// contrast=1 => original
// contrast=2 => twice as contrasty
// contrast=.5 => half as contrasty
static BWImage bwContrast(double contrast, BWImage bw) {
  if (contrast == 1 || bw == null) ret bw;
  int w = bw.getWidth(), h = bw.getHeight();
  BWImage bw2 = new BWImage(w, h);
  for y to h: for x to w:
    bw2.setPixel(x, y, (float) (0.5+(bw.getPixel(x, y)-0.5)*contrast));
  ret bw2;
}