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Riskiness score underestimates visible distortion #14
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Yes, this image is very challenging. The threshold between SharpYUV420 and YUV444 is set at the riskiness score of 70 (see kThreshSharpYU420 in jpeg_tools.cc:186).. Ideally, the threshold should be made dependent of the targeted quality. The current setting corresponds to some sweet-spot around quality 70-80, which is obviously not enough for this particular difficult image. hope it helps, Note that 'vjpeg' can show the riskiness-map (instead of just having the total riskiness score) by pressing the 'r' key. Here's the result: As can be seen, all of the image is problematic. Usually only few areas are (crisps edge, etc.). |
sjpeg recommends sharp YUV for this image (https://user-images.githubusercontent.com/19407943/33548263-53136816-d8ef-11e7-8994-6a6cfded7912.png):
But in fact only YUV444 gives transparent results for this image; any chroma subsampling adds very obvious changes to the image.
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