cjpeg -rgb compatibility

[Zsolt Ero]

Hi,

I'm trying to encode an image with some very fine line graphics, and it turns out that 50-60 quality range totally distorts the colors. Going up till quality 80 helps, but for HiDPI images for the web I never do that, the files are just very big. Also, I feel that even quality 80 changes the colors slightly.

I've found cjpeg's -rgb option and it seems to save the colors perfectly. Even using some low quality, like 30 preserves the colors (rgb's 30 is like a normal 80 in file size).

Can you tell me if I'm safe in using -rgb for web browsers?

Also, is there any better way to solve my issue, or I should just use -rgb?

Regards,
Zsolt

[DRC]

There are no compatibility concerns with using the RGB JPEG colorspace, but in general, if your goal is color fidelity, you will get better compression and approximately the same color fidelity by using the YCbCr JPEG colorspace with 4:4:4 chroma subsampling (AKA no chroma subsampling.) The default tables are designed to compress well with YCbCr, so using the RGB colorspace always produces a larger file than 4:4:4. The default subsampling level in the libjpeg API is 4:2:0, which is probably why you observe that YCbCr has much lower color fidelity than RGB. Disabling subsampling should make both colorspaces produce approximately the same fidelity (YCbCr has a tiny, imperceptible amount of roundoff error.)

On Feb 26, 2021, at 9:41 PM, Zsolt Ero <zsol...@gmail.com> wrote:

Hi,

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[Zsolt Ero]

Thanks. 

2 questions: 

1. I've read somewhere that for quality >= 80 cjpeg automatically selects 4:4:4, is that right?
   
2. How can I manually select 4:4:4 in cjpeg? Is it just "-sample 1x1" or I need something else?

Regards,

Zsolt

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[DRC]

No, cjpeg does not automatically select a different subsampling level based on quality. It will use 4:2:0 unless you specify otherwise. Passing -sample 1,1 is the correct way to enable 4:4:4.

On Feb 27, 2021, at 3:47 PM, Zsolt Ero <zsol...@gmail.com> wrote:



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