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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
man/dwebp.1
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@ -1,5 +1,5 @@
.\" Hey, EMACS: -*- nroff -*-
.TH DWEBP 1 "March 7, 2014"
.TH DWEBP 1 "June 13, 2014"
.SH NAME
dwebp \- decompress a WebP file to an image file
.SH SYNOPSIS
@ -68,6 +68,11 @@ Specify a dithering \fBstrength\fP between 0 and 100. Dithering is a
post-processing effect applied to chroma components in lossy compression.
It helps by smoothing gradients and avoiding banding artifacts.
.TP
.BI \-alpha_dither
If the compressed file contains a transparency plane that was quantized
during compression, this flag will allow dithering the reconstructed plane
in order to generate smoother transparency gradients.
.TP
.B \-nodither
Disable all dithering (default).
.TP