bytes.
To read a PNG file using the simplified API:
- 1) Declare a 'png_image' structure (see below) on the
- stack and memset() it to all zero.
+ 1) Declare a 'png_image' structure (see below) on the stack, set the
+ version field to PNG_IMAGE_VERSION and the 'opaque' pointer to NULL
+ (this is REQUIRED, your program may crash if you don't do it.)
2) Call the appropriate png_image_begin_read... function.
- 3) Set the png_image 'format' member to the required
- format and allocate a buffer for the image.
+ 3) Set the png_image 'format' member to the required sample format.
- 4) Call png_image_finish_read to read the image into
- your buffer.
+ 4) Allocate a buffer for the image and, if required, the color-map.
+
+ 5) Call png_image_finish_read to read the image and, if required, the
+ color-map into your buffers.
There are no restrictions on the format of the PNG input itself; all valid
color types, bit depths, and interlace methods are acceptable, and the
input image is transformed as necessary to the requested in-memory format
-during the png_image_finish_read() step.
+during the png_image_finish_read() step. The only caveat is that if you
+request a color-mapped image from a PNG that is full-color or makes
+complex use of an alpha channel the transformation is extremely lossy and the
+result may look terrible.
To write a PNG file using the simplified API:
2) Initialize the members of the structure that describe the
image, setting the 'format' member to the format of the
- image in memory.
+ image samples.
3) Call the appropriate png_image_write... function with a
- pointer to the image to write the PNG data.
+ pointer to the image and, if necessary, the color-map to write
+ the PNG data.
png_image is a structure that describes the in-memory format of an image
-when it is being read or define the in-memory format of an image that you
+when it is being read or defines the in-memory format of an image that you
need to write. The "png_image" structure contains the following members:
+ png_controlp opaque Initialize to NULL, free with png_image_free
png_uint_32 version Set to PNG_IMAGE_VERSION
png_uint_32 width Image width in pixels (columns)
png_uint_32 height Image height in pixels (rows)
png_uint_32 format Image format as defined below
png_uint_32 flags A bit mask containing informational flags
- png_controlp opaque Initialize to NULL, free with png_image_free
png_uint_32 colormap_entries; Number of entries in the color-map
png_uint_32 warning_or_error;
char message[64];
-In the event of an error or warning the following field warning_or_error
+In the event of an error or warning the "warning_or_error"
field will be set to a non-zero value and the 'message' field will contain
a '\0' terminated string with the libpng error or warning message. If both
warnings and an error were encountered, only the error is recorded. If there
are multiple warnings, only the first one is recorded.
-The upper 30 bits of this value are reserved; the low two bits contain
-a two bit code such that a value more than 1 indicates a failure in the API
-just called:
+The upper 30 bits of the "warning_or_error" value are reserved; the low two
+bits contain a two bit code such that a value more than 1 indicates a failure
+in the API just called:
0 - no warning or error
1 - warning
The color/gray channels are not scaled (pre-multiplied) by the alpha
channel and are suitable for passing to color management software.
- b) As a value in the range 0..65535, contained in a 2-byte integer. All
-channels can be converted to the original value by dividing by 65535; all
+ b) As a value in the range 0..65535, contained in a 2-byte integer, in
+the native byte order of the platform on which the application is running.
+All channels can be converted to the original value by dividing by 65535; all
channels are linear. Color channels use the RGB encoding (RGB end-points) of
the sRGB specification. This encoding is identified by the
PNG_FORMAT_FLAG_LINEAR flag below.
+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
+approximation used elsewhere in libpng.
+
When an alpha channel is present it is expected to denote pixel coverage
of the color or luminance channels and is returned as an associated alpha
channel: the color/gray channels are scaled (pre-multiplied) by the alpha
value.
-When a color-mapped image is used as a result of calling
-png_image_read_colormap or png_image_write_colormap the channels are encoded
-in the color-map and the descriptions above apply to the color-map entries.
-The image data is encoded as small integers, value 0..255, that index the
-entries in the color-map. One integer (one byte) is stored for each pixel.
+The samples are either contained directly in the image data, between 1 and 8
+bytes per pixel according to the encoding, or are held in a color-map indexed
+by bytes in the image data. In the case of a color-map the color-map entries
+are individual samples, encoded as above, and the image data has one byte per
+pixel to select the relevant sample from the color-map.
PNG_FORMAT_*
The #defines to be used in png_image::format. Each #define identifies a
particular layout of channel data and, if present, alpha values. There are
-separate defines for each of the two channel encodings.
+separate defines for each of the two component encodings.
