1 /*-
2 * sRGB.h
3 *
4 * Last changed in libpng 1.6.0 [February 14, 2013]
5 *
6 * COPYRIGHT: Written by John Cunningham Bowler, 2013.
7 * To the extent possible under law, the author has waived all copyright and
8 * related or neighboring rights to this work. This work is published from:
9 * United States.
10 *
11 * Utility file; not actually a header, this contains definitions of sRGB
12 * calculation functions for inclusion in those test programs that need them.
13 *
14 * All routines take and return a floating point value in the range
15 * 0 to 1.0, doing a calculation according to the sRGB specification
16 * (in fact the source of the numbers is the wikipedia article at
17 * http://en.wikipedia.org/wiki/SRGB).
18 */
19 static double
sRGB_from_linear(double l)20 sRGB_from_linear(double l)
21 {
22 if (l <= 0.0031308)
23 l *= 12.92;
24
25 else
26 l = 1.055 * pow(l, 1/2.4) - 0.055;
27
28 return l;
29 }
30
31 static double
linear_from_sRGB(double s)32 linear_from_sRGB(double s)
33 {
34 if (s <= 0.04045)
35 return s / 12.92;
36
37 else
38 return pow((s+0.055)/1.055, 2.4);
39 }
40
41 static double
YfromRGB(double r,double g,double b)42 YfromRGB(double r, double g, double b)
43 {
44 /* Use the sRGB (rounded) coefficients for Rlinear, Glinear, Blinear to get
45 * the CIE Y value (also linear).
46 */
47 return 0.2126 * r + 0.7152 * g + 0.0722 * b;
48 }
49