blackbody

Signed-off-by: Mattias Andrée <maandree@member.fsf.org>

1 files changed, 152 insertions, 0 deletions

diff --git a/src/blackbody.c b/src/blackbody.c
new file mode 100644
index 0000000..ca337ae
--- /dev/null
+++ b/src/blackbody.c
@@ -0,0 +1,152 @@
+/**
+ * Copyright © 2016  Mattias Andrée <maandree@member.fsf.org>
+ * 
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ * 
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ * 
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ */
+#include "blackbody.h"
+#include <unistd.h>
+#include <math.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 *g)
+{
+	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, 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 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) < 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;
+}
+