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

JavaScript library for color conversions

import { unpack, limit, TWOPI, PITHIRD } from '../../utils/index.js';
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];
};

export default hsi2rgb;