Issue 523 in webp: Animated WebP presents blocking artifacts

[perse… via monorail]

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

New issue 523 by perse...@gmail.com: Animated WebP presents blocking artifacts
https://bugs.chromium.org/p/webp/issues/detail?id=523

What steps will reproduce the problem?
1. img2webp -d 40 -lossy -o image-out.webp -q 90 image-in-*.webp
2. vwebp image-out.webp

What is the expected output? What do you see instead?
The image-out.webp has a lot of blocking artifacts.

What version of the product are you using? On what operating system?
OS: Linux Mint 19.3 Tricia x86_64
Base: Ubuntu 18.04 bionic
Kernel: 5.4.0-54-generic
Shell: bash 4.4.20

img2webp -version
WebP Encoder version: 1.2.0
WebP Mux version: 1.2.0

Attachments:
webp-issue-input-files.zip 4.7 MB
image-out.webp 4.4 MB

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[jz… via monorail]

Updates:
Owner: pasca...@gmail.com

Comment #1 on issue 523 by jz...@google.com: Animated WebP presents blocking artifacts
https://bugs.chromium.org/p/webp/issues/detail?id=523#c1

(No comment was entered for this change.)

[pasca… via monorail]

Comment #2 on issue 523 by pasca...@gmail.com: Animated WebP presents blocking artifacts
https://bugs.chromium.org/p/webp/issues/detail?id=523#c2

Thanks for the report, i can reproduce the problem.

It seems related to how blocks that are very similar to their colocated blocks in the previous frame are optimized in lossy coding.
The comparison for similarity is done on the original source frame, but not on the coded/decoded frame as will be seen by the decoder.
There's a progressive drift here, leading to visible artifact due to a mismatch between was the encoder and decoder consider to be the previous frame.

The encoder is not decoding the compressed frame to save CPU. We could try to add an option ("-slow" ?) to do that, but we'd need to improve the API first, i think.

For definiteness, i attach a problematic frame #18, as extracted by 'anim_dump'.

Attachments:
dump_0018.png 2.2 MB

[yaron… via monorail]

Comment #3 on issue 523 by yaron...@cloudinary.com: Animated WebP presents blocking artifacts
https://bugs.chromium.org/p/webp/issues/detail?id=523#c3

similar issue to https://bugs.chromium.org/p/webp/issues/detail?id=554

I've tested my suggested fix for issue id #554 with this issue attached input:
for q=90 there are much less blocking artifacts as result maxDiff return 2
for q=91 there are no artifacts as resulting maxDiff return 1 (see attached)

Attachments:
image-out.webp 4.7 MB