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