Quote:
Originally Posted by Asmodian
CRF like makes little sense for still images. CRF works by increasing QP during high motion scenes because they take a lot more bit rate and humans notice quality issues less during motion. In a still image there is no motion to hide extra artifacts, it is possible to increase the QP for larger blocks but that would be like AQ, not CRF.
Edit: And CRF isn't really based on a quality metric in that way, it is a rate control method not a quality setting method. QP would be just as good for setting quality if it wasn't for the time based bit rate curve compression.
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Well, x264/5 adaptive quantization is basically QP with some offsets based on visual complexity. In essence, it'll lower QP in smooth regions and increase it in complex regions, improving its results psychovisually versus just QP.
I've had good success using CRF like one would use the JPEG "quality" parameter (although on a very different scale), and have gotten superior results from CRF than QP.
Basically, your options are CRF, QP, bitrate, or lossless. CRF is the best choice for still images for most use cases.
Copying the common parameters from H.264's --tune stillimage can help, although I'm sure a well-optimized x265 --tune stillimage would have some adjustments.