hidden color leaks with lossy compression

[GoogleCodeExporter]

As reported by Jos on the mailing list:

invisible color (under the alpha=0 area) can 'bleed' into
visible pixel if they are part of the 2x2 block used during U/V downsampling.

Attached example 'alpha-testcase.png' with red invisible color.
the result 'alpha-testcase.webp' shows red outlines on the cross.

('cwebp alpha-testcase.png -o alpha-testcase.webp')

Original issue reported on code.google.com by pascal.m...@gmail.com on 12 Sep 2014 at 10:15

Attachments:

• alpha-testcase.png
• alpha-testcase.webp

[GoogleCodeExporter]

a tentative patch is cooking as https://gerrit.chromium.org/gerrit/#/c/71555/

Original comment by pascal.m...@gmail.com on 12 Sep 2014 at 10:19

[GoogleCodeExporter]

patch has been submitted. Took a while to make it equally fast than previous 
non-averaging version.
Should be ok now.

Original comment by pascal.m...@gmail.com on 19 Sep 2014 at 7:34

• Changed state: Fixed