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
      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
      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
      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  }