-A format is built up using single bit flag values. Not all combinations are
-valid: use the bit flag values below for testing a format returned by the
-read APIs, but set formats from the derived values.
+A format is built up using single bit flag values. All combinations are
+valid. Formats can be built up from the flag values or you can use one of
+the predefined values below. When testing formats always use the FORMAT_FLAG
+macros to test for individual features - future versions of the library may
+add new flags.
When reading or writing color-mapped images the format should be set to the
format of the entries in the color-map then png_image_{read,write}_colormap
called to read or write the color-map and set the format correctly for the
image data. Do not set the PNG_FORMAT_FLAG_COLORMAP bit directly!
-NOTE: libpng can be built with particular features disabled, if you see
+NOTE: libpng can be built with particular features disabled. If you see
compiler errors because the definition of one of the following flags has been
compiled out it is because libpng does not have the required support. It is
possible, however, for the libpng configuration to enable the format on just
-read or just write; in that case you may see an error at run time. You can
-guard against this by checking for the definition of:
+read or just write; in that case you may see an error at run time.
+You can guard against this by checking for the definition of the
+appropriate "_SUPPORTED" macro, one of:
PNG_SIMPLIFIED_{READ,WRITE}_{BGR,AFIRST}_SUPPORTED
- PNG_FORMAT_FLAG_ALPHA 0x01 format with an alpha channel
- PNG_FORMAT_FLAG_COLOR 0x02 color format: otherwise grayscale
- PNG_FORMAT_FLAG_LINEAR 0x04 png_uint_16 channels else png_byte
- PNG_FORMAT_FLAG_COLORMAP 0x08 libpng use only
- PNG_FORMAT_FLAG_BGR 0x10 BGR colors, else order is RGB
- PNG_FORMAT_FLAG_AFIRST 0x20 alpha channel comes first
+ PNG_FORMAT_FLAG_ALPHA format with an alpha channel
+ PNG_FORMAT_FLAG_COLOR color format: otherwise grayscale
+ PNG_FORMAT_FLAG_LINEAR 2-byte channels else 1-byte
+ PNG_FORMAT_FLAG_COLORMAP image data is color-mapped
+ PNG_FORMAT_FLAG_BGR BGR colors, else order is RGB
+ PNG_FORMAT_FLAG_AFIRST alpha channel comes first
Supported formats are as follows. Future versions of libpng may support more
formats; for compatibility with older versions simply check if the format
macro is defined using #ifdef. These defines describe the in-memory layout
of the components of the pixels of the image.
-First the single byte formats:
+First the single byte (sRGB) formats:
- PNG_FORMAT_GRAY 0
- PNG_FORMAT_GA PNG_FORMAT_FLAG_ALPHA
- PNG_FORMAT_AG (PNG_FORMAT_GA|PNG_FORMAT_FLAG_AFIRST)
- PNG_FORMAT_RGB PNG_FORMAT_FLAG_COLOR
- PNG_FORMAT_BGR (PNG_FORMAT_FLAG_COLOR|PNG_FORMAT_FLAG_BGR)
- PNG_FORMAT_RGBA (PNG_FORMAT_RGB|PNG_FORMAT_FLAG_ALPHA)
- PNG_FORMAT_ARGB (PNG_FORMAT_RGBA|PNG_FORMAT_FLAG_AFIRST)
- PNG_FORMAT_BGRA (PNG_FORMAT_BGR|PNG_FORMAT_FLAG_ALPHA)
- PNG_FORMAT_ABGR (PNG_FORMAT_BGRA|PNG_FORMAT_FLAG_AFIRST)
+ PNG_FORMAT_GRAY
+ PNG_FORMAT_GA
+ PNG_FORMAT_AG
+ PNG_FORMAT_RGB
+ PNG_FORMAT_BGR
+ PNG_FORMAT_RGBA
+ PNG_FORMAT_ARGB
+ PNG_FORMAT_BGRA
+ PNG_FORMAT_ABGR
Then the linear 2-byte formats. When naming these "Y" is used to
indicate a luminance (gray) channel. The component order within the pixel
is always the same - there is no provision for swapping the order of the
-components in the linear format.
+components in the linear format. The components are 16-bit integers in
+the native byte order for your platform, and there is no provision for
+swapping the bytes to a different endian condition.
