chroma-js/src/io/hsi/hsi2rgb.js

JavaScript library for color conversions

const {unpack, limit, TWOPI, PITHIRD} = require('../../utils');
const {cos} = Math;

/*
 * hue [0..360]
 * saturation [0..1]
 * intensity [0..1]
 */
const hsi2rgb = (...args) => {
    /*
    borrowed from here:
    http://hummer.stanford.edu/museinfo/doc/examples/humdrum/keyscape2/hsi2rgb.cpp
    */
    args = unpack(args, 'hsi');
    let [h,s,i] = args;
    let r,g,b;

    if (isNaN(h)) h = 0;
    if (isNaN(s)) s = 0;
    // normalize hue
    if (h > 360) h -= 360;
    if (h < 0) h += 360;
    h /= 360;
    if (h < 1/3) {
        b = (1-s)/3;
        r = (1+s*cos(TWOPI*h)/cos(PITHIRD-TWOPI*h))/3;
        g = 1 - (b+r);
    } else if (h < 2/3) {
        h -= 1/3
        r = (1-s)/3
        g = (1+s*cos(TWOPI*h)/cos(PITHIRD-TWOPI*h))/3
        b = 1 - (r+g)
    } else {
        h -= 2/3
        g = (1-s)/3
        b = (1+s*cos(TWOPI*h)/cos(PITHIRD-TWOPI*h))/3
        r = 1 - (g+b)
    }
    r = limit(i*r*3);
    g = limit(i*g*3);
    b = limit(i*b*3);
    return [r*255, g*255, b*255, args.length > 3 ? args[3] : 1];
}

module.exports = hsi2rgb;