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

> I would like to have a more authoritative source on this. The spec itself never talks about absolute luminance, it only assigns the reference display luminances to code points **which could be done** with any spec such as sRGB.

What do you mean "could be done"? How is that relevant?

> The PQ format handles display luminance values up to a maximum of 10,000 cd/m2 as absolute values. It introduces a new transfer function considering efficient bit allocation on the basis of human visual characteristics that cover a wide luminance range. As an absolute luminance format, video signals have a unique correspondence with luminance values reproduced on the display, which means that the range of video signals that can be displayed depends on the peak luminance of the display. The EOTF of this format was specified in ST 2084[12)](https://www.nhk.or.jp/strl/english/publica/bt/70/2.html#note-12) of the Society of Motion Picture and Television Engineers (SMPTE) in 2014 as a reference display standard for use in HDR production.

Source: https://www.nhk.or.jp/strl/english/publica/bt/70/2.html

NHK is basically the Japanese BBC. So it's absolute in the sense that there is a 1:1 mapping of video signal to luminance on the display. If a signal would result in a luminance that's too high for the display to show, the 2020 spec only allows for some way of falloff like clipping. One is not supposed to fit the luminance range into what the display can show _across the whole spectrum_. Only at the very top for out of range values.

Making it fit somehow is what HLG was made for.

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Received on Thursday, 20 June 2024 19:31:03 UTC