Conversion algorithm from LCH(ab) to RGB.

lch.c

/*
 * Conversion algorithm from LCH(ab) to RGB. Following the directives from w3.
 * https://www.w3.org/TR/css-color-4/#lab-to-rgb
 *
 * Tags: luma,chroma,hue,lab,lch,cielab,c,windows,win10
 * Author: Carlos Gajardo (github.com/cjgajard).
 * Date: 2020-09-04
 */
#define _USE_MATH_DEFINES
#include <math.h>

struct rgba {
	int r, g, b, a;
};

#define XD50 0.96422
#define YD50 1.0
#define ZD50 0.82521

#define CRE (6.0 / 29.0)
#define K 903.3
#define KE 8.0
#define F_XZ(n) ((n) > CRE ? (n) * (n) * (n) : (116 * (n) - 16) / K)

static double srgbgamma (double n)
{
	return n <= 0.0031308 ? 12.92 * n : 1.055 * pow(n, 1 / 2.4) - 0.055;
}

static struct rgba lab2rgb (double L, double a, double b)
{
/*
 * From Lab to D50-XYZ.
 * http://www.brucelindbloom.com/index.html?Eqn_Lab_to_XYZ.html
 */
	double y = (L + 16) / 116.0;
	double x = y + a / 500.0;
	double z = y - b / 200.0;

	x = XD50 * F_XZ(x);
	y = YD50 * (L > KE ? y * y * y : L / K);
	z = ZD50 * F_XZ(z);
/*
 * Chromatic adaptation from D50-XYZ to D65-XYZ.
 * http://www.brucelindbloom.com/index.html?Eqn_ChromAdapt.html
 */
	x =  0.9555766 * x - 0.0230393 * y + 0.0631636 * z;
	y = -0.0282895 * x + 1.0099416 * y + 0.0210077 * z;
	z =  0.0122982 * x - 0.0204830 * y + 1.3299098 * z;
/*
 * XYZ to RGB.
 * http://www.brucelindbloom.com/index.html?Eqn_XYZ_to_RGB.html
 *
 * Linear sRGB using [M]-1 for sRGB|D65
 * http://www.brucelindbloom.com/index.html?Eqn_RGB_XYZ_Matrix.html
 */
	double r_ = 3.2404542 * x - 1.5371385 * y - 0.4985314 * z;
	double g_ = -0.9692660 * x + 1.8760108 * y + 0.0415560 * z;
	double b_ = 0.0556434 * x - 0.2040259 * y + 1.0572252 * z;
	/* gamma companding */
	r_ = srgbgamma(r_);
	g_ = srgbgamma(g_);
	b_ = srgbgamma(b_);
	/* clipping */
	r_ = r_ > 1 ? 1 : (r_ < 0 ? 0 : r_);
	g_ = g_ > 1 ? 1 : (g_ < 0 ? 0 : g_);
	b_ = b_ > 1 ? 1 : (b_ < 0 ? 0 : b_);
	/* rgb 0-255 */
	struct rgba color = {
		(int)(0xff * r_),
		(int)(0xff * g_),
		(int)(0xff * b_),
		0xff,
	};
	return color;
}

/*
 * Transform lch to rgb.
 * l [0.0-1.0] Luma.
 * c [0.0-1.0] Chroma. This argument is actually unbound.
 * h [0-359] Hue. In degrees, 0deg is not #f00.
 */
struct rgba lch (double l, double c, double h)
{
	double hrad = h * M_PI / 180.0;
	double a = cos(hrad) * c * 100;
	double b = sin(hrad) * c * 100;
	return lab2rgb(l * 100, a, b);
}