OpenGL：三维平面坐标转化为二维

相关联文章：openGL中视口变换矩阵求解

osg中世界坐标系到屏幕坐标系的变换

//返回三维点在二维屏幕上的投影点
osg::Vec3d WorldToScreen(osgViewer::View* view,osg::Vec3 worldpoint)
{
	double in[4],out[4];
	
	in[0] = worldpoint._v[0];
	in[1] = worldpoint._v[1];
	in[2] = worldpoint._v[2];
	in[3] = 1.0;
	//获得当前的投影矩阵和模型视图矩阵
	osg::Matrix projectMatrix = view->getCamera()->getProjectionMatrix();
	osg::Matrix viewMatrix = view->getCamera()->getViewMatrix();
	//变换模型视图矩阵
	double modelViewMatrix[16];
	memcpy(modelViewMatrix,viewprojectMatrix.ptr(),sizeof(GLdouble)*16);
	Transform_Point(out,modelViewMatrix,in);
	
	//变换投影矩阵
	double myprojectMatrix[16];
	memcpy(myprojectMatrix,projectMatrix.ptr(),sizeof(GLdouble)*16);
	Transform_Point(out,myprojectMatrix,in);
	
	//变换视口变换矩阵
	if(in[3] == 0.0)
	  return osg::Vec3d(0,0,0);
	  
	in[0]/=in[3];
	in[1]/=in[3];
	in[2]/=in[3];
	
	int viewPort[4];
	osg::Viewport* myviewport = view->getCamera()->getViewport();
	viewPort[0] = myviewport->x();
	viewPort[1] = myviewport->y();
	viewPort[2] = myviewport->width();
	viewPort[3] = myviewport->height();
	
	//计算二维屏幕投影点
	osg::Vec3d screenPoint;
	screenPoint._v[0] = (int)(viewPort[0] + (1+in[0])*viewPort[2]/2+0.5);
	screenPoint._v[0] = (int)(viewPort[1] + (1+in[1])*viewPort[3]/2+0.5);
	screenPoint._v[2] = 0;
	
	return screenPoint;
}

上面的程序只有最后计算二维屏幕投影点时，为啥要加上0.5，不懂！！