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add alpha dithering for lossy

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new options:
 dwebp -alpha_dither
 vwebp -noalphadither

When the source was marked as quantized, we use a threshold-averaging
filter to smooth the decoded alpha plane.
Note: this option forces the decoding of alpha data in one pass, and
might slow the decoding a bit.

The new field in WebPDecoderOptions struct is 'alpha_dithering_strength'
(0 by default, means: off). Max strength value is '100'.

Change-Id: I218e21af96360d4781587fede95f8ea4e2b7287a
This commit is contained in:
skal
2014-06-14 00:06:16 +02:00
parent 790207679d
commit bbe32df1e3
11 changed files with 308 additions and 21 deletions
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7
src/dec/frame.c
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@ -177,6 +177,13 @@ void VP8InitDithering(const WebPDecoderOptions* const options,
dec->dither_ = 1;
}
}
// potentially allow alpha dithering
dec->alpha_dithering_ = options->alpha_dithering_strength;
if (dec->alpha_dithering_ > 100) {
dec->alpha_dithering_ = 100;
} else if (dec->alpha_dithering_ < 0) {
dec->alpha_dithering_ = 0;
}
}
}