Feature request: Quality-optimized subsampling

[tdaede]

There are methods for producing 4:2:0 subsampled video that aim to provide more optimal results once displayed (both subjectively, and via cross-coefficient PSNR and CIEDE2000 metrics):

https://norkin.org/pdf/SPIE_2016_HDR_conversion_metrics.pdf

Despite the HDR title, the methods are also useful for SDR content. The paper provides 3 methods, of which the "Closed Form 1" method is probably most suitable for a SIMD implementation.

Also, these algorithms currently only perturb the luma coefficient. This helps make the problem tractable but there are likely better algorithms that could potentially generate better chroma values as well.

HDRTools also has an open source implementation that can be checked against (HDRTools is more a reference implementation rather than a practical library like zimg).