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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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README
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@@ -270,6 +270,7 @@ Use following options to convert into alternate image formats:
-nofilter .... disable in-loop filtering
-nodither .... disable dithering
-dither <d> .. dithering strength (in 0..100)
-alpha_dither use alpha-plane dithering if needed.
-mt .......... use multi-threading
-crop <x> <y> <w> <h> ... crop output with the given rectangle
-scale <w> <h> .......... scale the output (*after* any cropping)
@@ -297,6 +298,7 @@ Options are:
-nofancy ..... don't use the fancy YUV420 upscaler.
-nofilter .... disable in-loop filtering.
-dither <int> dithering strength (0..100). Default=50.
-noalphadither disable alpha plane dithering.
-mt .......... use multi-threading.
-info ........ print info.
-h ....... this help message.