用 GDI+ 画 Chord

GDI+并没有提供GDI中的Chord函数（其它的还有RoundRect函数，可参考BobPowell 的这篇文章），只好自己动手了：

BOOL GDIplusChord( HDC hDC, INT x1, INT y1, INT x2, INT y2, INT x3, INT y3, INT x4, INT y4 )
{
	Graphics graphics(hDC);
	CRect rectBound(x1, y1, x2, y2);
	rectBound.NormalizeRect();
	Rect ellipseRect(rectBound.left, rectBound.top, rectBound.Width(), rectBound.Height());

	////////////////////////////////////////////////////////////////////////////////
	// For testing only.
	BYTE		byAlpha = 200;
	SolidBrush	fillBrush(Color(byAlpha, 0, 0, 255));
	Pen			redPen(Color(byAlpha, 255, 0, 0), 3);
	////////////////////////////////////////////////////////////////////////////////
	Status ret = InvalidParameter;

	if ( x3 == x4 && y3 == y4 )
	{
		// If the starting point and ending point of the curve are the same, a complete ellipse should be drawn. 
		ret = graphics.FillEllipse(&fillBrush, ellipseRect);
		ret = graphics.DrawEllipse(&redPen, ellipseRect);
		return Gdiplus::Ok == ret;
	}

	CPoint ptCenter(rectBound.CenterPoint());

#define PI 3.1415926

	REAL startAngle = atan2(y3-ptCenter.y, x3-ptCenter.x ) * 180.0f / PI;
	REAL sweepAngle = (atan2(y4-ptCenter.y, x4-ptCenter.x ) * 180.0f / PI) - startAngle - 360.0f;
	if ( sweepAngle < -360.0f )
		sweepAngle += 360.0f;
	
	GraphicsPath path;
	path.StartFigure();
	path.AddArc(ellipseRect, startAngle, sweepAngle);
	path.CloseFigure();

	ret = graphics.FillPath(&fillBrush, &path);
	ret = graphics.DrawPath(&redPen, &path);
	
	return Gdiplus::Ok == ret;
}