Quote:
Originally Posted by Cary Knoop
But that is not necessarily true 4:4:4.
In order to have true 4K/UHD 4:4:4 the camera needs a very high-resolution sensor due to the Bayer (or other) sensor design.
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And even that's not really accurate. A Bayer pattern has 2 green sensors for every blue and red, mimicing human visual sensitivities. As Y' is over half green, one could argue that there is chroma subsampling at the sensor, even!
The only TRUE 4:4:4 content is computer rendered, where it is actually created at native RGB 4:4:4.
Also, the human visual system is far more sensitive to edges in the luma domain than to chroma overall. While we have highly accurate color vision, we process it with much less spatial and temporal detail than basic black-and-white vision which is what most of our evolutionary ancestors only had. Luma is what keeps you from getting eaten by a tiger stalking you across a treeline. Chroma is what keeps you from eating unripe or poisonous fruit. But seeing precise color detail in something that's moving just isn't something we're wired for.
We've got some decades of digital image processing under our belt, and we've consistantly seen that 4:2:0 delivers more visual value per bit than 4:4:4 when bitrate is contstrained. Chroma subsampling was part of JPEG, even, at the birth of this technology.
Fortunately the misbegotten YUV-9 (one chroma sample per 4x4 block of luma) used in 90's codecs like Sorenson Video and Indeo died. That was definitively TOO supersampled. Especially with 320x240 video, which would have only 80x60 chroma samples. Colored text was a nightmare.
Another classic problem was going from NTSC DV25, which used 4:1:1 subsampling (one chroma per four pixels horizontally) to DVD (4:2:0) which netted out 4:1:0 color, so one chroma per 4x2 block of pixels. Key was to do all motion graphics after DV25, rendering as 4:2:0. Natural images were okay-ish with 4:1:1, as long as all graphics were done with more precision.