public void grow(double dw, double dh)
    {
        if (x1 <= x2)
        {
            x1 -= dw;
            x2 += dw;
        }
        else
        {
            x1 += dw;
            x2 -= dw;
        }
        if (y1 <= y2)
        {
            y1 -= dh;
            y2 += dh;
        }
        else
        {
            y1 += dh;
            y2 -= dh;
        }
            
        Canvas.getInstance().repaint();
    }