koying wrote: 06 May 2018, 07:45
No. If you want to call it upsampling/upscaling, it would be a very dumb algorithm where every chroma pixel is duplicated 2x2, without any interpolation

Not sure what's the fuzz about chroma upscaling, but any other algorithm is just making out stuff, imho
Chroma subsampling & upsampling terms have been in use in digital video processing for over 2 decades
You said it yourself- nearest neighbor is a dumb algorithm, but is the easiest to implement. I’m not sure why think duplication to fill in missing values is not considered as interpolation.
As for the fuzz about chroma upsampling algorithms, depending on the algorithm being used you can end up with artificats like jaggedness/boxy appearance, color-bleeding, out-of-gamut colors, aliasing & ringing. That’s why there is a whole slew of advanced filtering algorithms. Yes, all of them are making up missing stuff, the difference is how good they are in doing it.
Since at this time, there is no option for a user to change the chroma upsampling algorithm in Kodi, this may be a moot point for discussion now.
BTW, BT.601 and BT.709 recommendations do have filtering guidelines that ideally necessitates the use of multitap-FIR filters, but being computationally intensive is not implemented commonly.