wlsunset/color_math.c

#define _USE_MATH_DEFINES
#include <math.h>
#include <errno.h>
#include <time.h>
#include "color_math.h"

static int is_leap(int year) {
	return (year % 4 == 0 && year % 100 != 0) || year % 400 == 0;
}

static int days_in_year(int year) {
	return is_leap(year) ? 366 : 365;
}

static double radians(double degrees) {
	return degrees * M_PI / 180.0;
}

static double degrees(double radians) {
	return radians * 180.0 / M_PI;
}

void sun(struct tm *tm, double longitude, double latitude, time_t *sunrise, time_t *sunset) {
	// https://www.esrl.noaa.gov/gmd/grad/solcalc/solareqns.PDF
	double year_rad = 2 * M_PI /
		days_in_year(tm->tm_year) *
		(tm->tm_yday - 1 + (tm->tm_hour - 12)/24);
	double eqtime = 229.18 * (0.000075 +
		0.001868 * cos(year_rad) -
		0.032077 * sin(year_rad) -
		0.014615 * cos(2*year_rad) -
		0.040849 * sin(2*year_rad));
	double decl = 0.006918 -
		0.399912 * cos(year_rad) +
		0.070257 * sin(year_rad) -
		0.006758 * cos(2*year_rad) +
		0.000907 * sin(2*year_rad) -
		0.002697 * cos(3*year_rad) +
		0.00148 * sin(3*year_rad);
	double ha = degrees(acos(
		cos(radians(90.833)) / (cos(radians(latitude)) * cos(decl)) -
		tan(radians(latitude)) * tan(decl)));
	*sunrise = (720 - 4 * (longitude + fabs(ha)) - eqtime) * 60;
	*sunset = (720 - 4 * (longitude - fabs(ha)) - eqtime) * 60;
}

static int illuminant_d(int temp, double *x, double *y) {
	// https://en.wikipedia.org/wiki/Standard_illuminant#Illuminant_series_D
	if (temp >= 4000 && temp <= 7000) {
		*x = 0.244063 +
			0.09911e3 / temp +
			2.9678e6 / pow(temp, 2) -
			4.6070e9 / pow(temp, 3);
	} else if (temp > 7000 && temp <= 25000) {
		*x = 0.237040 +
			0.24748e3 / temp +
			1.9018e6 / pow(temp, 2) -
			2.0064e9 / pow(temp, 3);
	} else {
		errno = EINVAL;
		return -1;
	}
	*y = (-3 * pow(*x, 2)) + (2.870 * (*x)) - 0.275;
	return 0;
}

static int planckian_locus(int temp, double *x, double *y) {
	if (temp >= 1667 && temp <= 4000) {
		*x = -0.2661239e9 / pow(temp, 3) - 
			0.2343589e6 / pow(temp, 2) + 
			0.8776956e3 / temp +
			0.179910;
		if (temp <= 2222) {
			*y = -1.1064814 * pow(*x, 3) -
				1.34811020 * pow(*x, 2) + 
				2.18555832 * (*x) -
				0.20219683;
		} else {
			*y = -0.9549476 * pow(*x, 3) -
				1.37418593 * pow(*x, 2) + 
				2.09137015 * (*x) -
				0.16748867;
		}
	} else if (temp > 4000 && temp < 25000) {
		*x = -3.0258469e9 / pow(temp, 3) + 
			2.1070379e6 / pow(temp, 2) + 
			0.2226347e3 / temp +
			0.240390;
		*y = 3.0817580 * pow(*x, 3) -
			5.87338670 * pow(*x, 2) +
			3.75112997 * (*x) -
			0.37001483;
	} else {
		errno = EINVAL;
		return -1;
	}
	return 0;
}

static double srgb_gamma(double value, double gamma) {
	// https://en.wikipedia.org/wiki/SRGB
	if (value <= 0.0031308) {
		return 12.92 * value;
	} else {
		return pow(1.055 * value, 1.0/gamma) - 0.055;
	}
}

double clamp(double value) {
	if (value > 1.0) {
		return 1.0;
	} else if (value < 0.0) {
		return 0.0;
	} else {
		return value;
	}
}

static void xyz_to_srgb(double x, double y, double z, double *r, double *g, double *b) {
	// http://www.brucelindbloom.com/index.html?Eqn_RGB_XYZ_Matrix.html
	*r = srgb_gamma(clamp(3.2404542 * x - 1.5371385 * y - 0.4985314 * z), 2.2);
	*g = srgb_gamma(clamp(-0.9692660 * x + 1.8760108 * y + 0.0415560 * z), 2.2);
	*b = srgb_gamma(clamp(0.0556434 * x - 0.2040259 * y + 1.0572252 * z), 2.2);
}

static void srgb_normalize(double *r, double *g, double *b) {
	double maxw = fmaxl(*r, fmaxl(*g, *b));
	*r /= maxw;
	*g /= maxw;
	*b /= maxw;
}

void calc_whitepoint(int temp, double *rw, double *gw, double *bw) {
	if (temp == 6500) {
		*rw = *gw = *bw = 1.0;
		return;
	}

	double x = 1.0, y = 1.0;
	if (temp > 1667 && temp <= 6500) {
		planckian_locus(temp, &x, &y);
	} else if (temp >= 6500 && temp <= 25000) {
		illuminant_d(temp, &x, &y);
	}
	double z = 1.0 - x - y;

	xyz_to_srgb(x, y, z, rw, gw, bw);
	srgb_normalize(rw, gw, bw);
}