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Comment by crynix | Hacker News Reader

I hope you're aware imgur compresses images that you upload to it regardless of whether they were already compressed.

Maybe they do that for lossy images, but I just compared what's on imgur to the original on my computer, and it is pixel-for-pixel identical. They did shave about 7KB off of it though, so maybe they push PNGs through pngout or similar.

I mean, as far as I know, there aren't even any practically useful algorithms out there for doing lossy compression on PNGs (although there should be).

Fireworks, pngquant, and png-nq have quantization algorithms that will dither a 32-bit PNG with alpha down to 8-bit palletized PNGs with alpha. The palette selection algorithms the free tools use (I haven't used Fireworks) sometimes drop important colors used in only a small section of an image, resulting in a blue power LED losing its blue color.

Yeah, technically that's lossy compression, but what I meant was lossy 32-bit PNG; that is, a preprocessing step before the prediction step which makes the result more compressible by the final DEFLATE step while having a minimal impact on quality.

That sounds very interesting. I wonder what kinds of transforms would improve compressibility by DEFLATE. I know a bit about PNG's predictors, but not enough about DEFLATE to confidently guess. If you ever work on this, please let me know. I'd like to collaborate.

Even if imgur did use a lossy format (which it didn't), you can see a difference between the top and bottom halves of that single image.

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