share/png/contrib/tools/sRGB.h

   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 }