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Re: [csswg-drafts] [css-color-4] sRGB doesn't really use 80 cd/m^2 white luminance (#3435)

From: Chris Lilley via GitHub <sysbot+gh@w3.org>
Date: Mon, 01 Jun 2020 00:00:07 +0000
To: public-css-archive@w3.org
Message-ID: <issue_comment.created-636550793-1590969605-sysbot+gh@w3.org>
For displaying SDR content,it is common practice for the user to adjust screen brightness as they see fit and in response to (widely varying) viewing conditions. Thus the official 80 cd/m² has no impact on SDR usage.

It matters when SDR content is composited with HDR content that uses a absolute luminance scale (PQ); as far as I can see it does not matter with HDR content which uses a relative scale (HLG). And the important thing is to avoid the following obvious traps, in order of seriousness:

1. Mapping sRGB white to full peak brightness (10,000 cd/m²), which can't even be displayed full-screen on an HDR monitor and would be blinding if it could
2. Mapping sRGB white to max full-screen white, which [varies from 500 cd/m² to 1400 cd/m²](https://displayhdr.org/performance-criteria-cts1-1/) for commercial HDR displays; this will be too bright
3. Mapping sRGB white to the official 80 cd/m² which will be too dim.

Looking at the [Reference Level Guidelines for PQ (BT.2100)](https://www.dolby.com/us/en/technologies/dolby-vision/operational-guidelines-for-pq-2.pdf), from Dolby Laboratories, Aug. 9, 2016:

> reference 18% grey card, indoor scenes, 17 cd/m², PQ value of 34%
> diffuse white, indoor scenes, 140 cd/m², PQ value of 54%

That seems enough of a recommendation to put in a future CSS Color specification which includes HDR (Rec. BT.2100 PQ, Jzazbz which also uses PQ, etc) ; and to close the issue for CSS Color 4.

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Received on Monday, 1 June 2020 00:00:10 UTC

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