Comment by erincandescent

> In my tests with assorted 24-bit sRGB monitors, a difference of 1 in a single channel is almost always indistinguishable (and this might be a matter of monitor tuning); even a difference of 1 simultaneously in all three channels is only visible in a few places along the lerps. (Contrast all those common shitty 18-bit monitors. On those, even with temporal dithering, the contrast between adjacent colors is always glaringly distracting.)

Now swap the sRGB primaries for the Rec.2020 primaries. This gives you redder reds, greener greens, and slightly bluer blues (sRGB blue is already pretty good)

This is why Rec.2020 specifies a minimum of 10-bit per channel colour. It stretches out the chromaticity space and so you need additional precision.

This is "just" Wide Colour Gamut, not HDR. But even retaining the sRGB gamma curve, mapping sRGB/Rec.709 content into Rec.2020 without loss of precision requires 10-bit precision.

Swap out the gamma curve for PQ or HLG and then you have extended range at the top. Now you can go super bright without "bleeding" the intensity into the other colour channels. In other words: you can have really bright things without them turning white.

Defining things in terms of absolute brightness was a bit of a weird decision (probably influenced by how e.g. movie audio is mixed assuming the 0dBFS = 105dB(SPL) reference level that theaters are supposed to be callibrated to) but pushing additional range above the SDR reference levels is reasonable, especially if you expect that range to be used judiciously and/or you do not expect displays to be able to hit their maximum values on that across the whole screen continuously.