@mapbox/mapbox-gl-style-spec/util/color_spaces.ts

a specification for mapbox gl styles

import Color from './color';
import {number as interpolateNumber} from './interpolate';

type LABColor = {
    l: number;
    a: number;
    b: number;
    alpha: number;
};

type HCLColor = {
    h: number;
    c: number;
    l: number;
    alpha: number;
};

// Constants
const Xn = 0.950470, // D65 standard referent
    Yn = 1,
    Zn = 1.088830,
    t0 = 4 / 29,
    t1 = 6 / 29,
    t2 = 3 * t1 * t1,
    t3 = t1 * t1 * t1,
    deg2rad = Math.PI / 180,
    rad2deg = 180 / Math.PI;

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

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

function xyz2rgb(x: number) {
    return 255 * (x <= 0.0031308 ? 12.92 * x : 1.055 * Math.pow(x, 1 / 2.4) - 0.055);
}

function rgb2xyz(x: number) {
    x /= 255;
    return x <= 0.04045 ? x / 12.92 : Math.pow((x + 0.055) / 1.055, 2.4);
}

// LAB
function rgbToLab(rgbColor: Color): LABColor {
    const b = rgb2xyz(rgbColor.r),
        a = rgb2xyz(rgbColor.g),
        l = rgb2xyz(rgbColor.b),
        x = xyz2lab((0.4124564 * b + 0.3575761 * a + 0.1804375 * l) / Xn),
        y = xyz2lab((0.2126729 * b + 0.7151522 * a + 0.0721750 * l) / Yn),
        z = xyz2lab((0.0193339 * b + 0.1191920 * a + 0.9503041 * l) / Zn);

    return {
        l: 116 * y - 16,
        a: 500 * (x - y),
        b: 200 * (y - z),
        alpha: rgbColor.a
    };
}

function labToRgb(labColor: LABColor): Color {
    let y = (labColor.l + 16) / 116,
        x = isNaN(labColor.a) ? y : y + labColor.a / 500,
        z = isNaN(labColor.b) ? y : y - labColor.b / 200;
    y = Yn * lab2xyz(y);
    x = Xn * lab2xyz(x);
    z = Zn * lab2xyz(z);
    return new Color(
        xyz2rgb(3.2404542 * x - 1.5371385 * y - 0.4985314 * z), // D65 -> sRGB
        xyz2rgb(-0.9692660 * x + 1.8760108 * y + 0.0415560 * z),
        xyz2rgb(0.0556434 * x - 0.2040259 * y + 1.0572252 * z),
        labColor.alpha
    );
}

function interpolateLab(from: LABColor, to: LABColor, t: number): LABColor {
    return {
        l: interpolateNumber(from.l, to.l, t),
        a: interpolateNumber(from.a, to.a, t),
        b: interpolateNumber(from.b, to.b, t),
        alpha: interpolateNumber(from.alpha, to.alpha, t)
    };
}

// HCL
function rgbToHcl(rgbColor: Color): HCLColor {
    const {l, a, b} = rgbToLab(rgbColor);
    const h = Math.atan2(b, a) * rad2deg;
    return {
        h: h < 0 ? h + 360 : h,
        c: Math.sqrt(a * a + b * b),
        l,
        alpha: rgbColor.a
    };
}

function hclToRgb(hclColor: HCLColor): Color {
    const h = hclColor.h * deg2rad,
        c = hclColor.c,
        l = hclColor.l;
    return labToRgb({
        l,
        a: Math.cos(h) * c,
        b: Math.sin(h) * c,
        alpha: hclColor.alpha
    });
}

function interpolateHue(a: number, b: number, t: number) {
    const d = b - a;
    return a + t * (d > 180 || d < -180 ? d - 360 * Math.round(d / 360) : d);
}

function interpolateHcl(from: HCLColor, to: HCLColor, t: number): HCLColor {
    return {
        h: interpolateHue(from.h, to.h, t),
        c: interpolateNumber(from.c, to.c, t),
        l: interpolateNumber(from.l, to.l, t),
        alpha: interpolateNumber(from.alpha, to.alpha, t)
    };
}

export const lab = {
    forward: rgbToLab,
    reverse: labToRgb,
    interpolate: interpolateLab
} as const;

export const hcl = {
    forward: rgbToHcl,
    reverse: hclToRgb,
    interpolate: interpolateHcl
} as const;