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
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