Faster gamma conversion

posterize.c

@@ -72,11 +72,18 @@ inline static unsigned int linear_to_int(const double value)
  return g < 255.0 ? g : 255;
 }
 
+static double image_gamma, gamma_lut[256];
+static void set_gamma(const double gamma)
+{
+ image_gamma = gamma;
+ for(int i=0; i < 256; i++) gamma_lut[i] = pow(int_to_linear(i), gamma);
+}
+
 // Converts gamma 2.2 to linear unit value. Linear color is required for preserving brightness (esp. when dithering).
-double image_gamma = 2.2;
+// call set_gamma first
 inline static double gamma_to_linear(unsigned int value)
 {
- return pow(int_to_linear(value), image_gamma);
+ return gamma_lut[value];
 }
 
 // Reverses gamma_to_linear.
@@ -493,7 +500,7 @@ int main(int argc, char *argv[])
  }
  if (input != stdin) fclose(input);
 
- image_gamma = 1.0/img.gamma;
+ set_gamma(1.0/img.gamma);
 
  posterize(&img, maxlevels, maxerror, dither, verbose);