static L<IntRange> pixelSetBoundsByLine(PixelSet set) {
  Rect rect = pixelSetBounds(set);
  L<IntRange> l = repFunc(func { IntRange(rect.x+rect.width, rect.x), rect.h);
  for (Pixel p : set) {
    IntRange r = l.get(p.y-rect.y);
    int x = p.x-rect.x;
    r.start = min(x, r.start);
    r.end = max(x+1, r.end);
  }
  ret l;
}