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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
This commit is contained in:
Pascal Massimino
2013-04-02 19:14:14 -07:00
parent df4a406d8d
commit e7d9548c9b
8 changed files with 121 additions and 6 deletions
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5
src/webp/encode.h
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@ -456,6 +456,11 @@ WEBP_EXTERN(void) WebPCleanupTransparentArea(WebPPicture* picture);
// alpha plane can be ignored altogether e.g.).
WEBP_EXTERN(int) WebPPictureHasTransparency(const WebPPicture* picture);
// Remove the transparency information (if present) by blending the color with
// the background color 'background_rgb' (specified as 24bit RGB triplet).
// After this call, all alpha values are reset to 0xff.
WEBP_EXTERN(void) WebPBlendAlpha(WebPPicture* pic, uint32_t background_rgb);
//------------------------------------------------------------------------------
// Main call