Issue 308 in webp: grey incorrectly colorized

[pdk… via monorail]

Status: New
Owner: ----
Labels: Type-Defect Priority-Medium

New issue 308 by pdk...@gmail.com: grey incorrectly colorized
https://bugs.chromium.org/p/webp/issues/detail?id=308

http://cdn.pwmon.org/webp/r.84.webp
http://cdn.pwmon.org/webp/r.85.webp

Please review the images, which were made like this.

$ cwebp r.png -metadata icc -q 85 -o r.85.webp
$ cwebp r.png -metadata icc -q 84 -o r.84.webp

Very little difference one might assume. Strangely, 84 tints the left triangle noticeably purple, and the right triangle less noticeably green. (Use color picker to verify.) While 85 lacks any tint.

With different -m setting, the cut moves slightly from 84/85 but not significantly.

(Using -pre 4 makes no difference.)

http://cdn.pwmon.org/webp/r.png

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[pascal.m… via monorail]

Updates:
Status: Accepted

Comment #1 on issue 308 by pascal.m...@gmail.com: grey incorrectly colorized
https://bugs.chromium.org/p/webp/issues/detail?id=308#c1

confirmed: at q=84, the U/V values are changed to {127,129} above and below the red split, instead of initial uniform 128 values. For q=85, the quantization doesn't happen and

see pgm output (from: cwebp r.png -q 84 -d out_84.pgm) from q=84 encoding.

Attachments:
out_q84.pgm 600 KB

[s… via monorail]

Comment #3 on issue 308 by sk...@google.com: grey incorrectly colorized
https://bugs.chromium.org/p/webp/issues/detail?id=308#c3

update: this patch https://chromium-review.googlesource.com/#/c/webm/libwebp/+/813774/ will help in this case (see attached, the new version it would produce at q=84).

However, that patch will current be restricted to quality <= 30. Before extending it to higher quality, we need to assess the impact on filesize and speed (and possible use 'lighter' algo that still does the job at q >= 50 for difficult cases).


Attachments:
r.84b.webp 4.7 KB

[bugdro… via monorail]

Comment #4 on issue 308 by bugd...@chromium.org: grey incorrectly colorized
https://bugs.chromium.org/p/webp/issues/detail?id=308#c4

The following revision refers to this bug:
https://chromium.googlesource.com/webm/libwebp/+/815652de03377339aee55f048dd41f581853f0ef

commit 815652de03377339aee55f048dd41f581853f0ef
Author: Pascal Massimino <pascal.m...@gmail.com>
Date: Wed Dec 27 20:11:57 2017

enable dc error-diffusion always

for q<=98, we always enable error diffusion.

+ reduce storage 2x by using int8_t
+ make the error diffusion more robust

BUG=webp:340,308

Change-Id: I0608df839ff7b64d6843005a0f81d2577143af9e

[modify] https://crrev.com/815652de03377339aee55f048dd41f581853f0ef/src/enc/quant_enc.c
[modify] https://crrev.com/815652de03377339aee55f048dd41f581853f0ef/src/enc/vp8i_enc.h

[pascal.m… via monorail]

Updates:
Status: Verified

Comment #5 on issue 308 by pascal.m...@gmail.com: grey incorrectly colorized
https://bugs.chromium.org/p/webp/issues/detail?id=308#c5

for definiteness, here are the output of the current HEAD version (dd174caef).

Note also that the red line is better captured using -sharp_yuv option (see r.84.sharp.webp), but that's a separate topic.

Thanks for the report!

Attachments:
r.84.webp 7.7 KB
r.85.webp 7.8 KB
r.84.sharp.webp 7.9 KB