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bachphuc / webp

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visible luma ringing even at max quality '-q 100' #99

Closed GoogleCodeExporter closed 9 years ago

GoogleCodeExporter commented 9 years ago 
What steps will reproduce the problem?

cwebp-0.1.3 color-fade-test.png -q 100 -f 0 -sns 0 -o test.webp

The results (attached) demonstrates some ringing.
Looking at the compressed YUV planes (test.yuv.png), the ringing
is visible on the luma plane.

Possible example: even at q=100, (quant = 0), the minimal quantization step is 
still '4',
which caps the max achievable PSNR to something like ~56dB.

Possible solution: deringing post-processing?

Original issue reported on code.google.com by pascal.m...@gmail.com on 8 Dec 2011 at 4:40

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GoogleCodeExporter commented 9 years ago 
The following change should help with removing banding:
https://gerrit.chromium.org/gerrit/#/c/67545/

To activate dithering, you need to use '-pre 2' option (or '-preset photo'):

For instance:
cwebp color-fade-test.png -q 100 -f 0 -sns 0 -pre 2 -o test_new.webp

Original comment by s...@google.com on 18 Oct 2013 at 1:04

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