/**
* Copyright © 2016 Mattias Andrée <maandree@member.fsf.org>
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
#include "blackbody.h"
#include <unistd.h>
#include <math.h>
#include <errno.h>
/**
* The highest colour temperature in the table.
*/
#define HIGHEST 40000
/**
* The lowest colour temperature in the table.
*/
#define LOWEST 1000
/**
* The temperature difference between the colours in the table.
*/
#define DELTA 100
/**
* Convert from CIE xyY to [0, 1] sRGB.
*
* @param x The 'x' component.
* @param y The 'y' component.
* @param Y The 'Y' component.
* @param r Output parameter for the “red” value.
* (Seriously, sRGB red is orange, just look at it fullscreen.)
* @param g Output parameter for the green value.
* @param b Output parameter for the blue value.
*/
static void
ciexyy_to_srgb(double x, double y, double Y, double *r, double *g, double *b)
{
#define SRGB(C) (((C) <= 0.0031308) ? (12.92 * (C)) : ((1.0 + 0.055) * pow((C), 1.0 / 2.4) - 0.055))
double X, Z;
/* Convert CIE xyY to CIE XYZ. */
X = Y * (y == 0 ? 0 : (x / y));
Z = Y * (y == 0 ? 0 : ((1 - x - y) / y));
/* Convert CIE XYZ to [0, 1] linear RGB. (ciexyz_to_linear) */
*r = ( 3.240450 * X) + (-1.537140 * Y) + (-0.4985320 * Z);
*g = (-0.969266 * X) + ( 1.876010 * Y) + ( 0.0415561 * Z);
*b = (0.0556434 * X) + (-0.204026 * Y) + ( 1.0572300 * Z);
/* Convert [0, 1] linear RGB to [0, 1] sRGB. */
SRGB(*r), SRGB(*g), SRGB(*b);
}
/**
* Perform linear interpolation (considered very good)
* between the CIE xyY values for two colour temperatures
* and convert the result to sRGB. The two colours should
* be the closest below the desired colour temperature,
* and the closest above the desired colour temperature.
*
* @param x1 The 'x' component for the low colour.
* @param y1 The 'y' component for the low colour.
* @param x2 The 'x' component for the high colour.
* @param y2 The 'y' component for the high colour.
* @param temp The desired colour temperature.
* @param r Output parameter for the “red” value.
* @param g Output parameter for the green value.
* @param b Output parameter for the blue value.
*/
static void
interpolate(double x1, double y1, double x2, double y2, double temp, double *r, double *g, double *b)
{
double weight = fmod(temp, (double)DELTA) / (double)DELTA;
double x = x1 * (1 - weight) + x2 * weight;
double y = y1 * (1 - weight) + y2 * weight;
ciexyy_to_srgb(x, y, 1.0, r, g, b);
}
/**
* Get the [0, 1] sRGB values of a colour temperature.
*
* @param fd File descriptor for the colour table.
* @param temp The desired colour temperature.
* @param r Output parameter for the “red” value.
* @param g Output parameter for the green value.
* @param b Output parameter for the blue value.
* @return 0 on succeess, -1 on error.
*
* @throws 0 The file did not have the expected size.
* @throws EDOM The selected temperature is below 1000 K.
*/
int
get_colour(int fd, long int temp, double *r, double *g, double *b)
{
double values[10]; /* low:x,y,r,g,b + high:x,y,r,g,b */
off_t offset;
double max;
/* We do not have any values for above 40 000 K, but
* the differences will be unnoticeable, perhaps even
* unencodeable. */
if (temp > HIGHEST) temp = HIGHEST;
/* Things do not glow below 1000 K. Yes, fire is hot! */
if (temp < LOWEST) return errno = EDOM, -1;
/* Read table. */
offset = ((off_t)temp - LOWEST) / DELTA;
offset *= (off_t)(5 * sizeof(double));
errno = 0;
if (pread(fd, values, sizeof(values), offset) < (ssize_t)sizeof(values))
return -1;
/* Get colour. */
if (temp % DELTA)
interpolate(values[0], values[1], values[6], values[7], (double)temp, r, g, b);
else
*r = values[2], *g = values[3], *b = values[4];
/* Adjust colours for use. */
max = fmax(fmax(fabs(*r), fabs(*g)), fabs(*b));
if (max != 0)
*r /= max, *g /= max, *b /= max;
*r = *r > 0.0 ? *r : 0.0;
*g = *g > 0.0 ? *g : 0.0;
*b = *b > 0.0 ? *b : 0.0;
return 0;
}
