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Comment by mistercow | Hacker News Reader

>JPEGmini adaptively encodes each JPEG file to the minimum file size possible without affecting its original quality

That is indeed what the FAQ says, but that being the case, the tool does not actually work very well, and the presentation is incredibly dishonest. The fact is that the JPEGmini versions of these images do lose noticeable quality, but Beamr is hiding this by giving a demo where the images are shown at 25% scale.

Take the dog image, for example. Using the slider, you'd think that JPEGmini nailed it; no visible artifacts whatsoever. But let's look at a section of the image at 100% scale, and see if this tool is really that impressive: http://imgur.com/z12mHnd

...holy block artifacts Batman!

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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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