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[libpng17] Moved some documentation from png.h to libpng.3 and libpng-manual.txt

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Minor editing of contrib/arm-neon/README and contrib/examples/*.c
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Glenn Randers-Pehrson 2014-02-26 11:50:02 -06:00
parent 51ecc14a8a
commit 148cdac18f
4 changed files with 413 additions and 67 deletions
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9
ANNOUNCE
View File

@ -1,5 +1,5 @@
Libpng 1.7.0beta32 - February 23, 2014
Libpng 1.7.0beta32 - February 26, 2014
This is not intended to be a public release. It will be replaced
within a few weeks by a public version or by another test version.
@ -533,7 +533,7 @@ Version 1.7.0beta31 [February 6, 2014]
and it adds corresponding code to pngimage.c to handle such options
by not attempting to test them.
Version 1.7.0beta32 [February 23, 2014]
Version 1.7.0beta32 [February 26, 2014]
Moved redefines of png_error(), png_warning(), png_chunk_error(),
and png_chunk_warning() from pngpriv.h to png.h to make them visible
to libpng-calling applications.
@ -555,6 +555,11 @@ Version 1.7.0beta32 [February 23, 2014]
Added png_ptr->process_mode = PNG_READ_IDAT_MODE in png_push_read_chunk
after recognizing the IDAT chunk, which avoids an infinite loop while
reading a datastream whose first IDAT chunk is of zero-length.
This fixes CERT VU#684412 and CVE-2014-0333.
Don't recognize known sRGB profiles as sRGB if they have been hacked,
but don't reject them and don't issue a copyright violation warning.
Minor editing of contrib/arm-neon/README and contrib/examples/*.c
Moved some documentation from png.h to libpng.3 and libpng-manual.txt
Send comments/corrections/commendations to png-mng-implement at lists.sf.net
(subscription required; visit

7
CHANGES
View File

@ -4822,7 +4822,7 @@ Version 1.7.0beta31 [February 6, 2014]
and it adds corresponding code to pngimage.c to handle such options
by not attempting to test them.
Version 1.7.0beta32 [February 23, 2014]
Version 1.7.0beta32 [February 26, 2014]
Moved redefines of png_error(), png_warning(), png_chunk_error(),
and png_chunk_warning() from pngpriv.h to png.h to make them visible
to libpng-calling applications.
@ -4844,6 +4844,11 @@ Version 1.7.0beta32 [February 23, 2014]
Added png_ptr->process_mode = PNG_READ_IDAT_MODE in png_push_read_chunk
after recognizing the IDAT chunk, which avoids an infinite loop while
reading a datastream whose first IDAT chunk is of zero-length.
This fixes CERT VU#684412 and CVE-2014-0333.
Don't recognize known sRGB profiles as sRGB if they have been hacked,
but don't reject them and don't issue a copyright violation warning.
Minor editing of contrib/arm-neon/README and contrib/examples/*.c
Moved some documentation from png.h to libpng.3 and libpng-manual.txt
Send comments/corrections/commendations to png-mng-implement at lists.sf.net
(subscription required; visit

228
libpng-manual.txt
View File

@ -1,6 +1,6 @@
libpng-manual.txt - A description on how to use and modify libpng
libpng version 1.7.0beta32 - February 23, 2014
libpng version 1.7.0beta32 - February 26, 2014
Updated and distributed by Glenn Randers-Pehrson
<glennrp at users.sourceforge.net>
Copyright (c) 1998-2014 Glenn Randers-Pehrson
@ -11,7 +11,7 @@ libpng-manual.txt - A description on how to use and modify libpng
Based on:
libpng versions 0.97, January 1998, through 1.7.0beta32 - February 23, 2014
libpng versions 0.97, January 1998, through 1.7.0beta32 - February 26, 2014
Updated and distributed by Glenn Randers-Pehrson
Copyright (c) 1998-2014 Glenn Randers-Pehrson
@ -711,12 +711,12 @@ value. You can also specify a default encoding for the PNG file in
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
@ -744,6 +744,70 @@ component value whenever arithmetic is performed. A lot of graphics software
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
@ -768,11 +832,11 @@ by png_set_alpha_mode().
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
@ -789,11 +853,35 @@ be used!
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!)
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.
PNG_ALPHA_STANDARD: The data libpng produces
is encoded in the standard way
assumed by most correctly written graphics software.
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.
@ -822,9 +910,8 @@ dynamic range. To avoid problems, and if your software
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.
@ -843,18 +930,16 @@ representation of non-opaque pixels are irrelevant.
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():
@ -885,6 +970,89 @@ All you can do is compose the result onto a matching output. Since this
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
@ -1215,7 +1383,7 @@ png_set_rgb_to_gray()).
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
@ -5278,7 +5446,7 @@ Other rules can be inferred by inspecting the libpng source.
XVII. Y2K Compliance in libpng
February 23, 2014
February 26, 2014
Since the PNG Development group is an ad-hoc body, we can't make
an official declaration.

