source: node_modules/d3-color/src/lab.js

import define, {extend} from "./define.js";
import {Color, rgbConvert, Rgb} from "./color.js";
import {degrees, radians} from "./math.js";

// https://observablehq.com/@mbostock/lab-and-rgb
const K = 18,
    Xn = 0.96422,
    Yn = 1,
    Zn = 0.82521,
    t0 = 4 / 29,
    t1 = 6 / 29,
    t2 = 3 * t1 * t1,
    t3 = t1 * t1 * t1;

function labConvert(o) {
  if (o instanceof Lab) return new Lab(o.l, o.a, o.b, o.opacity);
  if (o instanceof Hcl) return hcl2lab(o);
  if (!(o instanceof Rgb)) o = rgbConvert(o);
  var r = rgb2lrgb(o.r),
      g = rgb2lrgb(o.g),
      b = rgb2lrgb(o.b),
      y = xyz2lab((0.2225045 * r + 0.7168786 * g + 0.0606169 * b) / Yn), x, z;
  if (r === g && g === b) x = z = y; else {
    x = xyz2lab((0.4360747 * r + 0.3850649 * g + 0.1430804 * b) / Xn);
    z = xyz2lab((0.0139322 * r + 0.0971045 * g + 0.7141733 * b) / Zn);
  }
  return new Lab(116 * y - 16, 500 * (x - y), 200 * (y - z), o.opacity);
}

export function gray(l, opacity) {
  return new Lab(l, 0, 0, opacity == null ? 1 : opacity);
}

export default function lab(l, a, b, opacity) {
  return arguments.length === 1 ? labConvert(l) : new Lab(l, a, b, opacity == null ? 1 : opacity);
}

export function Lab(l, a, b, opacity) {
  this.l = +l;
  this.a = +a;
  this.b = +b;
  this.opacity = +opacity;
}

define(Lab, lab, extend(Color, {
  brighter(k) {
    return new Lab(this.l + K * (k == null ? 1 : k), this.a, this.b, this.opacity);
  },
  darker(k) {
    return new Lab(this.l - K * (k == null ? 1 : k), this.a, this.b, this.opacity);
  },
  rgb() {
    var y = (this.l + 16) / 116,
        x = isNaN(this.a) ? y : y + this.a / 500,
        z = isNaN(this.b) ? y : y - this.b / 200;
    x = Xn * lab2xyz(x);
    y = Yn * lab2xyz(y);
    z = Zn * lab2xyz(z);
    return new Rgb(
      lrgb2rgb( 3.1338561 * x - 1.6168667 * y - 0.4906146 * z),
      lrgb2rgb(-0.9787684 * x + 1.9161415 * y + 0.0334540 * z),
      lrgb2rgb( 0.0719453 * x - 0.2289914 * y + 1.4052427 * z),
      this.opacity
    );
  }
}));

function xyz2lab(t) {
  return t > t3 ? Math.pow(t, 1 / 3) : t / t2 + t0;
}

function lab2xyz(t) {
  return t > t1 ? t * t * t : t2 * (t - t0);
}

function lrgb2rgb(x) {
  return 255 * (x <= 0.0031308 ? 12.92 * x : 1.055 * Math.pow(x, 1 / 2.4) - 0.055);
}

function rgb2lrgb(x) {
  return (x /= 255) <= 0.04045 ? x / 12.92 : Math.pow((x + 0.055) / 1.055, 2.4);
}

function hclConvert(o) {
  if (o instanceof Hcl) return new Hcl(o.h, o.c, o.l, o.opacity);
  if (!(o instanceof Lab)) o = labConvert(o);
  if (o.a === 0 && o.b === 0) return new Hcl(NaN, 0 < o.l && o.l < 100 ? 0 : NaN, o.l, o.opacity);
  var h = Math.atan2(o.b, o.a) * degrees;
  return new Hcl(h < 0 ? h + 360 : h, Math.sqrt(o.a * o.a + o.b * o.b), o.l, o.opacity);
}

export function lch(l, c, h, opacity) {
  return arguments.length === 1 ? hclConvert(l) : new Hcl(h, c, l, opacity == null ? 1 : opacity);
}

export function hcl(h, c, l, opacity) {
  return arguments.length === 1 ? hclConvert(h) : new Hcl(h, c, l, opacity == null ? 1 : opacity);
}

export function Hcl(h, c, l, opacity) {
  this.h = +h;
  this.c = +c;
  this.l = +l;
  this.opacity = +opacity;
}

function hcl2lab(o) {
  if (isNaN(o.h)) return new Lab(o.l, 0, 0, o.opacity);
  var h = o.h * radians;
  return new Lab(o.l, Math.cos(h) * o.c, Math.sin(h) * o.c, o.opacity);
}

define(Hcl, hcl, extend(Color, {
  brighter(k) {
    return new Hcl(this.h, this.c, this.l + K * (k == null ? 1 : k), this.opacity);
  },
  darker(k) {
    return new Hcl(this.h, this.c, this.l - K * (k == null ? 1 : k), this.opacity);
  },
  rgb() {
    return hcl2lab(this).rgb();
  }
}));