I'm still not 100% convinced that these still image comparisons are valid, I mean, are we sure that we aren't grabbing
an I frame from one video and a B frame from the other? But based on other anecdotal evidence, we have to conclude that the 2160p30 viewed at 1080p30 is marginally better-looking than the 1080p30 native. I, for one, think 1080
p50 looks far better than the indistuingishable (to me) 1080p30 and 2160p30. Even when it's just a talking head shot, it's just so fluid and life-like, I really like the 50fps.
As for how a smaller file (the 2160p transcoded to 1080p file) can appear better quality, here's some brand new speculation: If you allow a video compressor more
time do its compressing, it can achieve a higher quality for a given fixed (or even slightly smaller) bitrate. Two-pass encoding is just one example of this, but there are presumably many parameters to fiddle with. Given that only a negligible fraction of files uploaded are 4K at the moment; YouTube can afford to spend a
lot more CPU time transcoding each video. By contrast, every % of performance they can squeeze out of the 1080p compressor is worth gazillions of dollars, so that's going to be optimized down to a cost. I think that could be one explaination for what we're seeing here.
I only have a 1920x1080 monitor. Both 2160 and 1080 looks the same to me, although in the posted still images in this thread there does indeed seem to be a small difference. 2160p stutters even though my computer and broadband should be fast enough (in theory), not sure why this is yet but it's not watchable as it is.
Wait, are you playing the 2160p video AT 2160p? Dave's OP specifically instructs us to play the 2160p video at 1080p:
Can you see any difference between this video at 1080p and this other one uploaded at 1080p?
If so, that's
fascinating that the smaller file would be more demanding to decode AND higher quality to watch.