236
libpng.3
View File

@ -1,4 +1,4 @@
.TH LIBPNG 3 "February 23, 2014"
.TH LIBPNG 3 "February 26, 2014"
.SH NAME
libpng \- Portable Network Graphics (PNG) Reference Library 1.7.0beta32
.SH SYNOPSIS
@ -494,7 +494,7 @@ Following is a copy of the libpng-manual.txt file that accompanies libpng.
.SH LIBPNG.TXT
libpng-manual.txt - A description on how to use and modify libpng
libpng version 1.7.0beta32 - February 23, 2014
libpng version 1.7.0beta32 - February 26, 2014
Updated and distributed by Glenn Randers-Pehrson
<glennrp at users.sourceforge.net>
Copyright (c) 1998-2014 Glenn Randers-Pehrson
@ -505,7 +505,7 @@ libpng-manual.txt - A description on how to use and modify libpng
Based on:
libpng versions 0.97, January 1998, through 1.7.0beta32 - February 23, 2014
libpng versions 0.97, January 1998, through 1.7.0beta32 - February 26, 2014
Updated and distributed by Glenn Randers-Pehrson
Copyright (c) 1998-2014 Glenn Randers-Pehrson
@ -1205,12 +1205,12 @@ value. You can also specify a default encoding for the PNG file in
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
@ -1238,6 +1238,70 @@ component value whenever arithmetic is performed. A lot of graphics software
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
@ -1262,11 +1326,11 @@ by png_set_alpha_mode().
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
@ -1283,11 +1347,35 @@ be used!
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!)
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.
PNG_ALPHA_STANDARD: The data libpng produces
is encoded in the standard way
assumed by most correctly written graphics software.
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.
@ -1316,9 +1404,8 @@ dynamic range. To avoid problems, and if your software
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.
@ -1337,18 +1424,16 @@ representation of non-opaque pixels are irrelevant.
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():
@ -1379,6 +1464,89 @@ All you can do is compose the result onto a matching output. Since this
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
@ -1709,7 +1877,7 @@ png_set_rgb_to_gray()).
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
@ -5773,7 +5941,7 @@ Other rules can be inferred by inspecting the libpng source.
.SH XVII. Y2K Compliance in libpng
February 23, 2014
February 26, 2014
Since the PNG Development group is an ad-hoc body, we can't make
an official declaration.
@ -6043,7 +6211,7 @@ possible without all of you.
Thanks to Frank J. T. Wojcik for helping with the documentation.
Libpng version 1.7.0beta32 - February 23, 2014:
Libpng version 1.7.0beta32 - February 26, 2014:
Initially created in 1995 by Guy Eric Schalnat, then of Group 42, Inc.
Currently maintained by Glenn Randers-Pehrson (glennrp at users.sourceforge.net).
@ -6066,7 +6234,7 @@ this sentence.
This code is released under the libpng license.
libpng versions 1.2.6, August 15, 2004, through 1.7.0beta32, February 23, 2014, are
libpng versions 1.2.6, August 15, 2004, through 1.7.0beta32, February 26, 2014, are
Copyright (c) 2004,2006-2007 Glenn Randers-Pehrson, and are
distributed according to the same disclaimer and license as libpng-1.2.5
with the following individual added to the list of Contributing Authors
@ -6165,7 +6333,7 @@ certification mark of the Open Source Initiative.
Glenn Randers-Pehrson
glennrp at users.sourceforge.net
February 23, 2014
February 26, 2014
.\" end of man page