What subsampling factors are supported by JPEG/JFIF? E.g....

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Question by Nick H
Submitted on 7/19/2003
Related FAQ: JPEG image compression FAQ, part 1/2
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What subsampling factors are supported by JPEG/JFIF? E.g. I think 4:4:4(1,1;1,1;1,1), 4:2:2(2,1;1,1;1,1), 4:2:0(2,2;1,1;1,1), 4:1:1(4,1;1,1;1;1) are all supported; but what about 4:1:0(4,2;1,1;1,1) or (4,4;1,1;1,1).. The JFIF spec doesn't seem to say much about this, it even cites an example where Cb and Cr have different sizes. The JPEG book implies that all horizontal and vertical factors between 1 and 4 are allowed, including (bizarrely) 3. Does a JPEG decoder have to support every one of the hundred or so permutations, or is it more restricted? Can I assume at least that Cb and Cr sizes will be the same, and that factor 3 will never occur?

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