static float horizontalAutoSplit_threshold = 0.99f;

static L<Rect> horizontalAutoSplit(BWImage img) {
  ret horizontalAutoSplit(img, horizontalAutoSplit_threshold);
}

static L<Rect> horizontalAutoSplit(BWImage img, float threshold) {
  int w = img.getWidth(), h = img.getHeight();
  float[] f = bwImageColumnBrightnesses(img);
  
  new L<Rect> l;
  int j = -1;
  for (int i = 0; i < l(f); i++)
    if (f[i] < threshold) { // start / continue streak
      if (j < 0) j = i; 
    } else {
      if (j >= 0) { // end streak
        l.add(new Rect(j, 0, i-j, h));
        j = -1;
      }
    }
  if (j >= 0)
    l.add(new Rect(j, 0, w-j, h));
  ret l;
}

Began life as a copy of #1005703