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      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
     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