wlsunset/color_math.c

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#define _POSIX_C_SOURCE 200809L
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#include <math.h>
#include <errno.h>
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#include <time.h>
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#include "color_math.h"
static double SOLAR_START_TWILIGHT = RADIANS(90.833 + 6.0);
static double SOLAR_END_TWILIGHT = RADIANS(90.833 - 3.0);
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static int days_in_year(int year) {
int leap = (year % 4 == 0 && year % 100 != 0) || year % 400 == 0;
return leap ? 366 : 365;
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}
static double date_orbit_angle(struct tm *tm) {
return 2 * M_PI / (double)days_in_year(tm->tm_year + 1900) * tm->tm_yday;
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}
static double equation_of_time(double orbit_angle) {
// https://www.esrl.noaa.gov/gmd/grad/solcalc/solareqns.PDF
return 4 * (0.000075 +
0.001868 * cos(orbit_angle) -
0.032077 * sin(orbit_angle) -
0.014615 * cos(2*orbit_angle) -
0.040849 * sin(2*orbit_angle));
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}
static double sun_declination(double orbit_angle) {
// https://www.esrl.noaa.gov/gmd/grad/solcalc/solareqns.PDF
return 0.006918 -
0.399912 * cos(orbit_angle) +
0.070257 * sin(orbit_angle) -
0.006758 * cos(2*orbit_angle) +
0.000907 * sin(2*orbit_angle) -
0.002697 * cos(3*orbit_angle) +
0.00148 * sin(3*orbit_angle);
}
static double sun_hour_angle(double latitude, double declination, double target_sun) {
// https://www.esrl.noaa.gov/gmd/grad/solcalc/solareqns.PDF
return acos(cos(target_sun) /
cos(latitude) * cos(declination) -
tan(latitude) * tan(declination));
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}
static time_t hour_angle_to_time(double hour_angle, double eqtime) {
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// https://www.esrl.noaa.gov/gmd/grad/solcalc/solareqns.PDF
return DEGREES((4.0 * M_PI - 4 * hour_angle - eqtime) * 60);
}
static enum sun_condition condition(double latitude_rad, double sun_declination) {
int sign_lat = signbit(latitude_rad) == 0;
int sign_decl = signbit(sun_declination) == 0;
return sign_lat == sign_decl ? MIDNIGHT_SUN : POLAR_NIGHT;
}
enum sun_condition calc_sun(struct tm *tm, double latitude, struct sun *sun) {
double orbit_angle = date_orbit_angle(tm);
double decl = sun_declination(orbit_angle);
double eqtime = equation_of_time(orbit_angle);
double ha_twilight = sun_hour_angle(latitude, decl, SOLAR_START_TWILIGHT);
double ha_daylight = sun_hour_angle(latitude, decl, SOLAR_END_TWILIGHT);
sun->dawn = hour_angle_to_time(fabs(ha_twilight), eqtime);
sun->dusk = hour_angle_to_time(-fabs(ha_twilight), eqtime);
sun->sunrise = hour_angle_to_time(fabs(ha_daylight), eqtime);
sun->sunset = hour_angle_to_time(-fabs(ha_daylight), eqtime);
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return isnan(ha_twilight) || isnan(ha_daylight) ?
condition(latitude, decl) : NORMAL;
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}
/*
* Illuminant D, or daylight locus, is is a "standard illuminant" used to
* describe natural daylight. It is on this locus that D65, the whitepoint used
* by most monitors and assumed by wlsunset, is defined.
*
* This approximation is strictly speaking only well-defined between 4000K and
* 25000K, but we stretch it a bit further down for transition purposes.
*/
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static int illuminant_d(int temp, double *x, double *y) {
// https://en.wikipedia.org/wiki/Standard_illuminant#Illuminant_series_D
if (temp >= 2500 && temp <= 7000) {
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*x = 0.244063 +
0.09911e3 / temp +
2.9678e6 / pow(temp, 2) -
4.6070e9 / pow(temp, 3);
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} else if (temp > 7000 && temp <= 25000) {
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*x = 0.237040 +
0.24748e3 / temp +
1.9018e6 / pow(temp, 2) -
2.0064e9 / pow(temp, 3);
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} else {
errno = EINVAL;
return -1;
}
*y = (-3 * pow(*x, 2)) + (2.870 * (*x)) - 0.275;
return 0;
}
/*
* Planckian locus, or black body locus, describes the color of a black body at
* a certain temperatures. This is not entirely equivalent to daylight due to
* atmospheric effects.
*
* This approximation is only valid from 1667K to 25000K.
*/
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static int planckian_locus(int temp, double *x, double *y) {
// https://en.wikipedia.org/wiki/Planckian_locus#Approximation
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if (temp >= 1667 && temp <= 4000) {
*x = -0.2661239e9 / pow(temp, 3) -
0.2343589e6 / pow(temp, 2) +
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0.8776956e3 / temp +
0.179910;
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if (temp <= 2222) {
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*y = -1.1064814 * pow(*x, 3) -
1.34811020 * pow(*x, 2) +
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2.18555832 * (*x) -
0.20219683;
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} else {
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*y = -0.9549476 * pow(*x, 3) -
1.37418593 * pow(*x, 2) +
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2.09137015 * (*x) -
0.16748867;
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}
} else if (temp > 4000 && temp < 25000) {
*x = -3.0258469e9 / pow(temp, 3) +
2.1070379e6 / pow(temp, 2) +
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0.2226347e3 / temp +
0.240390;
*y = 3.0817580 * pow(*x, 3) -
5.87338670 * pow(*x, 2) +
3.75112997 * (*x) -
0.37001483;
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} 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;
}
}
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static double clamp(double value) {
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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 >= 25000) {
illuminant_d(25000, &x, &y);
} else if (temp >= 4000) {
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illuminant_d(temp, &x, &y);
} else if (temp >= 2500) {
double x1, y1, x2, y2;
illuminant_d(temp, &x1, &y1);
planckian_locus(temp, &x2, &y2);
double factor = (4000 - temp) / 1500;
double sinefactor = (cos(M_PI*factor) + 1.0) / 2.0;
x = x1 * sinefactor + x2 * (1.0 - sinefactor);
y = y1 * sinefactor + y2 * (1.0 - sinefactor);
} else if (temp >= 1667) {
planckian_locus(temp, &x, &y);
} else {
planckian_locus(1667, &x, &y);
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}
double z = 1.0 - x - y;
xyz_to_srgb(x, y, z, rw, gw, bw);
srgb_normalize(rw, gw, bw);
}