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Re: [csswg-drafts] [css-color] Generic Transform Function for RGB Color Spaces (#4488)

From: Chris Lilley via GitHub <sysbot+gh@w3.org>
Date: Wed, 13 Nov 2019 20:31:35 +0000
To: public-css-archive@w3.org
Message-ID: <issue_comment.created-553588769-1573677094-sysbot+gh@w3.org>
> Each of these spaces is completely defined by an ICC profile. So conversion to or from any of these spaces is going to be done by the ICC code, rather than by doing the maths by hand.

That may well be the case for some implementations. For example, Apple stated clearly that they will implement this in Safari by using ICC profiles.

In other cases, and Chrome springs to mind, from what I understand they plan to do the matric math themselves rather than loading profiles. TV and other video-oriented implementations also historically do not use ICC profiles for colorspace conversions.

> I don't think there's any value in trying to reproduce the ICC profile in CSS.

I disagree. The specification needs to state these values, because they are normative. Forcing people to dig into ICC profiles to find the values is counter productive and prone to error (for example ICC v.2 profiles typically have unadapted primaries while ICC v.4 profiles with a `chad` tag will have the primaries adapted to D50.

< That said, in the PDF world defining ColorSpaces without reference to an ICC profile is extremely common. It's not used for RGB as you've done here, but to define custom separations - a single separation is often called a "spot color", but they can be combined with other custom separations or the normal process colors (cyan, magenta, yellow and black) to form what PDF call "DeviceN" ColorSpaces.

Yes, I'm aware of them.

In CSS Color 4, multichannel calibrated spaces are already possible. It would be possible to add a `Device-N()` alongside `Device-CMYK()` but both suffer from not being a colorimetric specification and thus, not composable with other colors (opacity, blending, interpolation as a color stop, etc).

> The ability to define custom separations is absolutely critical for any CSS to PDF engine. It's not a nice-to-have, and it's not something that can be done with ICC profiles.

I'd like to see a fuller proposal on how that would fit into CSS.

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Received on Wednesday, 13 November 2019 20:31:37 UTC

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