Color Spaces: Units in color channels

The current definition of colors says that channel values are floating-point—that is, they’re stored without reference to a unit. However, there’s no explicit conversion from Sass numbers (which may have units) to floating-point numbers (which may not) either in Parsing Color Components or color().

This raises a few questions:

• CSS Colors 5 only allows raw numbers or percentages for custom colors. Should we do the same for unknown color spaces? Or is the risk of forwards-incompatibility with potential new predefined polar color spaces too great?

• If we do allow unknown color spaces to specify channels with circular units, do we have to store that fact? Can we just serialize those colors with a unitless value for that channel? Is there a risk of a new predefiend color space with a polar channel that defaults to turns instead of degrees?

• If we allow unknown color spaces to specify channels with circular units, do we allow any units, or just unitless/percent/circular?

• The existing channel-access functions color.hue(), color.saturation(), color.lightness(), color.whiteness(), and color.blackness() return values with units. However, color.channel() is specified never to do so—so color.channel($color, 'hue') isn’t a drop-in replacement for color.hue($color). Is this a problem?

[nex3]

What’s the best way to capture that final part? I’m not familiar with the term ‘raw double’. Is it fair to keep channel values as floating-point numbers, and then make sure we have guidance for adding/removing units as appropriate?

Yes, that’s what I mean. The term “double” (in a Sass context) is defined in the Number spec and basically just means “a 64-bit floating point number”. We should try to use “double” in the spec rather than “floating-point number” because it’s more explicit about the size and gamut of the number in question.

The channel normalization/validation processes (which could technically be combined) already now convert all values into unitless values where possible, or use % or deg in a few cases. Appending them where appropriate is simple enough. I think we would ideally want to remove them as part of ‘creating a color’ - maybe we can note that in the definition of the color object, that units are removed on creation? Or do we need to build that into various processes?

I think generally it’s a good idea to try to avoid having logic in definitions, so if it’s feasible to keep that in processes that would be ideal. It shouldn’t require too much verbosity—just saying “hue’s value” is enough to make it clear that you mean the double it contains.

[mirisuzanne]

• add units in: channels() and serialization
• remove units in normalization