540 lines of those components, while with the 4k stream it might be: 2160/2 => back to 1080. So your encoded 1080p might be able to render after decoding only e.g. If it were perceptible, I think the codec designers wouldn't feel it was an acceptable tradeoff. The eye is much less sensitive to colors than intensity, as you point out. I would be interested in a double blind experiment confirming that their specific implementation of chroma subsampling is even detectable.