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static BWImage bwAutoContrast(BWImage bw) {
  ret bwAutoContrast(bw, 0f);
}

// def value is usually irrelevant
static BWImage bwAutoContrast(BWImage bw, float def) {
  if (bw == null) null;
  int w = bw.getWidth(), h = bw.getHeight();
  float min = 1, max = 0;
  for y to h: for x to w: {
    float f = bw.getPixel(x, y);
    min = min(min, f);
    max = max(max, f);
  }
  if (min == max) ret new BWImage(w, h, def);
  if (min == 0 && max == 1) ret bw;
  float factor = floatRatio(1, max-min);
  BWImage bw2 = new BWImage(w, h);
  for y to h: for x to w:
    bw2.setPixel(x, y, (bw.getPixel(x, y)-min)*factor);
  ret bw2;
}