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Comment by enriquto | Hacker News Reader

> Instead of trying to smooth the entire image, it reduces blocking artifacts by finding plausible coefficients that reduce discontinuities at block boundaries

This is essentially the same thing, but using slightly different regularizers. In both cases, an image is found that whose jpeg compression is identical to the given one, by selecting the "most regular" image among the large family of such images. Jpeg2png minimizes the total variation over the whole image, and knusperly only along the boundaries of the blocks. The effect is quite similar at the end.

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The effects are quite different, since reducing total variation also reduces texture, which is important to perceived quality. Here's a direct comparison-- note how knusperli retains slightly more detail instead of oversmoothing: https://mod.ifies.com/f/200710_lena_decode_comparison.png

Good point. Unfortunately, the lena image is quite bad for comparison, there's almost no texture (except for the blue kerchief). I wonder what would happen for a heavily textured image; say, white noise. Would knusperly smooth it only along the block boundaries and leave it alone elsewhere?

Use "-w 0.0" switch to jpeg2png, gives slightly sharper results.

Also try with: https://github.com/ilyakurdyukov/jpeg-quantsmooth with "-q6" (default setting without luma-aware chroma upsampling)

My guess is that quantsmooth won't handle squares in the background well, but will be better at the edges (without excessive blurring).

Update: I tested it, it's about 10% quality, jpeg-quantsmooth doesn't handle this quality. At 25% quality - result is fine, starting at 20% and less - not good.

Perhaps the synthesis of all three projects combined into one (taking best of each) could give a great result.

So then...

jpeg2png: overblurs and slow

knusperli: does only deblocking, not removes artifacts

jpegqs: fails to deblock low frequencies (less 25% quality)

I wish people would stop using that image as an example. Not because of its content (what a stupid beat-up that was) but because the source file is of such terrible quality that it isn’t representative of the images we routinely handle today. And the extreme colour cast makes it useless for judging skin tones.

The jpeg2png should have noise (grain) simulated back.

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