- PNG_FORMAT_LINEAR_Y PNG_FORMAT_FLAG_LINEAR
+ PNG_FORMAT_LINEAR_Y
PNG_FORMAT_LINEAR_Y_ALPHA
- (PNG_FORMAT_FLAG_LINEAR|PNG_FORMAT_FLAG_ALPHA)
PNG_FORMAT_LINEAR_RGB
- (PNG_FORMAT_FLAG_LINEAR|PNG_FORMAT_FLAG_COLOR)
PNG_FORMAT_LINEAR_RGB_ALPHA
- (PNG_FORMAT_FLAG_LINEAR|PNG_FORMAT_FLAG_COLOR|
- PNG_FORMAT_FLAG_ALPHA)
-Color-mapped formats are obtained by calling png_image_{read,write}_colormap,
-as appropriate after setting png_image::format to the format of the color-map
-to be read or written. Applications may check the value of
-PNG_FORMAT_FLAG_COLORMAP to see if they have called the colormap API. The
-format of the color-map may be extracted using the following macro.
+With color-mapped formats the image data is one byte for each pixel. The byte
+is an index into the color-map which is formatted as above. To obtain a
+color-mapped format it is sufficient just to add the PNG_FOMAT_FLAG_COLORMAP
+to one of the above definitions, or you can use one of the definitions below.
- PNG_FORMAT_OF_COLORMAP(fmt) ((fmt) & ~PNG_FORMAT_FLAG_COLORMAP)
+ PNG_FORMAT_RGB_COLORMAP
+ PNG_FORMAT_BGR_COLORMAP
+ PNG_FORMAT_RGBA_COLORMAP
+ PNG_FORMAT_ARGB_COLORMAP
+ PNG_FORMAT_BGRA_COLORMAP
+ PNG_FORMAT_ABGR_COLORMAP
PNG_IMAGE macros
structure. The PNG_IMAGE_SAMPLE_ macros return values appropriate to the
actual image sample values - either the entries in the color-map or the
pixels in the image. The PNG_IMAGE_PIXEL_ macros return corresponding values
-for the pixels and will always return 1 after a call to
-png_image_{read,write}_colormap. The remaining macros return information
-about the rows in the image and the complete image.
+for the pixels and will always return 1 for color-mapped formats. The
+remaining macros return information about the rows in the image and the
+complete image.
NOTE: All the macros that take a png_image::format parameter are compile time
constants if the format parameter is, itself, a constant. Therefore these
Similarly the macros are also pre-processor constants (sizeof is not used) so
they can be used in #if tests.
-First the information about the samples.
-
PNG_IMAGE_SAMPLE_CHANNELS(fmt)
Returns the total number of channels in a given format: 1..4
PNG_IMAGE_SAMPLE_COMPONENT_SIZE(fmt)
Returns the size in bytes of a single component of a pixel or color-map
- entry (as appropriate) in the image.
+ entry (as appropriate) in the image: 1 or 2.
PNG_IMAGE_SAMPLE_SIZE(fmt)
This is the size of the sample data for one sample. If the image is
color-mapped it is the size of one color-map entry (and image pixels are
one byte in size), otherwise it is the size of one image pixel.
+ PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(fmt)
+ The maximum size of the color-map required by the format expressed in a
+ count of components. This can be used to compile-time allocate a
+ color-map:
+
+ png_uint_16 colormap[PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(linear_fmt)];
+
+ png_byte colormap[PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(sRGB_fmt)];
+
+ Alternatively use the PNG_IMAGE_COLORMAP_SIZE macro below to use the
+ information from one of the png_image_begin_read_ APIs and dynamically
+ allocate the required memory.
+
PNG_IMAGE_COLORMAP_SIZE(fmt)
The size of the color-map required by the format; this is the size of the
- color-map buffer passed to the png_image_{read,write}_colormap APIs, it is
+ color-map buffer passed to the png_image_{read,write}_colormap APIs. It is
a fixed number determined by the format so can easily be allocated on the
stack if necessary.
-#define PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(fmt)\
- (PNG_IMAGE_SAMPLE_CHANNELS(fmt) * 256)
- /* The maximum size of the color-map required by the format expressed in a
- * count of components. This can be used to compile-time allocate a
- * color-map:
- *
- * png_uint_16 colormap[PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(linear_fmt)];
- *
- * png_byte colormap[PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(sRGB_fmt)];
- *
- * Alternatively use the PNG_IMAGE_COLORMAP_SIZE macro below to use the
- * information from one of the png_image_begin_read_ APIs and dynamically
- * allocate the required memory.
- */
-
-
Corresponding information about the pixels
- PNG_IMAGE_PIXEL_(test,fmt)
-
PNG_IMAGE_PIXEL_CHANNELS(fmt)
The number of separate channels (components) in a pixel; 1 for a
color-mapped image.
row. For a color-mapped image this is the minimum number of bytes in a
row.
