persistable sclass ScanlineBitMatrix extends AbstractMatrix<Bool> is BitMatrix {
  int bx1 = -1, by1, bx2, by2; // bounding box of positive bits
  new IntBuffer scanlineData;
  new IntBuffer scanlineStarts;
  
  *(BitMatrix matrix) {
    super(matrix.getWidth(), matrix.getHeight());
    
    int ptr = 0;
    for y to h: {
      scanlineStarts.add(ptr));
    }
  }
  
  public Bool get(int x, int y) { ret points.contains(pt(x, y)); }
  public void set(int x, int y, Bool a) { points.add(pt(x, y)); }
  
  Rect boundingBoxOfTrueBits() { ret bx1 < 0 ? null : rect(bx1, by1, bx2, by2); }
}