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Subsampling Chroma in the Compressed Domian.

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Subsampling Chroma in the Compressed Domian. Azhar Aga 12 Apr 06:59
  Subsampling Chroma in the Compressed Domian. Loren Merritt 13 Apr 03:57
Azhar Aga
2007-04-12 06:59:42 UTC (about 17 years ago)

Subsampling Chroma in the Compressed Domian.

Hi ,
Can any one tell me whether it is possible to sub-sample the Chroma in a Compressed Image Format like JPEG. What I mean is that if a JPEG file is say 4:4:4 Can I subsample the Chroma to 4:2:2 whithout actually decoding the Image i.e in the Compressed domain. Azhar


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Loren Merritt
2007-04-13 03:57:07 UTC (about 17 years ago)

Subsampling Chroma in the Compressed Domian.

On Wed, 11 Apr 2007, Azhar Aga wrote:

Can any one tell me whether it is possible to sub-sample the Chroma in a Compressed Image Format like JPEG. What I mean is that if a JPEG file is say 4:4:4 Can I subsample the Chroma to 4:2:2 whithout actually decoding the Image i.e in the Compressed domain.

JPEG uses only 8x8 DCT. While you can downscale just by dropping some coefficients, that leaves you with 8x4 or 4x4 DCTs. To put the result into another JPEG you need to regroup the coefficients, compute an 8x8 DCT, and requantize.
Potential shortcuts:
If you only subsample in 1 dimension (4:4:4->4:2:2 or 4:2:2->4:2:0), then you can perform only a 1 dimensional iDCT + DCT. I think it's possible to convert two 4-point DCT blocks into one 8-point DCT in fewer operations than a full iDCT + DCT. And of course you can leave luma in the compressed domain.

--Loren Merritt