osg中实现如果AutoCAD中左下角的坐标轴

class HUDAxis:public Camera
{
public:
HUDAxis();
HUDAxis(HUDAxis const& copy, CopyOp copyOp = CopyOp::SHALLOW_COPY);
META_Node(osg, HUDAxis);
inline  void setMainCamera(Camera* camera){ _mainCamera = camera;}
virtual void traverse(NodeVisitor& nv);
protected:
virtual ~HUDAxis();
observer_ptr<Camera>       _mainCamera;
};
/////////////////////////////////////cpp///////////////////////////////////////////////////

HUDAxis::HUDAxis()
{
//可以在这直接读取axes.osgt;
//  this->addChild(osgDB::readNodeFile("axes.osgt"));
}

HUDAxis::HUDAxis(HUDAxis const& copy, CopyOp copyOp /* = CopyOp::SHALLOW_COPY */):Camera(copy, copyOp),
_mainCamera(copy._mainCamera)
{

}

void HUDAxis::traverse(NodeVisitor& nv)
{
double fovy, aspectRatio, vNear, vFar;
_mainCamera->getProjectionMatrixAsPerspective(fovy, aspectRatio, vNear, vFar);
this->setprojectionMatrixAsOrtho(-10.0*aspectRatio, 10.0*aspectRatio, -10.0, 10.0, 2.0, -2.0);

Vec3 trans(8.5*aspectRatio, -8.5, -8.0);
if(_mainCamera.valid() && nv.getVisitorType() == NodeVisitor::CULL_VISITOR)
{
Matrix matrix =  _mainCamera->getViewMatrix();
matrix.setTrans(trans);
this->setViewMatrix(matrix);
}//if
osg::Camera::traverse(nv);
}

HUDAxis::~HUDAxis()
{

}

ref_ptr<HUDAxis> hudAxes = new HUDAxis;
ref_ptr(Node)  _axes = osgDB::readNodeFile("axes.osgt");
hudAxes->addChild(_axes);
hudAxes->setMainCamera(camera);
hudAxes->setRenderOrder(Camera:OST_RENDER);
hudAxes->setClearMask(GL_DEPTH_BUFFER_BIT);
hudAxes->setAllowEventFocus(false);
hudAxes->setReferenceFrame(Transform::ABSOLUTE_RF);
scene->asGroup()->addChild(hudAxes);