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add a -dither dithering option to the decoder

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Even at high quality setting, the U/V quantizer step is limited
to 4 which can lead to banding on gradient.
This option allows to selectively apply some randomness to
potentially flattened-out U/V blocks and attenuate the banding.

This option is off by default in 'dwebp', but set to -dither 50
by default in 'vwebp'.

Note: depending on the number of blocks selectively dithered,
we can have up to a 10% slow-down in decoding speed it seems.

Change-Id: Icc2446007f33ddacb60b3a80a9e63f2d5ad162de
This commit is contained in:
skal
2013-11-26 22:59:02 +01:00
committed by Pascal Massimino
parent e812401299
commit cbdd3e6e53
11 changed files with 138 additions and 11 deletions
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12
man/dwebp.1
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@ -1,5 +1,5 @@
.\" Hey, EMACS: -*- nroff -*-
.TH DWEBP 1 "May 10, 2013"
.TH DWEBP 1 "November 26, 2013"
.SH NAME
dwebp \- decompress a WebP file to an image file
.SH SYNOPSIS
@ -55,7 +55,15 @@ edges (especially the red ones), but should be faster.
.B \-nofilter
Don't use the in-loop filtering process even if it is required by
the bitstream. This may produce visible blocks on the non-compliant output,
but will make the decoding faster.
but it will make the decoding faster.
.TP
.B \-dither " strength
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
.B \-nodither
Disable all dithering (default).
.TP
.B \-mt
Use multi-threading for decoding, if possible.