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[csswg-drafts] [css-color-4] Clarify which transfer function to use for color(rec2020 ...) (#5907)

From: weinig via GitHub <sysbot+gh@w3.org>
Date: Fri, 29 Jan 2021 02:34:38 +0000
To: public-css-archive@w3.org
Message-ID: <issues.opened-796526607-1611887678-sysbot+gh@w3.org>
weinig has just created a new issue for https://github.com/w3c/csswg-drafts:

== [css-color-4] Clarify which transfer function to use for color(rec2020 ...) ==
The normative definition for color(rec2020 ...) at https://drafts.csswg.org/css-color-4/#valdef-color-rec2020 says to use a transfer function value of `1/2.4` (with a note) whereas the sample conversion code at https://drafts.csswg.org/css-color-4/#color-conversion-code uses a different value of `0.45`. (This could just an issue of updating the sample code to account for this).

A few things to note:
- It looks like color.js (https://github.com/LeaVerou/color.js/blob/master/src/spaces/rec2020.js) uses `1/2.4`.
- From testing, it seems like CoreGraphics, the graphics library on macOS and iOS, uses the `0.45` when using the kCGColorSpaceITUR_2020 named color space. (This is not a blocker for us, I can perform the conversion manually, but it would be a bit unfortunate for CSS and CoreGraphics to disagree here, if people are trying to use color values interchangeably.) 

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/5907 using your GitHub account

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