How to get display extent in real time

osgEarth::GeoExtent getExtent(osgViewer::View* view)
{
    // Get the corners of all points on the view frustum.  Mostly modified from osgthirdpersonview
    osg::Matrixd proj = view->getCamera()->getProjectionMatrix();
    osg::Matrixd mv = view->getCamera()->getViewMatrix();
    osg::Matrixd invmv = osg::Matrixd::inverse( mv );

    double nearPlane = proj(3,2) / (proj(2,2)-1.0);
    double farPlane = proj(3,2) / (1.0+proj(2,2));

    // Get the sides of the near plane.
    double nLeft = nearPlane * (proj(2,0)-1.0) / proj(0,0);
    double nRight = nearPlane * (1.0+proj(2,0)) / proj(0,0);
    double nTop = nearPlane * (1.0+proj(2,1)) / proj(1,1);
    double nBottom = nearPlane * (proj(2,1)-1.0) / proj(1,1);

    // Get the sides of the far plane.
    double fLeft = farPlane * (proj(2,0)-1.0) / proj(0,0);
    double fRight = farPlane * (1.0+proj(2,0)) / proj(0,0);
    double fTop = farPlane * (1.0+proj(2,1)) / proj(1,1);
    double fBottom = farPlane * (proj(2,1)-1.0) / proj(1,1);

    double dist = farPlane - nearPlane;

    std::vector< osg::Vec3d > verts;
    verts.reserve(9);


    // Include origin?
    //verts.push_back(osg::Vec3d(0., 0., 0. ));
    verts.push_back(osg::Vec3d( nLeft, nBottom, -nearPlane ));
    verts.push_back(osg::Vec3d( nRight, nBottom, -nearPlane ));
    verts.push_back(osg::Vec3d( nRight, nTop, -nearPlane ));
    verts.push_back(osg::Vec3d( nLeft, nTop, -nearPlane ));
    verts.push_back(osg::Vec3d( fLeft, fBottom, -farPlane ));
    verts.push_back(osg::Vec3d( fRight, fBottom, -farPlane ));
    verts.push_back(osg::Vec3d( fRight, fTop, -farPlane ));
    verts.push_back(osg::Vec3d( fLeft, fTop, -farPlane ));

    const osgEarth::SpatialReference* srs = osgEarth::SpatialReference::create("epsg:4326");

    // Compute the bounding sphere of the frustum.
    osg::BoundingSphered bs;
    for (unsigned int i = 0; i < verts.size(); i++)
    {
        osg::Vec3d world = verts[i] * invmv;
        bs.expandBy( world );
    }
 
    // Get the center of the bounding sphere
    osgEarth::GeoPoint center;
    center.fromWorld(srs, bs.center());

    double radiusDegrees = bs.radius() /= 111000.0;
    double minLon = center.x() - radiusDegrees;
    double minLat = osg::clampAbove(center.y() - radiusDegrees, -90.0);
    double maxLon = center.x() + radiusDegrees;
    double maxLat = osg::clampBelow(center.y() + radiusDegrees, 90.0);

    osgEarth::GeoExtent extent(srs, minLon, minLat, maxLon, maxLat);
    extent.normalize();

    return extent;
}
the result is not correct