Re: [csswg-drafts] Define rec2020 color space to use 2.4 gamma (#12574)

This is a pretty **substantial and breaking change**, but I agree with your analysis and CSS Color 4 is certainly incorrect (it uses the Table 4 inverse OETF, but also says that rec2020 is display-referred which is inconsistent.

The [ICC documentation for BT.2020](https://www.color.org/chardata/rgb/BT2020.xalter) uses the 2.4 gamma and states that the color space is display referred, noting that "some implementations" use the inverse OETF.

ITU-R BT.2087-0 whimsically notes that there are two cases in 709 to 2020 conversion which give "slightly different" colors (the difference is pretty substantial, as you note):

> Non-linear to linear conversion from normalized R′G′B′ colour signals E′RE′GE′B (Rec. 709) to linearly represented, normalized RGB colour signals EREGEB (Rec. 709) is accomplished by one of two equations which produce slightly different colours from each other:
>
> Case #1: In the case where the goal is to preserve colours seen on a Rec. 709 display1 when displayed on a Rec. 2020 display2, an approximation of the electro-optical transfer function (EOTF) from Recommendation ITU-R BT.1886 (Rec. 1886) is used:
> 𝐸=(𝐸′)2.40,0≤𝐸′≤1
>
> Case #2: In the case where the source is a direct camera output and the goal is to match the colours of a direct Rec. 2020 camera output, an approximation of the Rec. 709 inverse opto-electronic transfer function (OETF) is used (see Annex 2):
> 𝐸=(𝐸′)2,0≤𝐸′≤1

It also notes that BT.1886 uses a more complex formula which takes into account a non-zero black luminance and describes the form with L<sub>W</sub> = 1 and L<sub>B</sub> = 0 as an "approximated, normalized form".

If we do change the transfer function, it should continue to be defined over the extended range not just 0 to 1.

Do we know why Apple uses the  γ = 1.961 form and where that figure comes from? Are they rolling in an overall system gamma for non-reference viewing conditions? It would be very useful to know where this comes from and confirmation that it is now being changed. @smfr can you shed light on this?

It doesn't help that genuine full-gamut 2020 reference displays are exotic equipment that is rarer than hen's teeth. Makes it hard to do real, live color matching comparisons.

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Received on Tuesday, 5 August 2025 13:33:09 UTC