Re: [csswg-drafts] [compositing] SDR and HDR compositing (#554)

This is [now solved in CSS Color HDR](https://drafts.csswg.org/css-color-hdr/#Compositing-SDR-HDR) with a section entitled **Compositing SDR and HDR content** which in turn references ITU Rec.2408-0.

This places the diffuse white for all SDR RGB colorspaces as defined in Rec.2408:

> Relative HDR, using the HLG transfer function, must map SDR media white to the same luminance as is used to display the 75% HLG value.

and

> Absolute HDR, using the PQ transfer function, should map SDR media white to 140 cd/m², the same luminance as is used to display the 54% PQ value. However, implementations may chose to incorporate a color re-rendering step (OOTF) to account for non-reference viewing conditions.

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Received on Tuesday, 8 September 2020 15:01:09 UTC