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