Support CSS Color Level 4 hwb/lab/lch/gray functions (and related adjustments)

[mirisuzanne]

• Proposal (Language)
• Proposal (JS API)
• Embedded Protocol ([Color 4] Add JS type definitions and docs for CSS Color Level 4. #3718)
• Request for Comments (Write a blog post for Color Spaces sass-site#671)
• Implementation
  • Dart Sass (Merge feature.color-4 into main dart-sass#2341)
  • Node.js Embedded Host (Merge feature.color-4 into main embedded-host-node#330)
  • Tests (Merge feature.color-4 into main sass-spec#2013)
  • Documentation (Document new color spaces (#1055) sass-site#1115)
  • Specification (Merge feature.color-4 into main #3930, ...)
  • Migrator (Add legacy color function migrator migrator#260)
• Breaking Changes
  • Dart Sass
  • Tests
  • Documentation

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See Color Level 4

Having access to CIE colors/adjustments would be particularly useful for design systems, and it seems like we could provide the syntax in sass:color without waiting on browsers to support the CSS variant.

When I say adjustments, I'm not referring to the new CSS color functions in Level 5. I think we need to wait for those to solidify. I'm thinking of something similar to, or extending, color.adjust().

[nex3]

I'd definitely like to expand the set of built-in color functions, like we did for math functions in #851. Miriam, are you interested in writing up a proposal?

[mirisuzanne]

Yeah, I'm happy to. I see a few potential issues right away, and I'd love your thoughts:

Currently, color-adjust provides parameters for individually adjusting RGB or HSL values (both in an sRGB color space). LAB & LCH have values like lightness and hue that use the same name, even a similar syntax, but a different meaning in CIE color space. That's not a problem for the lch() and lab() functions, but I'm not sure how to address it with e.g. color.adjust(lch(…), $hue: …).

The level 5 spec provides a clever solution with the relative colors syntax, defining adjustments inside the respective color functions (hsl(from var(--accent) calc(h+180) s l) but I'm not sure if it's safe to rely on a proposed syntax like that.

Also: since no browsers support the new color formats, we're likely converting everything to sRGB for output. Since sRGB and CIE provide different color gamuts, that may not be a good long term solution, once CIE is supported in browsers. Do we need to plan ahead for that change in some way?

[nex3]

Good questions! I think my first high-level response is another question: what's the benefit of providing LAB- and LCH-friendly functions if we can't accurately represent their color spaces? Does it even make sense to start adding those when we know we'll probably want to support them differently in the future?

[mirisuzanne]

The main advantage is that values & adjustments in the CIE color space are perceptually uniform. So blue and yellow hues at the same chroma & lightness (in LCH) will look similarly bright to the eye, and provide similar contrast ratios against a background color. That's a big advantage for doing any sort of automated color management… assuming you stay in the rgb gamut.

I think that conversion will likely still happen for most users even once browsers support CIE formats, because most monitors use sRGB for display. If that's true, the conversion would only become an issue for people designing for wide-gamut displays…? I could do a bit more research…

[mirisuzanne]

ok, I have confirmation from Tab that it's a reasonable idea to do some conversion:

Most monitors can only display approximately the sRGB gamut (or less!) anyway, so lab() is just a fun new way to write rgb() (the coordinates have better behavior, that's all).
And for higher-def monitors, only the out-of-sRGB colors will be affected, and you can warn about that.
(And provide strategies for how to squish them into sRGB; there are several possibilities with different trade-offs.)
(We're discussing high-end color support in Chrome right now, and it looks like we'll be eagerly converting everything into the extended-sRGB colorspace to work with anyway, since that's what Skia uses.)
https://drafts.csswg.org/css-color-4/#descdef-color-profile-rendering-intent for more details
(linking over to some papers for the actual more details)
Since people probably expect their in-gamut colors to stay as specified, you're gonna be doing some variety of relative-colorimetric fixing.
But lots of variety there still.

There's a lot of detail in the spec, including sample conversion functions.

[nex3]

I'm not sure how safe it is for us to rely on "most monitors". Sass is generally pretty conservative with its polyfilling, for a couple reasons:

• We want the same Sass file to behave the same way everywhere, so we avoid configuration that affects the way the language works. This means that the whole language has to work for everyone—including people working with wide-gamut monitors, people targeting non-screen media, and so on.

• The status quo is always changing. Back in the day, Sass polyfilling hsl() was a feature—now, people are clamoring for us not to compile it out because they (mistakenly) believe that it has a wider gamut than RGB colors. Any behavior we add now will be very painful to change later, even if five or ten years down the line wide-gamut monitors and native browser support for lab() are the norm.

These factors make me want to avoid eagerly or silently converting colors to RGB. I think you're probably right that it's useful to do transformations and definitions in these color spaces, but we need to find a way to do that that will continue to make sense if wide-gamut colors become widespread.

[mirisuzanne]

Yeah, I think we should not be adding these to the global namespace. But if we put them in the sass:color module, they can provide both consistent sRGB output, and a straight-forward optional upgrade path. Authors can start using the plain CSS functions on their own schedule, without our intervention, and the module functions will continue to be useful for supporting old browsers.

I'm thinking of them like the sass:math functions that overlap CSS – a bit more limited than what the eventual CSS functions will provide, but a consistent and backwards-compatible pre-compiled option. No config required, and no more per-screen variation than using hsl() or rgb() now.