+ If you need the stride measured in bytes, row_stride_bytes is
+ PNG_IMAGE_ROW_STRIDE(image) * PNG_IMAGE_PIXEL_COMPONENT_SIZE(fmt)
+ plus any padding bytes that your application might need, for example
+ to start the next row on a 4-byte boundary.
+
PNG_IMAGE_BUFFER_SIZE(image, row_stride)
- Returns the size, in bytes, of an image buffer given a png_image and a row
- stride - the number of components to leave space for in each row.
+ Return the size, in bytes, of an image buffer given a png_image and a row
+ stride - the number of components to leave space for in each row.
+
+ PNG_IMAGE_SIZE(image)
+ Return the size, in bytes, of the image in memory given just a png_image;
+ the row stride is the minimum stride required for the image.
+
+ PNG_IMAGE_COLORMAP_SIZE(image)
+ Return the size, in bytes, of the color-map of this image. If the image
+ format is not a color-map format this will return a size sufficient for
+ 256 entries in the given format; check PNG_FORMAT_FLAG_COLORMAP if
+ you don't want to allocate a color-map in this case.
+
+PNG_IMAGE_FLAG_*
+
+Flags containing additional information about the image are held in
+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
correspond to the red, green and blue end-points defined by sRGB.
- PNG_IMAGE_FLAG_COLORMAP == 0x02
- The PNG is color-mapped. If this flag is set png_image_read_colormap
- can be used without further loss of image information. If it is not set
- png_image_read_colormap will cause significant loss if the image has any
+ PNG_IMAGE_FLAG_FAST == 0x02
+ On write emphasise speed over compression; the resultant PNG file will be
+ larger but will be produced significantly faster, particular for large
+ images. Do not use this option for images which will be distributed, only
+ used it when producing intermediate files that will be read back in
+ repeatedly. For a typical 24-bit image the option will double the read
+ speed at the cost of increasing the image size by 25%, however for many
+ more compressible images the PNG file can be 10 times larger with only a
+ slight speed gain.
+
+ PNG_IMAGE_FLAG_16BIT_sRGB == 0x04
+ On read if the image is a 16-bit per component image and there is no gAMA
+ or sRGB chunk assume that the components are sRGB encoded. Notice that
+ images output by the simplified API always have gamma information; setting
+ this flag only affects the interpretation of 16-bit images from an
+ external source. It is recommended that the application expose this flag
+ to the user; the user can normally easily recognize the difference between
+ linear and sRGB encoding. This flag has no effect on write - the data
+ passed to the write APIs must have the correct encoding (as defined
+ above.)
+
+ If the flag is not set (the default) input 16-bit per component data is
+ assumed to be linear.
+
+ NOTE: the flag can only be set after the png_image_begin_read_ call,
+ because that call initializes the 'flags' field.
READ APIs
With all APIs row_stride is handled as in the read APIs - it is the spacing
from one row to the next in component sized units (float) and if negative
-indicates a bottom-up row layout in the buffer.
+indicates a bottom-up row layout in the buffer. If you pass zero, libpng will
+calculate the row_stride for you from the width and number of channels.
Note that the write API does not support interlacing, sub-8-bit pixels,
-and indexed (paletted) images.
+indexed (paletted) images, or most ancillary chunks.
VI. Modifying/Customizing libpng
is not the same as the calloc(number, size) function provided by stdlib.h.
There is limited support for certain systems with segmented memory
architectures and the types of pointers declared by png.h match this; you
-will have to use appropriate pointers in your application. Since it is
-unlikely that the method of handling memory allocation on a platform
-will change between applications, these functions must be modified in
-the library at compile time. If you prefer to use a different method
-of allocating and freeing data, you can use png_create_read_struct_2() or
-png_create_write_struct_2() to register your own functions as described
-above. These functions also provide a void pointer that can be retrieved
-via
+will have to use appropriate pointers in your application. If you prefer
+to use a different method of allocating and freeing data, you can use
+png_create_read_struct_2() or png_create_write_struct_2() to register your
+own functions as described above. These functions also provide a void
+pointer that can be retrieved via
mem_ptr=png_get_mem_ptr(png_ptr);
transformation to the one you want to add and copy off of it. More details
can be found in the comments inside the code itself.
-Configuring for 16-bit platforms
-
-You will want to look into zconf.h to tell zlib (and thus libpng) that
-it cannot allocate more then 64K at a time. Even if you can, the memory
-won't be accessible. So limit zlib and libpng to 64K by defining MAXSEG_64K.
