1 /* sRGB.h
2 *
3 * COPYRIGHT: Written by John Cunningham Bowler, 2013.
4 * To the extent possible under law, the author has waived all copyright and
5 * related or neighboring rights to this work. This work is published from:
6 * United States.
7 *
8 * Utility file; not actually a header, this contains definitions of sRGB
9 * calculation functions for inclusion in those test programs that need them.
10 *
11 * All routines take and return a floating point value in the range
12 * 0 to 1.0, doing a calculation according to the sRGB specification
13 * (in fact the source of the numbers is the wikipedia article at
14 * https://en.wikipedia.org/wiki/SRGB).
15 */
16
17 static double
sRGB_from_linear(double l)18 sRGB_from_linear(double l)
19 {
20 if (l <= 0.0031308)
21 l *= 12.92;
22
23 else
24 l = 1.055 * pow(l, 1/2.4) - 0.055;
25
26 return l;
27 }
28
29 static double
linear_from_sRGB(double s)30 linear_from_sRGB(double s)
31 {
32 if (s <= 0.04045)
33 return s / 12.92;
34
35 else
36 return pow((s+0.055)/1.055, 2.4);
37 }
38
39 static double
YfromRGB(double r,double g,double b)40 YfromRGB(double r, double g, double b)
41 {
42 /* Use the sRGB (rounded) coefficients for Rlinear, Glinear, Blinear to get
43 * the CIE Y value (also linear).
44 */
45 return 0.2126 * r + 0.7152 * g + 0.0722 * b;
46 }
47