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Use deterministic random-dithering during RGB->YUV conversion

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-> helps debanding (sky, gradients, etc.)

This dithering can only be triggered when using -preset photo
or -pre 2 (as a preprocessing). Everything is unchanged otherwise.

Note that this change is likely to make the perceived PSNR/SSIM drop
since we're altering the input internally.

Change-Id: Id8d4326245d9b828141de162c94ba381b1fa5813
This commit is contained in:
skal
2013-10-17 22:36:49 +02:00
parent 8a2fa099cc
commit 0b2b05049f
7 changed files with 136 additions and 43 deletions
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7
man/cwebp.1
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@ -1,5 +1,5 @@
.\" Hey, EMACS: -*- nroff -*-
.TH CWEBP 1 "March 28, 2013"
.TH CWEBP 1 "October 17, 2013"
.SH NAME
cwebp \- compress an image file to a WebP file
.SH SYNOPSIS
@ -168,8 +168,9 @@ Output additional ASCII-map of encoding information. Possible map values
range from 1 to 6. This is only meant to help debugging.
.TP
.BI \-pre " int
Specify a pre-processing filter. This option is a placeholder
and has currently no effect.
Specify some pre-processing steps. Using a value of '2' will trigger
quality-dependent pseudo-random dithering during RGBA->YUVA conversion
(lossy compression only).
.TP
.BI \-alpha_filter " string
Specify the predictive filtering method for the alpha plane. One of 'none',