-
-Configuring for DOS
-
-For DOS users who only have access to the lower 640K, you will
-have to limit zlib's memory usage via a png_set_compression_mem_level()
-call. See zlib.h or zconf.h in the zlib library for more information.
-
-Configuring for Medium Model
-
-Libpng's support for medium model has been tested on most of the popular
-compilers. Make sure MAXSEG_64K gets defined, USE_FAR_KEYWORD gets
-defined, and FAR gets defined to far in pngconf.h, and you should be
-all set. Everything in the library (except for zlib's structure) is
-expecting far data. You must use the typedefs with the p or pp on
-the end for pointers (or at least look at them and be careful). Make
-note that the rows of data are defined as png_bytepp, which is
-an "unsigned char far * far *".
-
Configuring for gui/windowing platforms:
You will need to write new error and warning functions that use the GUI
They can be changed later via png_set_error_fn(). On some compilers,
you may also have to change the memory allocators (png_malloc, etc.).
-Configuring for compiler xxx:
-
-All includes for libpng are in pngconf.h. If you need to add, change
-or delete an include, this is the place to do it.
-The includes that are not needed outside libpng are placed in pngpriv.h,
-which is only used by the routines inside libpng itself.
-The files in libpng proper only include pngpriv.h and png.h, which
-in turn includes pngconf.h and, as of libpng-1.5.0, pnglibconf.h.
-As of libpng-1.5.0, pngpriv.h also includes three other private header
-files, pngstruct.h, pnginfo.h, and pngdebug.h, which contain material
-that previously appeared in the public headers.
-
Configuring zlib:
There are special functions to configure the compression. Perhaps the
png_set_compression_method(png_ptr, method);
+This controls the size of the IDAT chunks (default 8192):
+
png_set_compression_buffer_size(png_ptr, size);
As of libpng version 1.5.4, additional APIs became
same as the value of filter_method used
in png_set_IHDR().
-It is also possible to influence how libpng chooses from among the
-available filters. This is done in one or both of two ways - by
-telling it how important it is to keep the same filter for successive
-rows, and by telling it the relative computational costs of the filters.
-
- double weights[3] = {1.5, 1.3, 1.1},
- costs[PNG_FILTER_VALUE_LAST] =
- {1.0, 1.3, 1.3, 1.5, 1.7};
-
- png_set_filter_heuristics(png_ptr,
- PNG_FILTER_HEURISTIC_WEIGHTED, 3,
- weights, costs);
-
-The weights are multiplying factors that indicate to libpng that the
-row filter should be the same for successive rows unless another row filter
-is that many times better than the previous filter. In the above example,
-if the previous 3 filters were SUB, SUB, NONE, the SUB filter could have a
-"sum of absolute differences" 1.5 x 1.3 times higher than other filters
-and still be chosen, while the NONE filter could have a sum 1.1 times
-higher than other filters and still be chosen. Unspecified weights are
-taken to be 1.0, and the specified weights should probably be declining
-like those above in order to emphasize recent filters over older filters.
-
-The filter costs specify for each filter type a relative decoding cost
-to be considered when selecting row filters. This means that filters
-with higher costs are less likely to be chosen over filters with lower
-costs, unless their "sum of absolute differences" is that much smaller.
-The costs do not necessarily reflect the exact computational speeds of
-the various filters, since this would unduly influence the final image
-size.
-
-Note that the numbers above were invented purely for this example and
-are given only to help explain the function usage. Little testing has
-been done to find optimum values for either the costs or the weights.
-
-Removing unwanted object code
-
-There are a bunch of #define's in pngconf.h that control what parts of
-libpng are compiled. All the defines end in _SUPPORTED. If you are
-never going to use a capability, you can change the #define to #undef
-before recompiling libpng and save yourself code and data space, or
-you can turn off individual capabilities with defines that begin with
-PNG_NO_.
-
-In libpng-1.5.0 and later, the #define's are in pnglibconf.h instead.
-
-You can also turn all of the transforms and ancillary chunk capabilities
-off en masse with compiler directives that define
-PNG_NO_READ[or WRITE]_TRANSFORMS, or PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS,
-or all four,
-along with directives to turn on any of the capabilities that you do
-want. The PNG_NO_READ[or WRITE]_TRANSFORMS directives disable the extra
-transformations but still leave the library fully capable of reading
-and writing PNG files with all known public chunks. Use of the
-PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS directive produces a library
-that is incapable of reading or writing ancillary chunks. If you are
-not using the progressive reading capability, you can turn that off
-with PNG_NO_PROGRESSIVE_READ (don't confuse this with the INTERLACING
-capability, which you'll still have).
