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 * https://en.wikipedia.org/wiki/SRGB). 18 */ 19 static double 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 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 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
