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[libpng16] Document alpha_mode choices in the man page

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Glenn Randers-Pehrson 2017-09-23 21:23:59 -05:00
parent 3d2d0b52a3
commit e4c8312d31
2 changed files with 32 additions and 14 deletions
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19
libpng-manual.txt
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@ -1,6 +1,6 @@
libpng-manual.txt - A description on how to use and modify libpng libpng-manual.txt - A description on how to use and modify libpng
libpng version 1.6.33rc02 - September 20, 2017 libpng version 1.6.33rc02 - September 23, 2017
Updated and distributed by Glenn Randers-Pehrson Updated and distributed by Glenn Randers-Pehrson
<glennrp at users.sourceforge.net> <glennrp at users.sourceforge.net>
Copyright (c) 1998-2017 Glenn Randers-Pehrson Copyright (c) 1998-2017 Glenn Randers-Pehrson
@ -11,7 +11,7 @@ libpng-manual.txt - A description on how to use and modify libpng
Based on: Based on:
libpng versions 0.97, January 1998, through 1.6.33rc02 - September 20, 2017 libpng versions 0.97, January 1998, through 1.6.33rc02 - September 23, 2017
Updated and distributed by Glenn Randers-Pehrson Updated and distributed by Glenn Randers-Pehrson
Copyright (c) 1998-2017 Glenn Randers-Pehrson Copyright (c) 1998-2017 Glenn Randers-Pehrson
@ -986,8 +986,17 @@ premultiplication.
png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB); png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB);
This is the default libpng handling of the alpha channel - it is not Choices for the alpha_mode are
pre-multiplied into the color components. In addition the call states
PNG_ALPHA_PNG 0 /* according to the PNG standard */
PNG_ALPHA_STANDARD 1 /* according to Porter/Duff */
PNG_ALPHA_ASSOCIATED 1 /* as above; this is the normal practice */
PNG_ALPHA_PREMULTIPLIED 1 /* as above */
PNG_ALPHA_OPTIMIZED 2 /* 'PNG' for opaque pixels, else 'STANDARD' */
PNG_ALPHA_BROKEN 3 /* the alpha channel is gamma encoded */
PNG_ALPHA_PNG is the default libpng handling of the alpha channel. It is not
pre-multiplied into the color components. In addition the call states
that the output is for a sRGB system and causes all PNG files without gAMA that the output is for a sRGB system and causes all PNG files without gAMA
chunks to be assumed to be encoded using sRGB. chunks to be assumed to be encoded using sRGB.
@ -1002,7 +1011,7 @@ early Mac systems behaved.
This is the classic Jim Blinn approach and will work in academic This is the classic Jim Blinn approach and will work in academic
environments where everything is done by the book. It has the shortcoming 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 of assuming that input PNG data with no gamma information is linear - this
is unlikely to be correct unless the PNG files where generated locally. is unlikely to be correct unless the PNG files were generated locally.
Most of the time the output precision will be so low as to show Most of the time the output precision will be so low as to show
significant banding in dark areas of the image. significant banding in dark areas of the image.

27
libpng.3
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@ -1,4 +1,4 @@
.TH LIBPNG 3 "September 20, 2017" .TH LIBPNG 3 "September 23, 2017"
.SH NAME .SH NAME
libpng \- Portable Network Graphics (PNG) Reference Library 1.6.33rc02 libpng \- Portable Network Graphics (PNG) Reference Library 1.6.33rc02
.SH SYNOPSIS .SH SYNOPSIS
@ -518,7 +518,7 @@ Following is a copy of the libpng-manual.txt file that accompanies libpng.