-
-All the reading and writing specific code are in separate files, so the
-linker should only grab the files it needs. However, if you want to
-make sure, or if you are building a stand alone library, all the
-reading files start with "pngr" and all the writing files start with "pngw".
-The files that don't match either (like png.c, pngtrans.c, etc.)
-are used for both reading and writing, and always need to be included.
-The progressive reader is in pngpread.c
-
-If you are creating or distributing a dynamically linked library (a .so
-or DLL file), you should not remove or disable any parts of the library,
-as this will cause applications linked with different versions of the
-library to fail if they call functions not available in your library.
-The size of the library itself should not be an issue, because only
-those sections that are actually used will be loaded into memory.
-
Requesting debug printout
The macro definition PNG_DEBUG can be used to request debugging
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.
-Prepending a prefix to exported symbols
-
-Starting with libpng-1.6.0, you can configure libpng (when using the
-"configure" script) to prefix all exported symbols by means of the
-configuration option "--with-libpng-prefix=FOO_", where FOO_ can be any
-string beginning with a letter and containing only uppercase
-and lowercase letters, digits, and the underscore (i.e., a C language
-identifier). This creates a set of macros in pnglibconf.h, so this is
-transparent to applications; their function calls get transformed by
-the macros to use the modified names.
-
VII. MNG support
The MNG specification (available at http://www.libpng.org/pub/mng) allows
bKGD chunk; you must check those separately to determine the maximum
palette index actually used.
-A. Changes that affect users of libpng
-
There are no substantial API changes between the non-deprecated parts of
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,
png_user_chunk_malloc_max 0 (unlimited) 8,000,000
The png_set_option() function (and the "options" member of the png struct) was
-added to libpng-1.5.15.
-
-B. Changes to the build and configuration of libpng
-
-Details of internal changes to the library code can be found in the CHANGES
-file and in the GIT repository logs. These will be of no concern to the vast
-majority of library users or builders; however, the few who configure libpng
-to a non-default feature set may need to change how this is done.
-
-There should be no need for library builders to alter build scripts if
-these use the distributed build support - configure or the makefiles -
-however, users of the makefiles may care to update their build scripts
-to build pnglibconf.h where the corresponding makefile does not do so.
-
-Building libpng with a non-default configuration has changed completely.
-The old method using pngusr.h should still work correctly even though the
-way pngusr.h is used in the build has been changed; however, library
-builders will probably want to examine the changes to take advantage of
-new capabilities and to simplify their build system.
-
-B.1 Specific changes to library configuration capabilities
+added to libpng-1.5.15, with option PNG_ARM_NEON.
The library now supports a complete fixed point implementation and can
thus be used on systems that have no floating point support or very
missing fixed point APIs have been implemented.
The exact mechanism used to control attributes of API functions has
-changed. A single set of operating system independent macro definitions
-is used and operating system specific directives are defined in
-pnglibconf.h
-
-As part of this the mechanism used to choose procedure call standards on
-those systems that allow a choice has been changed. At present this only
-affects certain Microsoft (DOS, Windows) and IBM (OS/2) operating systems
-running on Intel processors. As before, PNGAPI is defined where required
-to control the exported API functions; however, two new macros, PNGCBAPI
-and PNGCAPI, are used instead for callback functions (PNGCBAPI) and
-(PNGCAPI) for functions that must match a C library prototype (currently
-only png_longjmp_ptr, which must match the C longjmp function.) The new
-approach is documented in pngconf.h
-
-Despite these changes, libpng 1.5.0 only supports the native C function
-calling standard on those platforms tested so far (__cdecl on Microsoft
-Windows). This is because the support requirements for alternative
-calling conventions seem to no longer exist. Developers who find it
-necessary to set PNG_API_RULE to 1 should advise the mailing list
-(png-mng-implement) of this and library builders who use Openwatcom and
-therefore set PNG_API_RULE to 2 should also contact the mailing list.
+changed, as described in the INSTALL file.
A new test program, pngvalid, is provided in addition to pngtest.
pngvalid validates the arithmetic accuracy of the gamma correction
to choose at app buildtime whether or not to use macros (previously
impossible because the functions weren't in the default build.)
