Chroma downsampling and upsampling algorithms dont't work with heif_chroma_422

[homm]

Expected

heif_chroma_422 is one of the downsampling options for RGB → YCbCr conversion. Like heif_chroma_420, it also removes data from CbCr channels. So there are should be a way to enable the average downsampling and upsampling.

Actual result

$ heif-enc ./cat.png -o ./cat.ss1.avg.avif -q 89 -p chroma=422 -p speed=6 -C average
Could not encode HEIF/AVIF file: Unsupported feature: Unsupported color conversion

Additional

Combining with #935 this leads to that there is no way to decode the image with bilinear algorithm. If you set only_use_preferred_chroma_algorithm, it will breaks on heif_chroma_422 images and there is not way to find actual subsampling of the image before calling heif_decode_image, just as there is no way to set preferred_chroma_downsampling_algorithm after calling heif_decode_image. If you woudn't set only_use_preferred_chroma_algorithm, the preferred_chroma_downsampling_algorithm will be ignored.

[homm]

@farindk Are you aware of the problem? Does it seem meaningful to you?

[farindk]

Yes, we don't have a 4:2:2 to 4:4:4 bilinear upsampling algorithm implemented. I thought that was low priority because 4:2:2 is basically never used for images. It was meant for interlaced video, but is pretty useless for still images.

Usually only_use_preferred_chroma_algorithm=false should be the safe option to use. I'll have a closer look at #935 whether there is a problem when choosing the upsampling algorithm.

I could add a 4:2:2 to 4:4:4 bilinear implementation, just in case.

[homm]

I thought that was low priority because 4:2:2 is basically never used for images.

Why do you think so? It reduces amount of data a lot with moderate impact on visual quality. We use it for most our saved images.

[farindk]

Seems that we need a 4:2:2 to 4:4:4 implementation to be able to set use only_use_preferred_chroma_algorithm=true. I'll add that.

[farindk]

Please also check that these conversions work as expected. Especially at the image borders and images with odd width.
These are special cases.

[homm]

@farindk Thanks! I will try to do this within the day

[shaiksuhel1999]

@farindk is gpu hardware accelerators supports for chroma subsampling 422 files?

[farindk]

@farindk is gpu hardware accelerators supports for chroma subsampling 422 files?

No. There is no GPU acceleration for any chroma format yet.