Fast formula for a "high contrast" curve.

Answer 1

Trivially you could simply threshold, but I imagine this is too dumb:

return i < 0.5 ? 0.0 : 1.0;

Since you mention 'increasing contrast' I assume the input values are luminance values. If so, and they are discrete (perhaps it's an 8-bit value), you could use a lookup table to do this quite quickly.

Your 'mulContrastize' looks reasonably quick. One optimization would be to use integer math. Let's say, again, your input values could actually be passed as an 8-bit unsigned value in [0..255]. (Again, possibly a fine assumption?) You could do something roughly like...

int mulContrastize(int i) {
  if (i < 128) return (i * i) >> 7; 
  // The shift is really: * 2 / 256
  i = 255 - i;
  return 255 - ((i * i) >> 7);