Built SDL2_image and _mixer static

libsdl2_image/external/libpng-1.6.37/contrib/tools/sRGB.h

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+/*-
+ * sRGB.h
+ *
+ * Last changed in libpng 1.6.0 [February 14, 2013]
+ *
+ * COPYRIGHT: Written by John Cunningham Bowler, 2013.
+ * To the extent possible under law, the author has waived all copyright and
+ * related or neighboring rights to this work.  This work is published from:
+ * United States.
+ *
+ * Utility file; not actually a header, this contains definitions of sRGB
+ * calculation functions for inclusion in those test programs that need them.
+ *
+ * All routines take and return a floating point value in the range
+ * 0 to 1.0, doing a calculation according to the sRGB specification
+ * (in fact the source of the numbers is the wikipedia article at
+ * https://en.wikipedia.org/wiki/SRGB).
+ */
+static double
+sRGB_from_linear(double l)
+{
+   if (l <= 0.0031308)
+      l *= 12.92;
+
+   else
+      l = 1.055 * pow(l, 1/2.4) - 0.055;
+
+   return l;
+}
+
+static double
+linear_from_sRGB(double s)
+{
+   if (s <= 0.04045)
+      return s / 12.92;
+
+   else
+      return pow((s+0.055)/1.055, 2.4);
+}
+
+static double
+YfromRGB(double r, double g, double b)
+{
+   /* Use the sRGB (rounded) coefficients for Rlinear, Glinear, Blinear to get
+    * the CIE Y value (also linear).
+    */
+   return 0.2126 * r + 0.7152 * g + 0.0722 * b;
+}