colorjs.io/src/deltaE/deltaEJz.js

Let’s get serious about color

import jzczhz from "../spaces/jzczhz.js";
import getColor from "../getColor.js";
import { isNone } from "../util.js";

/**
 * More accurate color-difference formulae
 * than the simple 1976 Euclidean distance in Lab
 *
 * Uses JzCzHz, which has improved perceptual uniformity
 * and thus a simple Euclidean root-sum of ΔL² ΔC² ΔH²
 * gives good results.
 * @param {import("../types.js").ColorTypes} color
 * @param {import("../types.js").ColorTypes} sample
 * @returns {number}
 */
export default function (color, sample) {
	[color, sample] = getColor([color, sample]);

	// Given this color as the reference
	// and a sample,
	// calculate deltaE in JzCzHz.
	let [Jz1, Cz1, Hz1] = jzczhz.from(color);
	let [Jz2, Cz2, Hz2] = jzczhz.from(sample);

	// Lightness and Chroma differences
	// sign does not matter as they are squared.
	let ΔJ = Jz1 - Jz2;
	let ΔC = Cz1 - Cz2;

	// length of chord for ΔH
	if (isNone(Hz1) && isNone(Hz2)) {
		// both undefined hues
		Hz1 = 0;
		Hz2 = 0;
	}
	else if (isNone(Hz1)) {
		// one undefined, set to the defined hue
		Hz1 = Hz2;
	}
	else if (isNone(Hz2)) {
		Hz2 = Hz1;
	}

	let Δh = Hz1 - Hz2;
	let ΔH = 2 * Math.sqrt(Cz1 * Cz2) * Math.sin((Δh / 2) * (Math.PI / 180));

	return Math.sqrt(ΔJ ** 2 + ΔC ** 2 + ΔH ** 2);
}