hsi2rgbw.js

function hsi2rgbw(H, S, I) {
	let rgbw = {}, cos_h, cos_1047_h;

	H = H % 360;
	H = Math.PI * H / 180;

	S = S > 0 ? (S < 1 ? S : 1) : 0;
	I = I > 0 ? (I < 1 ? I : 1) : 0;

	if(H < 2.09439) {
	    cos_h = Math.cos(H);
	    cos_1047_h = Math.cos(1.047196667 - H);
	    rgbw.r = S * 255 * I / 3 * (1 + cos_h / cos_1047_h);
	    rgbw.g = S * 255 * I / 3 * (1 + (1 - cos_h / cos_1047_h));
	    rgbw.b = 0;
	    rgbw.w = 255 * (1 - S) * I;
	} else if(H < 4.188787) {
	    H = H - 2.09439;
	    cos_h = Math.cos(H);
	    cos_1047_h = Math.cos(1.047196667 - H);
	    rgbw.r = 0;
	    rgbw.g = S * 255 * I / 3 * (1 + cos_h / cos_1047_h);
	    rgbw.b = S * 255 * I / 3 * (1 + (1 - cos_h / cos_1047_h));
	    rgbw.w = 255 * (1 - S) * I;
	} else {
	    H = H - 4.188787;
	    cos_h = Math.cos(H);
	    cos_1047_h = Math.cos(1.047196667 - H);
	    rgbw.r = S * 255 * I / 3 * (1 + (1 - cos_h / cos_1047_h));
	    rgbw.g = 0;
	    rgbw.b = S * 255 * I / 3 * (1 + cos_h / cos_1047_h);
	    rgbw.w = 255 * (1 - S) * I;
	}

	rgbw.r = +rgbw.r.toFixed(0);
	rgbw.g = +rgbw.g.toFixed(0);
	rgbw.b = +rgbw.b.toFixed(0);
	rgbw.w = +rgbw.w.toFixed(0);

	return rgbw;
}