Chroma subsampling implicitly set for certain QFs in version 4.

Question

Chroma subsampling implicitly set for certain QFs in version 4.

NoraH2004 opened this issue 2 years ago · comments

NoraH2004 commented 2 years ago

I compressed color images with different quality factors and looked at their DCT values.
The chroma subsampling setting is left as default. However, I found that chroma subsampling is still applied for certain QFs.

(Specifically: EITHER for QFs of 75 and below (tested 50, 64, 75), OR higher than 75 (tested 95, 99, 100), for both modes, baseline and progressive.)

I also tested mozjpeg version 3.0 and did not observe this behavior, so it must have been introduced recently.

I was not able to locate it in jcparam.c. Can you give me a reference to this behavior in the source code?
Moreover, I was wondering if this is intended to happen with the default settings, as it is not mentioned in the release notes.

Thanks,
Nora

Kornel · Answer 1 · Sun Sep 11 2022 01:19:29 GMT+0800 (China Standard Time)

This behavior is limited to the cjpeg demo too. It's here: 3a86fa5

NoraH2004 · Answer 2 · Mon Sep 12 2022 16:30:48 GMT+0800 (China Standard Time)

Thanks for your answer and clarification.

From what I see, it was introduced in v3.2. In my opinion, it is an unfortunate feature. It breaks backward compatibility with cjpeg from previous releases / libjpeg-turbo and "silently" changes the value of an essential parameter without the developer noticing.

Best,
Nora

Kornel · Answer 3 · Mon Sep 12 2022 19:31:37 GMT+0800 (China Standard Time)

I don't treat cjpeg as a public API. It's a deprecated outdated demo tool. You should use MozJPEG as a library compiled into other libjpeg-based programs.