[brigade]

Progressive JPEG needs a minimum of 2 x width x height additional bytes over baseline to decode an image (maybe more, definitely 1.5-3x more than that if you're displaying coarse scans), regardless of how many scans you have or display, as it needs to save coefficients for N-1 scans over the entire image, whereas baseline needs only to save the coefficients for a couple 8x8 blocks at a time. Though if you're clever about and sacrifice displaying coarse cans you could reduce this some.

If you don't display coarse scans (and if you're comparing progressive vs. baseline CPU usage then counting such isn't fair), then approximately the only additional CPU time progressive should take is the time additional cache misses take. It's probably a wash considering that decoding fewer coefficients / less Huffman data takes less CPU.

Maybe I'll get some numbers, I'm curious now... But unless you're serving multi-megapixel images the additional CPU and memory doesn't matter. Probably not even until you're in the double digits, if then.

[aerobson]

Thanks for the comment. Whatever detail you can add to this conversation is much appreciated. It's a neglected topic, and it's important for us to understand it better.