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| author | Glenn Randers-Pehrson <glennrp at users.sourceforge.net> | 2017-09-23 21:23:59 -0500 |
|---|---|---|
| committer | Glenn Randers-Pehrson <glennrp at users.sourceforge.net> | 2017-09-23 21:23:59 -0500 |
| commit | e4c8312d31c335113d6decd780599f6a5d944df5 (patch) | |
| tree | 0c3895446440bade55e4ea23deda46e7b3cba0e1 /libpng-manual.txt | |
| parent | 3d2d0b52a3f36f90d74701a620980bf360cf7368 (diff) | |
| download | libpng-e4c8312d31c335113d6decd780599f6a5d944df5.tar.gz | |
[libpng16] Document alpha_mode choices in the man page
Diffstat (limited to 'libpng-manual.txt')
| -rw-r--r-- | libpng-manual.txt | 19 |
1 files changed, 14 insertions, 5 deletions
diff --git a/libpng-manual.txt b/libpng-manual.txt index 2f5c589e7..e306978a2 100644 --- a/libpng-manual.txt +++ b/libpng-manual.txt @@ -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. |