[shawn]

I would be interested in a double blind experiment confirming that their specific implementation of chroma subsampling is even detectable. The eye is much less sensitive to colors than intensity, as you point out. If it were perceptible, I think the codec designers wouldn't feel it was an acceptable tradeoff.

So your encoded 1080p might be able to render after decoding only e.g. 540 lines of those components, while with the 4k stream it might be: 2160/2 => back to 1080.

I'm not sure that's accurate -- whatever downscaling process was used to convert from 8k to 1080p on Google's servers is probably the same process to convert from 8k to 1080p in the youtube player, isn't it? At least perceptually.

I would agree that if they convert from 8k (compressed) to 4k (compressed), then 4k to 1080p (compressed), then that would introduce perceptible differences. But in general reencoding video multiple times is fail, so that would be a bug in the encoding process server side. They should be going from the source material directly to 1080p, which would give the encoder a chance to employ precisely the situation you mention.

Either way, you should totes email me or shoot me a keybase message. It's not every day that I find someone to debate human perceptual differences caused by esoteric encoding minutiae.