Re: [csswg-drafts] [css-color-hdr] Absolute luminance of PQ (#10460)

> I think this is at best very confusing. For example, `rec2100-linear` has the exact same peak white luminance, black luminance and white luminance but isn't said to have "absolute values". Are the values behaving differently? I don't believe they should.

Yes, they are - and the difference is the PQ transfer function which is defined in terms of absolute luminance values.

Contrast that with 2100 HLG, which is relative (0.75 code value is media white, which can be made brighter or dimmer).

> sRGB according to the "Controlling Dynamic Range" section (https://drafts.csswg.org/css-color-hdr/#controlling-dynamic-range) has (reference and peak) white defined at a luminance of 80 cd/m² and black at 0.2 cd/m². Is this not absolute? How is this different than the PQ absoluteness?

That definition is also present in CSS Color 4 [0.2. The Predefined sRGB Color Space: the [sRGB](https://drafts.csswg.org/css-color-4/#valdef-color-srgb) keyword](https://drafts.csswg.org/css-color-4/#predefined-sRGB). It is relative colorimetry: the reference display does indeed have a media white at 80 cd/m<sup>2</sup> but industry practice is to turn this up or down to suit personal preference or viewing conditions.

 - https://github.com/w3c/csswg-drafts/issues/3435

> This has been abused to not properly anchor SDR and HDR content and, at the actual display, have HDR content at a fixed luminance, while SDR content will have different luminances depending on some brightness setting.

It isn't clear how that is _abuse_.

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Received on Thursday, 20 June 2024 17:49:31 UTC