.SH LIBPNG.TXT .SH LIBPNG.TXT
libpng-manual.txt - A description on how to use and modify libpng libpng-manual.txt - A description on how to use and modify libpng
libpng version 1.6.33rc02 - September 20, 2017 libpng version 1.6.33rc02 - September 23, 2017
Updated and distributed by Glenn Randers-Pehrson Updated and distributed by Glenn Randers-Pehrson
<glennrp at users.sourceforge.net> <glennrp at users.sourceforge.net>
Copyright (c) 1998-2017 Glenn Randers-Pehrson Copyright (c) 1998-2017 Glenn Randers-Pehrson
@ -529,7 +529,7 @@ libpng-manual.txt - A description on how to use and modify libpng
Based on: Based on:
libpng versions 0.97, January 1998, through 1.6.33rc02 - September 20, 2017 libpng versions 0.97, January 1998, through 1.6.33rc02 - September 23, 2017
Updated and distributed by Glenn Randers-Pehrson Updated and distributed by Glenn Randers-Pehrson
Copyright (c) 1998-2017 Glenn Randers-Pehrson Copyright (c) 1998-2017 Glenn Randers-Pehrson
@ -1504,8 +1504,17 @@ premultiplication.
png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB); png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB);
This is the default libpng handling of the alpha channel - it is not Choices for the alpha_mode are
pre-multiplied into the color components. In addition the call states
PNG_ALPHA_PNG 0 /* according to the PNG standard */
PNG_ALPHA_STANDARD 1 /* according to Porter/Duff */
PNG_ALPHA_ASSOCIATED 1 /* as above; this is the normal practice */
PNG_ALPHA_PREMULTIPLIED 1 /* as above */
PNG_ALPHA_OPTIMIZED 2 /* 'PNG' for opaque pixels, else 'STANDARD' */
PNG_ALPHA_BROKEN 3 /* the alpha channel is gamma encoded */
PNG_ALPHA_PNG is the default libpng handling of the alpha channel. It is not
pre-multiplied into the color components. In addition the call states
that the output is for a sRGB system and causes all PNG files without gAMA that the output is for a sRGB system and causes all PNG files without gAMA
chunks to be assumed to be encoded using sRGB. chunks to be assumed to be encoded using sRGB.
@ -1520,7 +1529,7 @@ early Mac systems behaved.
This is the classic Jim Blinn approach and will work in academic This is the classic Jim Blinn approach and will work in academic
environments where everything is done by the book. It has the shortcoming 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 of assuming that input PNG data with no gamma information is linear - this
is unlikely to be correct unless the PNG files where generated locally. is unlikely to be correct unless the PNG files were generated locally.
Most of the time the output precision will be so low as to show Most of the time the output precision will be so low as to show
significant banding in dark areas of the image. significant banding in dark areas of the image.
@ -6081,7 +6090,7 @@ possible without all of you.
Thanks to Frank J. T. Wojcik for helping with the documentation. Thanks to Frank J. T. Wojcik for helping with the documentation.
Libpng version 1.6.33rc02 - September 20, 2017: Libpng version 1.6.33rc02 - September 23, 2017:
Initially created in 1995 by Guy Eric Schalnat, then of Group 42, Inc. Initially created in 1995 by Guy Eric Schalnat, then of Group 42, Inc.
Currently maintained by Glenn Randers-Pehrson (glennrp at users.sourceforge.net). Currently maintained by Glenn Randers-Pehrson (glennrp at users.sourceforge.net).
@ -6106,7 +6115,7 @@ this sentence.
This code is released under the libpng license. This code is released under the libpng license.
libpng versions 1.0.7, July 1, 2000 through 1.6.33rc02, September 20, 2017 are libpng versions 1.0.7, July 1, 2000 through 1.6.33rc02, September 23, 2017 are
Copyright (c) 2000-2002, 2004, 2006-2017 Glenn Randers-Pehrson, are Copyright (c) 2000-2002, 2004, 2006-2017 Glenn Randers-Pehrson, are
derived from libpng-1.0.6, and are distributed according to the same derived from libpng-1.0.6, and are distributed according to the same
disclaimer and license as libpng-1.0.6 with the following individuals disclaimer and license as libpng-1.0.6 with the following individuals
@ -6234,7 +6243,7 @@ files "pngbar.png" and "pngbar.jpg (88x31) and "pngnow.png" (98x31).
Glenn Randers-Pehrson Glenn Randers-Pehrson
glennrp at users.sourceforge.net glennrp at users.sourceforge.net
September 20, 2017 September 23, 2017
.\" end of man page .\" end of man page