利用橡皮筋技术画正圆,该怎么解决

利用橡皮筋技术画正圆
如题，如何利用橡皮筋技术画正圆。
画椭圆的代码如下：

C/C++ code

m_dcMem.SetROP2(R2_NOTXORPEN);m_dcMem.Ellipse(CRect(m_srcPoint,m_destPoint));m_dcMem.Ellipse(CRect(m_srcPoint,point));m_destPoint = point;Invalidate();

C/C++ code

int dx = point.x - m_srcPoint.x;int dy = point.y - m_srcPoint.y;int r = sqrt( dx*dx + dy*dy );m_dcMem.SetROP2(R2_NOTXORPEN);m_dcMem.Ellipse(m_srcPoint.x - r, m_srcPoint.y - r, m_srcPoint.x + r, m_srcPoint.y + r);Invalidate();