-B.2 Changes to the configuration mechanism
-
-Prior to libpng-1.5.0 library builders who needed to configure libpng
-had either to modify the exported pngconf.h header file to add system
-specific configuration or had to write feature selection macros into
-pngusr.h and cause this to be included into pngconf.h by defining
-PNG_USER_CONFIG. The latter mechanism had the disadvantage that an
-application built without PNG_USER_CONFIG defined would see the
-unmodified, default, libpng API and thus would probably fail to link.
-
-These mechanisms still work in the configure build and in any makefile
-build that builds pnglibconf.h, although the feature selection macros
-have changed somewhat as described above. In 1.5.0, however, pngusr.h is
-processed only once, when the exported header file pnglibconf.h is built.
-pngconf.h no longer includes pngusr.h, therefore pngusr.h is ignored after the
-build of pnglibconf.h and it is never included in an application build.
-
-The rarely used alternative of adding a list of feature macros to the
-CFLAGS setting in the build also still works; however, the macros will be
-copied to pnglibconf.h and this may produce macro redefinition warnings
-when the individual C files are compiled.
-
-All configuration now only works if pnglibconf.h is built from
-scripts/pnglibconf.dfa. This requires the program awk. Brian Kernighan
-(the original author of awk) maintains C source code of that awk and this
-and all known later implementations (often called by subtly different
-names - nawk and gawk for example) are adequate to build pnglibconf.h.
-The Sun Microsystems (now Oracle) program 'awk' is an earlier version
-and does not work; this may also apply to other systems that have a
-functioning awk called 'nawk'.
-
-Configuration options are now documented in scripts/pnglibconf.dfa. This
-file also includes dependency information that ensures a configuration is
-consistent; that is, if a feature is switched off dependent features are
-also removed. As a recommended alternative to using feature macros in
-pngusr.h a system builder may also define equivalent options in pngusr.dfa
-(or, indeed, any file) and add that to the configuration by setting
-DFA_XTRA to the file name. The makefiles in contrib/pngminim illustrate
-how to do this, and a case where pngusr.h is still required.
-
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
png_info_init_3()
png_convert_to_rfc1123() which has been replaced
with png_convert_to_rfc1123_buffer()
- png_data_freer()
png_malloc_default()
png_free_default()
png_reset_zstream()
png_infop became png_inforp or png_const_inforp
where "rp" indicates a "restricted pointer".
+Dropped support for 16-bit platforms. The support for FAR/far types has
+been eliminated and the definition of png_alloc_size_t is now controlled
+by a flag so that 'small size_t' systems can select it if necessary.
+
Error detection in some chunks has improved; in particular the iCCP chunk
reader now does pretty complete validation of the basic format. Some bad
profiles that were previously accepted are now accepted with a warning or
-rejected, depending upon the png_set_benign_errors() setting, in particular the
-very old broken Microsoft/HP 3144-byte sRGB profile. The PNG spec requirement
-that only grayscale profiles may appear in images with color type 0 or 4 and
-that even if the image only contains gray pixels, only RGB profiles may appear
-in images with color type 2, 3, or 6, is now enforced. The sRGB chunk
-is allowed to appear in images with any color type.
+rejected, depending upon the png_set_benign_errors() setting, in particular
+the very old broken Microsoft/HP 3144-byte sRGB profile. Starting with
+libpng-1.6.11, recognizing and checking sRGB profiles can be avoided by
+means of
+
+ #if defined(PNG_SKIP_sRGB_CHECK_PROFILE) && \
+ defined(PNG_SET_OPTION_SUPPORTED)
+ png_set_option(png_ptr, PNG_SKIP_sRGB_CHECK_PROFILE,
+ PNG_OPTION_ON);
+ #endif
+
+It's not a good idea to do this if you are using the "simplified API",
+which needs to be able to recognize sRGB profiles conveyed via the iCCP
+chunk.
+
+The PNG spec requirement that only grayscale profiles may appear in images
+with color type 0 or 4 and that even if the image only contains gray pixels,
+only RGB profiles may appear in images with color type 2, 3, or 6, is now
+enforced. The sRGB chunk is allowed to appear in images with any color type
+and is interpreted by libpng to convey a one-tracer-curve gray profile or a
+three-tracer-curve RGB profile as appropriate.
Prior to libpng-1.6.0 a warning would be issued if the iTXt chunk contained
an empty language field or an empty translated keyword. Both of these
stream to set the size of the sliding window for reading instead of using the
default 32-kbyte sliding window size. It was discovered that there are
hundreds of PNG files in the wild that have incorrect CMF bytes that caused
-libpng to issue a "too far back" error and reject the file. Libpng-1.6.3 and
-later calculate their own safe CMF from the image dimensions, provide a way
-to revert to the libpng-1.5.x behavior (ignoring the CMF bytes and using a
-32-kbyte sliding window), by using
+zlib to issue the "invalid distance too far back" error and reject the file.
