Re: [csswg-drafts] [css-color-hdr-1] Compositing SDR in HDR outputs (#5323)

> For example, a transform cannot simply scale SDR images in linear colour space, as the increased brightness and, if composited with HDR video, the presence of specular highlights will affect the perception of shadows and midtones - a perceptual gamma adjustment is required.

This is clearly true, but I wonder which part of the specification text triggered the worry that SDR content would be linearly scaled to put diffuse media white at HDR peak white?

Currently the [spec says](https://drafts.csswg.org/css-color-hdr/#Compositing-SDR-HDR), firstly, that compositing happens in CIE XYZ and is thus in a linear-light space. Then:

> 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 203 cd/m², the same luminance as is used to display the 58% PQ value. However, implementations may chose to incorporate a color re-rendering step (OOTF) to account for non-reference viewing conditions.

Both of these values are from BT.2408 and neither maps diffuse white to peak white.

Transfer function happens after that compositing, and depends on the destination colorspace.

I also agree that display-referred is a better approach, because scene-referred and artistic intent somewhat interact so there is no one true scene lighting.

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Received on Thursday, 10 September 2020 13:38:31 UTC