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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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7
man/cwebp.1
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@ -1,5 +1,5 @@
.\" Hey, EMACS: -*- nroff -*-
.TH CWEBP 1 "March 13, 2013"
.TH CWEBP 1 "March 28, 2013"
.SH NAME
cwebp \- compress an image file to a WebP file
.SH SYNOPSIS
@ -187,6 +187,11 @@ no compression, 1 uses WebP lossless format for compression. The default is 1.
Modify unseen RGB values under fully transparent area, to help compressibility.
The default is off.
.TP
.BI \-blend_alpha " int
This option blends the alpha channel (if present) with the source using the
background color specified in hexadecimal as 0xrrggbb. The alpha channel is
afterward reset to the opaque value 255.
.TP
.B \-noalpha
Using this option will discard the alpha channel.
.TP