[...] JPEG data is DCT of 8x8 pixel chunks. If you can get at that, you can compare the DC terms of each chunk with minimal decoding. Various thumbnailers do this for speed already.
I really doubt that this would work. It seems to me that you can have very different DC terms with very similar results. In other words, even a little noise in the picture might produce very different DC terms.
Instead of comparing the DC terms, you could compare just the luminance. You would have to decompress just half the data for that and you wouldn't need to make the YUV->RGB conversion. That would probably save a few cycles.
-- Paulo Marques - www.grupopie.com
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