Issue 604 in webp: Strange behavior of terminal screenshots

[gedeo… via monorail]

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

New issue 604 by gedeo...@gmail.com: Strange behavior of terminal screenshots
https://bugs.chromium.org/p/webp/issues/detail?id=604

I created a screenshot of the Windows terminal in PNG format and added it to squoosh.app to convert to WebP in lossless mode.

The output file in the "lossless" mode had a larger size than in the lossy mode.

I repeated this action using a simple desktop screenshot and everything went as expected.

After that, I took a screenshot of another Windows terminal again, and again the output file in "lossless" mode had a larger size than in lossy mode.

As a result, any screenshot other than the terminal screenshot behaves as expected. This situation is reproduced not only in squoosh.app, but also in the XnView application. Magic.

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[gedeo… via monorail]

Comment #1 on issue 604 by gedeo...@gmail.com: Strange behavior of terminal screenshots
https://bugs.chromium.org/p/webp/issues/detail?id=604#c1

(No comment was entered for this change.)

Attachments:
png-to-webp-lossless.png 1.3 MB
png-to-webp-lossy.png 559 KB

[gedeo… via monorail]

Comment #2 on issue 604 by gedeo...@gmail.com: Strange behavior of terminal screenshots
https://bugs.chromium.org/p/webp/issues/detail?id=604#c2

(No comment was entered for this change.)

Attachments:

png-to-webp-lossless.png 351 KB

[jz… via monorail]

Comment #3 on issue 604 by jz...@google.com: Strange behavior of terminal screenshots
https://bugs.chromium.org/p/webp/issues/detail?id=604#c3

Thanks for the report. Lossless has different techniques than lossy (palette mode being one that might help here) and isn't limited to block based encoding [1][2]. So some content like this, as well as computer generated content (icons, etc) may compress better with lossless than lossy at a high or medium quality setting.

If you try the same encodes with cwebp it will output the tools used for lossless, which might give a hint.

[1] https://developers.google.com/speed/webp/docs/compression
[2] https://developers.google.com/speed/webp/docs/webp_lossless_bitstream_specification

[jz… via monorail]

Updates:
Status: Invalid

Comment #4 on issue 604 by jz...@google.com: Strange behavior of terminal screenshots
https://bugs.chromium.org/p/webp/issues/detail?id=604#c4

I'm going to mark this as invalid based on the comment above and the additional commentary on the mailing list [1]. If you have a sample image you'd like to attach we can verify what tools are being used with the image.

[1] https://groups.google.com/a/webmproject.org/g/webp-discuss/c/ppzuzKnWICc/m/wDxqGbsjDQAJ