vs的opencv用ellipse函数画出来的椭圆是实心的

代码是
CBrush *brush = CBrush::FromHandle((HBRUSH)GetStockObject(NULL_BRUSH));
pDC->SelectObject(brush);

    memDC.Ellipse(ell.p1.x, ell.p1.y, ell.p2.x, ell.p2.y);

我也加过 pDC->SelectObject(brush);
没用

http://blog.csdn.net/gxiaob/article/details/9396955