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Re: [csswg-drafts] [css-color-4] extend rgb() for HDR

From: Chris Lilley via GitHub <sysbot+gh@w3.org>
Date: Fri, 26 Oct 2018 13:46:55 +0000
To: public-css-archive@w3.org
Message-ID: <issue_comment.created-433413236-1540561614-sysbot+gh@w3.org>
> What does a negative value mean here? You aren't intending to imply the existence of a display that can suck light in as well as emitting it?

Nothing of the sort. But once a colorspace is linearized (and thus, is additive), and assuming sufficient precision is available,  it can be shown that any 3-component colorspace can be transformed into another one by a simple matrix multiplication. If the destination colorspace has a smaller gamut, then some values may be greater than 100% or less than 0%.
As an example, the original RGB colorspace used for color matching experiments in the 1930s required negative red values to match some spectral colors. This is (one of the reasons) why the CIE XYZ colorspace was develped, so all possible spctral colors could be represented with positive values of the X, Y and Z primaries. (As a consequence, the X Y and Z primaries are supersaturated and not physically realizable, which does not matter as long as the math works out).
And more recently, it was found that even XYZ sometimes needs slightly negative values (if colors measured under one lighting are chromatically adapted to a different lighting condition). And again, existing systems often failed to handle those negative values.

So to summarize the issue - extending below 0% and above 100% seems like it might work, and can work in a theoretical context with linearized (non gamma corrected) colorspaces and high precision; but is not a good way forward and is better handled in other ways. Primarily, by using a wider color gamut (like the P3 gamut used on many Apple, Samsung, Dell and HP devices and by digital cinema projectors; or the Rec.2020 system used by broadcast television and movie studios). 

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Received on Friday, 26 October 2018 13:46:56 UTC

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