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by astrange 2176 days ago
ffmpeg/mplayer has some more principled smoothing filters than these (especially spp); they're made for MPEG4 but JPEG is pretty similar.

Another algorithm that should work but I haven't seen tried is edge-aware chroma upsampling. libjpeg uses nearest neighbor upscaling for the subsampled color planes, which is why JPEG looks so bad when there's a transition from eg saturated red to black.
1 comments
ache7 2176 days ago
Both of these projects have chroma upsampling.
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astrange 2176 days ago
All JPEG decoders have chroma upsampling. Here jpeg2png only seems to actually optimize it though; seems like it works but optimizing for "smoothness" could maybe be improved.

I'm suggesting something like eedi3 (http://avisynth.nl/index.php/Eedi3) but guided by the luma plane.

Oh, you can also jointly optimize the YUV-RGB conversion because if the pixel ends up out of range in RGB space it is probably a compression artifact. I read a paper on this once but haven't seen it implemented.

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ache7 2176 days ago
"All JPEG decoders have chroma upsampling."

Well, I meant better upsampling than bilinear interpolation, which blurs chroma.

"Oh, you can also jointly optimize the YUV-RGB conversion because if the pixel ends up out of range in RGB space it is probably a compression artifact."

Probably not, I suspect that the mozjpeg encoder may use this, so images with black on white text look better (artifact noise go beyond of range).

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