struct MyComputeRangeCallback : public osgEarth::ComputeRangeCallback

{

    virtual float operator()(osg::Node* node, osg::NodeVisitor& nv)

    {

        osgUtil::CullVisitor* cv = dynamic_cast<osgUtil::CullVisitor*>(&nv);

        if (cv)

        {

            // This code assumes that the range_mode is PIXEL_SIZE_ON_SCREEN and computes the pixel size on screen

            // without just using the vertical field of view.

            float distance = nv.getDistanceToViewPoint(node->getBound().center(),true);

            float radius = node->getBound().radius();

            double fov, ar, near, far;

            cv->getCurrentCamera()->getProjectionMatrixAsPerspective(fov, ar, near,far);

            osg::Viewport* viewPort = cv->getCurrentCamera()->getViewport();

            double angularSize = osg::RadiansToDegrees( 2.0*atan(radius/distance) );

            double dpp = osg::maximum(fov, 1.0e-17) / viewPort->height();

            float pixelSize = angularSize / dpp;

            return pixelSize;

        }

        // Fall back to the base calculation.

        return -1.0;

    }

};