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Lanczos 2 upsampling algorithm, recommended in ITU-T H series Supplement 18
As you may know SDR BT.2020 spec. and HDR (PQ and HLG transfer) stuff introduced in BT.2100 all require top-left chroma sitting (type 2, where possible types are 0, 1, 2, 3, 4, 5) if subsampled to 4:2:0. Yet not only I do not see it, but I also cannot see the use of normal 444 chroma upsampler a.k.a. FIR. ITU-T H series Supplement 18 recommends at least Lanczos 2.
Describe the solution you'd like Better use ffmpeg? Zimg uses Lanczos 2 by default and supports all chroma locations, but so does swscaler. I recently wrote how to do this stuff in swscale. Last paragraph https://trac.ffmpeg.org/wiki/Scaling
Some obvious logic, R'G'B' does not have chroma siting, since no chroma, 4:4:4 does not have it either. 4:2:2 has only one siting.
It may be good to do Lanczos 4 (that means with 4 taps). https://forum.doom9.org/archive/index.php/t-161263.html and
https://github.com/FFmpeg/FFmpeg/commit/58e59396f5fe93f0606dc458d84c609b5d23ea1c#diff-1664bc2b6de9114b685897e0152a71de38d58acdfb642639bd8c2210ef4cc68fR904
zscale already uses 3 taps by default (in mpv in particular).
Hi!
The default if you open a 4:2:0 YUV file is ChromaLocType=0 as in left. You can actually set this position in the YUV format selection:
So if you set both offsets to 0 you get type 2 (top left).
For the chroma interpolation algorithm, however, we only have the ones available under Chroma Interpolation. So no Lanczos yet.
So if you set both offsets to 0 you get type 2 (top left).
Oh! Thank you!
For the chroma interpolation algorithm, however, we only have the ones available under
Chroma Interpolation. So no Lanczos yet.
Understood! Will rename the issue!
I just looked through the code of zimg a bit on the weekend and that may be very interesing to integrate here. They support basically all relevant conversions and on top also SIMDI accelerated code.