static L<IntRange> findBrightHorizontalStreaks(BWImage img) {
  ret findBrightHorizontalStreaks(img, 0.5f);
}

static L<IntRange> findBrightHorizontalStreaks(BWImage img, float threshold) {
  new L<IntRange> out;
  int i = 0, n = img.getWidth();
  while (i < n) {
    int j = i;
    while (j < n && img.getPixel(j, 0) >= threshold) ++j;
    if (j > i)
      out.add(intRange(i, j));
    i = j+1;
  }
  ret out;
}

Began life as a copy of #1008769