Gamut mapping: clarification of chroma reduction algorithm

Hello, I’m coming to the repo from @svgeesus’s comment regarding color clamping in the CSSWG repository. I’ve finally had the chance to look into the proposed algorithm, and I just wanted to confirm some details.

A basic bisection method for reducing the LCH chroma:

let low = 0;
let high = color.chroma;
while (high - low > ε) {
	color.chroma = (high + low) / 2;
	if (displayable(color)) {
		low = color.chroma;
	} else {
		high = color.chroma;
	}
}

color.js compares the color at each iteration with its clipped version:

while ((high - low > ε) && (error < base_error)) {
	let clipped = mappedColor.toGamut({space, method: "clip"});
	let deltaE = mappedColor.deltaE(clipped, {method: "2000"});
	error = color.deltaE(mappedColor, {method: "2000"});
	if (deltaE - 2 < ε) {
		low = mappedColor[coordName];
		// console.log(++i, "in", mappedColor.chroma, mappedColor.srgb, error);
	}
	else {
		// console.log(++i, "out", mappedColor.chroma, mappedColor.srgb, clipped.srgb, deltaE, error);
		if (Math.abs(deltaE - 2) < ε) {
			// We've found the boundary
			break;
		}
		high = mappedColor[coordName];
	}
	mappedColor[coordName] = (high + low) / 2;
}

The current implementation seems to have diverged from the description in the docs:

Instead, the default algorithm reduces chroma (by binary search) and also, at each stage, calculates the deltaE2000 between the current estimate and a channel-clipped version of that color. If the deltaE is less than 2, the clipped color is displayed.

At a first pass through the function, what I see is the definition of displayable() / inGamut() relaxed to include colors at a deltaE2000 distance smaller than 2 to an in-gamut color (which, even by itself, sounds like a great idea). What I’m not seeing, unless I’m missing something, is how the condition Math.abs(deltaE - 2) < ε) can ever be fulfilled (and the clipped color returned early) without deltaE - 2 < ε catching it first.

[svgeesus]

Note that CSS Color 4 now describes gamut mapping. Much of that is just explanation, but it is followed by the actual algorithm which is constant-lightness, constant-hue chroma reduction in OKLCH using deltaE OK as the difference metric.

[svgeesus]

I would like to prototype the CSS Color 4 gamut mapping algorithm in color.js and have it available as an option. (It should probably the default, as it is clearly better than CIE LCH chroma reduction).