getViewMatrixAsLookAt的问题

widowss

各位好。新人遇到一个困惑的问题

//将参数设置给相机，并立即获取相机参数  
viewer.setCameraManipulator(NULL);
viewer.getCamera()->setViewMatrixAsLookAt(eye,center,up);
viewer.getCamera()->getViewMatrixAsLookAt(eye,center,up);  

//将各参数打印出来  
    printf("eye: %f,%f,%f new\n",eye._v[0],eye._v[1],eye._v[2]);  
    printf("center: %f,%f,%f new\n",center._v[0],center._v[1],center._v[2]);  
    printf("up: %f,%f,%f new\n",up._v[0],up._v[1],up._v[2]);  

问题是，getViewMatrixAsLookAt修改了我eye,center,up参数值，我f5调试跟，发现是在这个方法里面
void Matrix_implementation::getLookAt(Vec3d& eye,Vec3d& center,Vec3d& up,value_type lookDistance) const
{
    Matrix_implementation inv;
    inv.invert(*this);

    // note: e and c variables must be used inside this method instead of eye and center
    // because eye and center are references and they may point to the same variable.
    Vec3d e = osg::Vec3d(0.0,0.0,0.0)*inv;
    up = transform3x3(*this,osg::Vec3d(0.0,1.0,0.0));
    Vec3d c = transform3x3(*this,osg::Vec3d(0.0,0.0,-1));
    c.normalize();
    c = e + c*lookDistance;

    // assign the results
    eye = e;
    center = c;
}

不是很懂原理，请大侠指点。