[css-color-4] Do we need API for converting between colorspaces?

[grorg]

At the moment, a developer will have to include their own code for converting a color between two spaces. Is this something we are interested in providing?

e.g. color(srgb 1 0 0) is color(display-p3 0.9175 0.2003 0.1387)

[css-meeting-bot]

The CSS Working Group just discussed API for converting between color spaces, and agreed to the following resolutions:

• RESOLVED: Add an API for converting between the browser's known color spaces; dino will bring a proposal to the group.

The full IRC log of that discussion

<dino> Topic: API for converting between color spaces
<dino> GitHub Topic: https://github.com/w3c/csswg-drafts/issues/1248
<TabAtkins> dino: So say you're writing an imaging tool and you want to have some code that converts between color spaces.
<dbaron> TabAtkins: we should.  I don't like it when there's complicated math the browser can do, but there's no API for it.
<dbaron> TabAtkins: I've had to write this code in JS by looking up papers.
<TabAtkins> RESOLVED: Add an API for converting between the browser's known color spaces; dino will bring a proposal to the group.

[svgeesus]

An update to this issue is long overdue, sorry! @grorg

• I wrote a non-normative JavaScript Sample code for color conversions appendix, which has been in the spec for a while now. I have used this for generating examples and tests, both in this spec and also in CSS Color 5. It has also been ported by others, eg to SASS. The code is believed correct, but is written to be simple and clear rather than efficient or object oriented. For example, the developer is expected to call functions in the correct order.

let myP3 = gam_P3(XYZ_to_lin_P3(lin_sRGB_to_XYZ(lin_sRGB([1,0,0]))));
let myLCH = Lab_to_LCH(XYZ_to_Lab(D65_to_D50(lin_sRGB_to_XYZ(lin_sRGB([1,0,0])))));

• Lea and I wrote an object-oriented Color API color.js which is extensible, accepts any CSS color specification from CSS Color 4 (plus some more) and provides easy colorspace conversions.

let color= new Color("rgb(100% 0% 0%");
color.p3;
color.lch;

all the gamma correcting and un-correcting, any chromatic adaptation, is handled automatically. Color difference (deltaE, in assorted flavors), luminance contrast ratio, and interpolation (in various colorspaces) is also available. Lastly, there is control over gamut mapping. It can be used in the browser or on the server, e.g. in Node.

• Color.js was presented to the CSS WG in the context of Houdini Typed OM, and it was resolved to make a color object with colorspace conversions, WCAG contrast ratio, etc for Typed OM based on this work.

So the API will happen, but as part of Typed OM.