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add WebPBlendAlpha() function to blend colors against background

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new option: -blend_alpha 0xrrggbb
also: don't force picture.use_argb value for lossless. Instead,
delay the YUVA<->ARGB conversion till WebPEncode() is called.
This make the blending more accurate when source is ARGB
and lossy compression is used (YUVA).
This has an effect on cropping/rescaling. E.g. for PNG, these
are now done in ARGB colorspace instead of YUV when lossy compression
is used.

Change-Id: I18571f1b1179881737a8dbd23ad0aa8cddae3c6b
(cherry picked from commit e7d9548c9b)
This commit is contained in:
Pascal Massimino
2013-04-02 19:14:14 -07:00
committed by James Zern
parent bc9f5fbe0f
commit dcbb1ca54a
8 changed files with 121 additions and 6 deletions
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@ -170,6 +170,10 @@ options:
-alpha_filter <string> . predictive filtering for alpha plane.
One of: none, fast (default) or best.
-alpha_cleanup ......... Clean RGB values in transparent area.
-blend_alpha <hex> ..... Blend colors against background color
expressed as RGB values written in
hexadecimal, e.g. 0xc0e0d0 for red=0xc0
green=0xe0 and blue=0xd0.
-noalpha ............... discard any transparency information.
-lossless .............. Encode image losslessly.
-hint <string> ......... Specify image characteristics hint.