why in JPEG ,while taking DCT we are taking matrix of 8x8...

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Question by mohan
Submitted on 8/5/2003
Related FAQ: JPEG image compression FAQ, part 1/2
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why in JPEG ,while taking DCT we are taking matrix of 8x8 dimension. also why we are not using DFT instead of DCT. please revert me with the answer. mohan_sanodia@yahoomail.com

Answer by Nick H
Submitted on 8/5/2003
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I guess it was a tradeoff between compression, image quality and complexity. Smaller blocks are simpler to implement but less scope for compression. Larger blocks are more complicated but there are more frequency coefficients so more scope for compression; also more risk of visible artefacts... DFT treats a finite signal as if it was repeating -- a difference in level between the edges of the block will give rise to high frequencies; quantisation of high frequencies can cause blocky artefacts. (DFT with overlapping windows might work, but there would be redundancy.) DCT is a bit better behaved. Wavelets (as in JPEG2000) are also a good approach to eliminating blockiness.

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