TypeScript / ciede-2000-driver.ts 💾

// Limited Use License – March 1, 2025

// This source code is provided for public use under the following conditions :
// It may be downloaded, compiled, and executed, including in publicly accessible environments.
// Modification is strictly prohibited without the express written permission of the author.

// © Michel Leonard 2025

// The classic CIE ΔE2000 implementation, which operates on two L*a*b* colors, and returns their difference.
// "l" ranges from 0 to 100, while "a" and "b" are unbounded and commonly clamped to the range of -128 to 127.
function ciede_2000(l_1: number, a_1: number, b_1: number, l_2: number, a_2: number, b_2: number): number {
	// Working in TypeScript with the CIEDE2000 color-difference formula.
	// k_l, k_c, k_h are parametric factors to be adjusted according to
	// different viewing parameters such as textures, backgrounds...
	const k_l = 1.0, k_c = 1.0, k_h = 1.0;
	let n = (Math.sqrt(a_1 * a_1 + b_1 * b_1) + Math.sqrt(a_2 * a_2 + b_2 * b_2)) * 0.5;
	n = n * n * n * n * n * n * n;
	// A factor involving chroma raised to the power of 7 designed to make
	// the influence of chroma on the total color difference more accurate.
	n = 1.0 + 0.5 * (1.0 - Math.sqrt(n / (n + 6103515625.0)));
	// Application of the chroma correction factor.
	const c_1 = Math.sqrt(a_1 * a_1 * n * n + b_1 * b_1);
	const c_2 = Math.sqrt(a_2 * a_2 * n * n + b_2 * b_2);
	// atan2 is preferred over atan because it accurately computes the angle of
	// a point (x, y) in all quadrants, handling the signs of both coordinates.
	let h_1 = Math.atan2(b_1, a_1 * n), h_2 = Math.atan2(b_2, a_2 * n);
	if (h_1 < 0.0)
		h_1 += 2.0 * Math.PI;
	if (h_2 < 0.0)
		h_2 += 2.0 * Math.PI;
	n = Math.abs(h_2 - h_1);
	// Cross-implementation consistent rounding.
	if (Math.PI - 1E-14 < n && n < Math.PI + 1E-14)
		n = Math.PI;
	// When the hue angles lie in different quadrants, the straightforward
	// average can produce a mean that incorrectly suggests a hue angle in
	// the wrong quadrant, the next lines handle this issue.
	let h_m = (h_1 + h_2) * 0.5, h_d = (h_2 - h_1) * 0.5;
	if (Math.PI < n) {
		h_d += Math.PI;
		// 📜 Sharma’s formulation doesn’t use the next line, but the one after it,
		// and these two variants differ by ±0.0003 on the final color differences.
		h_m += Math.PI;
		// h_m += h_m < Math.PI ? Math.PI : -Math.PI;
	}
	const p = 36.0 * h_m - 55.0 * Math.PI;
	n = (c_1 + c_2) * 0.5;
	n = n * n * n * n * n * n * n;
	// The hue rotation correction term is designed to account for the
	// non-linear behavior of hue differences in the blue region.
	const r_t = -2.0 * Math.sqrt(n / (n + 6103515625.0))
		* Math.sin(Math.PI / 3.0 * Math.exp(p * p / (-25.0 * Math.PI * Math.PI)));
	n = (l_1 + l_2) * 0.5;
	n = (n - 50.0) * (n - 50.0);
	// Lightness.
	const l = (l_2 - l_1) / (k_l * (1.0 + 0.015 * n / Math.sqrt(20.0 + n)));
	// These coefficients adjust the impact of different harmonic
	// components on the hue difference calculation.
	const t = 1.0	+ 0.24 * Math.sin(2.0 * h_m + Math.PI * 0.5)
			+ 0.32 * Math.sin(3.0 * h_m + 8.0 * Math.PI / 15.0)
			- 0.17 * Math.sin(h_m + Math.PI / 3.0)
			- 0.20 * Math.sin(4.0 * h_m + 3.0 * Math.PI / 20.0);
	n = c_1 + c_2;
	// Hue.
	const h = 2.0 * Math.sqrt(c_1 * c_2) * Math.sin(h_d) / (k_h * (1.0 + 0.0075 * n * t));
	// Chroma.
	const c = (c_2 - c_1) / (k_c * (1.0 + 0.0225 * n));
	// Returning the square root ensures that dE00 accurately reflects the
	// geometric distance in color space, which can range from 0 to around 185.
	return Math.sqrt(l * l + h * h + c * c + c * h * r_t);
}

// GitHub Project : https://github.com/michel-leonard/ciede2000-color-matching
//   Online Tests : https://michel-leonard.github.io/ciede2000-color-matching

// L1 = 78.9   a1 = 29.5   b1 = 2.8
// L2 = 78.7   a2 = 34.9   b2 = -2.5
// CIE ΔE00 = 3.9137503016 (Bruce Lindbloom, Netflix’s VMAF, ...)
// CIE ΔE00 = 3.9137358886 (Gaurav Sharma, OpenJDK, ...)
// Deviation between implementations ≈ 1.4e-5

// See the source code comments for easy switching between these two widely used ΔE*00 implementation variants.

/////////////////////////////////////////////////
/////////////////////////////////////////////////
////////////                         ////////////
////////////    CIEDE2000 Driver     ////////////
////////////                         ////////////
/////////////////////////////////////////////////
/////////////////////////////////////////////////

// Reads a CSV file specified as the first command-line argument. For each line, this program
// in TypeScript displays the original line with the computed Delta E 2000 color difference appended.
// The C driver can offer CSV files to process and programmatically check the calculations performed there.

//  Example of a CSV input line : 48.5,92.7,-35,62,95,6
//    Corresponding output line : 48.5,92.7,-35,62,95,6,18.430816887860782649626625331919

import * as fs from 'fs';
import * as readline from 'readline';

if (process.argv.length < 3) {
	console.error("Usage: ts-node ciede-2000-driver.ts <filename>");
	process.exit(1);
}

const filename: string = process.argv[2];

const lineReader = readline.createInterface({
	input: fs.createReadStream(filename)
});

lineReader.on('line', (line: string) => {
	const [l1, a1, b1, l2, a2, b2] = line.split(',').map(parseFloat);
	const result = ciede_2000(l1, a1, b1, l2, a2, b2);
	process.stdout.write(`${line},${result}\n`);
});

lineReader.on('close', () => {
	// Nothing to do here
});