While there are several methods for handling out-of-gamut colors, it's clear (and defined in the CSS spec) that we should be using "relative-colorimetric" rendering. That doesn't leave much that we need to decide. At this point, I think if we can find a good method for calculating relative-colorimetric adjustments, I can finish putting together a proposal.

[mirisuzanne]

hwb() is an sRGB format already, so the conversion would be lossless. But we might want to namespace it anyway, to avoid complaints similar to the current hsl()-conversion…

[mirisuzanne]

I'm told by the editors of CSS Color level 4 that the best solution is to lock hue & lightness, then reduce chroma until the color is in-gamut.

I've created a (rough, slow) demo of that approach.

[mirisuzanne]

Ok, colors are complex. I think we can break this down a bit more, and take it a step at a time

1. hwb() is part of the sRGB color space we use for other functions. We could implement it with minimal considerations. I'm going to break this out into a stand-alone proposal while we discuss the other features.

2. CSS is planning to use lch() as the default space for color-mixing and adjustment functions. It is not gamut-limited, and it is perceptually uniform, so it makes a lot of sense as the default. I'm not sure we can change our adjustment defaults at this point, but we should be working towards full support of Lab/LCH colors asap – and there's a lot we can provide without getting in the way of forwards-compatibility. I'm feeling good about my progress on this proposal so far, but it involves some big questions about how we want to handle multiple colorspaces in adjust/scale/change/mix as well as lightness/hue functions.

3. Going further, CSS is moving towards full support of ICC color profiles. Some are predefined (srgb, display-p3, a98-rgb, prophoto-rgb, and rec-2020), but new profiles can be attached with the @color-profile rule. The color() function is proposed to set colors using any profile. I think that could be useful for us to support, and is related to this issue, but will take more significant research. I'd like to create a new issue for that, and handle it later.

4. The "relative colors" proposal in level 5 is also very interesting. Allows adjustments inside color functions, where the space is already implicit: hsl(from <color> calc(h+180) s l). But that proposal is more invasive, and still in flux. We probably want to hold off a bit.

[nex3]

Sorry for taking so long to come back to this.

Yeah, I think we should not be adding these to the global namespace. But if we put them in the sass:color module, they can provide both consistent sRGB output, and a straight-forward optional upgrade path. Authors can start using the plain CSS functions on their own schedule, without our intervention, and the module functions will continue to be useful for supporting old browsers.

I'm thinking of them like the sass:math functions that overlap CSS – a bit more limited than what the eventual CSS functions will provide, but a consistent and backwards-compatible pre-compiled option. No config required, and no more per-screen variation than using hsl() or rgb() now.

While there are several methods for handling out-of-gamut colors, it's clear (and defined in the CSS spec) that we should be using "relative-colorimetric" rendering. That doesn't leave much that we need to decide. At this point, I think if we can find a good method for calculating relative-colorimetric adjustments, I can finish putting together a proposal.

Cordoning functions in a module is beneficial because it makes it less likely that we'll need to migrate away from those functions, but it doesn't really make it less painful if we do end up needing to migrate. I want to make sure (as much as possible) that any functions we add are future-proof in and of themselves, even if there's a reasonable migration path.

I'm not saying there's no way it makes sense to fake wider gamuts in the short term, but whatever function names and syntax we come up with for doing so need to be able to do the same thing, and still be comprehensible, in a hypothetical future world where 99% of monitors and browsers support wide-gamut colors. If we end up in that world and Sass has a color.lab() function that returns an sRGB color, that's going to be very unexpected for users.

One possibility would be to define a function like color.lab($lightness, $a, $b, $alpha, $to), where the $to argument specifies a target colorspace. Initially users would be required to specify $to: srgb, but once we support native LAB colors that could be relaxed.

Ok, colors are complex. I think we can break this down a bit more, and take it a step at a time

1. hwb() is part of the sRGB color space we use for other functions. We could implement it with minimal considerations. I'm going to break this out into a stand-alone proposal while we discuss the other features.

👍

2. CSS is planning to use lch() as the default space for color-mixing and adjustment functions. It is not gamut-limited, and it is perceptually uniform, so it makes a lot of sense as the default. I'm not sure we can change our adjustment defaults at this point, but we should be working towards full support of Lab/LCH colors asap – and there's a lot we can provide without getting in the way of forwards-compatibility. I'm feeling good about my progress on this proposal so far, but it involves some big questions about how we want to handle multiple colorspaces in adjust/scale/change/mix as well as lightness/hue functions.

Looking forward to reading your proposals!

3. Going further, CSS is moving towards full support of ICC color profiles. Some are predefined (srgb, display-p3, a98-rgb, prophoto-rgb, and rec-2020), but new profiles can be attached with the @color-profile rule. The color() function is proposed to set colors using any profile. I think that could be useful for us to support, and is related to this issue, but will take more significant research. I'd like to create a new issue for that, and handle it later.

My guess is that this is something we'll have to punt to render-time for the most part—it's unlikely that we'll be able to accurately resolve at least URLs in @color-profile rules, and possibly other declarations as well.

4. The "relative colors" proposal in level 5 is also very interesting. Allows adjustments inside color functions, where the space is already implicit: hsl(from <color> calc(h+180) s l). But that proposal is more invasive, and still in flux. We probably want to hold off a bit.

Yeah, it would be cool to support this in Sass but also a LOT of work.