[lstamour]

Isn’t that kind of like saying Real numbers are complete? Yes, it’s true, but we don’t have an infinite level of precision for each colour. So we have to, practically speaking, limit how many colours can be specified to some reasonable (and arbitrary) number, e.g. 10 million or 1 billion or more (in terms of number of colours in a colour space) and representation in digital bits. Also, since I conflated the two a bit, it is often helpful to restrict what the max range of a colour is so that you can get more useful data out of the same precision. It’s kind of like creating a dictionary to compress data, you can express more useful values with less data by choosing to display a subset of colours and by picking where the white point is for your display. These optimizations used to mean more than they do today, perhaps, but it can still take a lot of effort to transfer 10-bit or 12-bit raw pixel data at 4K resolution, 120 times a second… And it’s worth pointing out that while 8-bit is all you might need as precision for perceptually lossless HDR (with dithering), that kind of range isn’t good enough for colour editing work. It’s like starting a photo edit from the JPEG rather than the RAW file.

[adgjlsfhk1]

8 bit just objectively isn't enough for hdr. 8 bits is few enough that you can pretty easily see banding on a simple color gradient in sdr. Dithering helps a lot, but once you get to HDR 10 bits is pretty much the minimum.

[Dylan16807]

> Isn’t that kind of like saying Real numbers are complete?

It's more like saying the range from 0..2 is complete so don't use an encoding that caps out at 0.8 or 1.1

The difference between all visible colors and a more constrained color space is only a factor of two or three. Less than half a bit per channel.

[BlueTemplar]

How do you explain then that even 10 bits aren't enough for a colour space like Rec.2020, which doesn't even feature all colours ?

[Dylan16807]

What are you comparing it with?

If you compare Rec.2020 with 10 bits per channel and sRGB at 9.5 bits per channel, both of them using the same peak brightness, you should see similar amounts of banding on both.

Increasing the maximum brightness can require a lot of extra bits to keep things smooth, but that's a separate thing from which colors are possible.

[BlueTemplar]

Ah, I see your point, but I still don't understand this argument, because in practice not only we want extra brightness in colours around the usual white points, but also we need the extra potential physical brightness to extend the colour space itself in terms of "colours only" (not sure that this even makes sense ?) because of the differing needs of different colours.

(I'd assume that this is especially relevant for backlit displays like the liquid crystal ones, since they have to use the same potential maximum source power for all of the channels.)

[Dylan16807]

Gamut is affected by brightness but not that much.

Rec.709 has 10 and 12 bit modes too, and it needs them, even though it has the same gamut as sRGB.

The reason 10 bits isn't enough for Rec.2020 is because 10 bits was never enough to fully represent any gamut. Once you add enough bits to properly support a gamut, then extending it to match human vision is a tiny cost.

And to be extra clear, when I initially said "The difference between all visible colors and a more constrained color space", I was talking about taking a color space and expanding its gamut without changing anything else about it.