+Libpng-1.6.3 and later calculate their own safe CMF from the image dimensions,
+provide a way to revert to the libpng-1.5.x behavior (ignoring the CMF bytes
+and using a 32-kbyte sliding window), by using
png_set_option(png_ptr, PNG_MAXIMUM_INFLATE_WINDOW,
PNG_OPTION_ON);
-and provide a tool (contrib/tools/pngfix) for optimizing the CMF bytes
-correctly.
+and provide a tool (contrib/tools/pngfix) for rewriting a PNG file while
+optimizing the CMF bytes in its IDAT chunk correctly.
Libpng-1.6.0 and libpng-1.6.1 wrote uncompressed iTXt chunks with the wrong
length, which resulted in PNG files that cannot be read beyond the bad iTXt
chunk. This error was fixed in libpng-1.6.3, and a tool (called
contrib/tools/png-fix-itxt) has been added to the libpng distribution.
+Starting with libpng-1.6.17, the PNG_SAFE_LIMITS macro was eliminated
+and safe limits are used by default (users who need larger limits
+can still override them at compile time or run time, as described above).
+
+The new limits are
+ default spec limit
+ png_user_width_max 1,000,000 2,147,483,647
+ png_user_height_max 1,000,000 2,147,483,647
+ png_user_chunk_cache_max 128 unlimited
+ png_user_chunk_malloc_max 8,000,000 unlimited
+
+Starting with libpng-1.6.18, a PNG_RELEASE_BUILD macro was added, which allows
+library builders to control compilation for an installed system (a release build).
+It can be set for testing debug or beta builds to ensure that they will compile
+when the build type is switched to RC or STABLE. In essence this overrides the
+PNG_LIBPNG_BUILD_BASE_TYPE definition which is not directly user controllable.
+
+Starting with libpng-1.6.19, attempting to set an over-length PLTE chunk
+is an error. Previously this requirement of the PNG specification was not
+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
The png_get_io_ptr() function has been present since libpng-0.88, has never
XV. Coding style
-Our coding style is similar to the "Allman" style, with curly
+Our coding style is similar to the "Allman" style
+(See http://en.wikipedia.org/wiki/Indent_style#Allman_style), with curly
braces on separate lines:
if (condition)
body;
}
+The return type and decorations are placed on a separate line
+ahead of the function name, as illustrated above.
+
The prototypes for all exported functions appear in png.h,
above the comment that says
}
The prototypes for non-exported functions (except for those in
-pngtest) appear in
-pngpriv.h
-above the comment that says
+pngtest) appear in pngpriv.h above the comment that says
/* Maintainer: Put new private prototypes here ^ */
+To avoid polluting the global namespace, the names of all exported
+functions and variables begin with "png_", and all publicly visible C
+preprocessor macros begin with "PNG". We request that applications that
+use libpng *not* begin any of their own symbols with either of these strings.
+
We put a space after the "sizeof" operator and we omit the
optional parentheses around its argument when the argument
is an expression, not a type name, and we always enclose the
Prior to libpng-1.6.0 we used a "png_sizeof()" macro, formatted as
though it were a function.
-To avoid polluting the global namespace, the names of all exported
-functions and variables begin with "png_", and all publicly visible C
-preprocessor macros begin with "PNG". We request that applications that
-use libpng *not* begin any of their own symbols with either of these strings.
+Control keywords if, for, while, and switch are always followed by a 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
when there is only one macro being tested. We always use parentheses
with "defined".
-We prefer to express integers that are used as bit masks in hex format,
-with an even number of lower-case hex digits (e.g., 0x00, 0xff, 0x0100).
+We express integer constants that are used as bit masks in hex format,
+with an even number of lower-case hex digits, and to make them unsigned
+(e.g., 0x00U, 0xffU, 0x0100U) and long if they are greater than 0x7fff
+(e.g., 0xffffUL).
-We prefer to use underscores in variable names rather than camelCase, except
+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 do not use the TAB character for indentation in the C sources.
Lines do not exceed 80 characters.
XVI. Y2K Compliance in libpng
-February 6, 2014
-
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 1.6.9 are Y2K compliant. It is my belief that earlier
+upward through 1.6.19 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
There are seven time-related functions:
- png_convert_to_rfc_1123() in png.c
- (formerly png_convert_to_rfc_1152() in error)
+ 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
projects / l4.git / blobdiff