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By the way, you can decode JPEG with less artifacts using these projects: https://github.com/ilyakurdyukov/jpeg-quantsmooth https://github.com/ilyakurdyukov/jp

by ache7 6y ago

By the way, you can decode JPEG with less artifacts using these projects: https://github.com/ilyakurdyukov/jpeg-quantsmooth https://github.com/ilyakurdyukov/jpeg-quantsmooth (good for most mid-quality and high-quality images) https://github.com/victorvde/jpeg2png https://github.com/victorvde/jpeg2png (use for very low-quality images, slower) jpeg-quantsmooth gives a sharper output (use "-q6" option). jpeg2png output is a little blurry, but it does a better job of unblocking very low-quality images.

astrange 6y 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.
- ache7 6y ago
  Both of these projects have chroma upsampling.
  - astrange 6y 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 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.
    - ache7 6y 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).

by ache7 6y ago

astrange 6y 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.

ache7 6y ago
Both of these projects have chroma upsampling.
- astrange 6y 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 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.
  - ache7 6y 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).