怎样在osg中动态的设置drawable的最近最远裁剪面

// draw callback that will tweak the far clipping plane just
    // before rendering a drawable.
    struct OverrideNearFarValuesCallback : public osg::Drawable::DrawCallback
    {
        OverrideNearFarValuesCallback(double radius)
            : _radius(radius) {}

virtual void drawImplementation(osg::RenderInfo& renderInfo,
            const osg::Drawable* drawable) const
        {
            osg::Camera* currentCamera = renderInfo.getCurrentCamera();
            if (currentCamera)
            {
                // Get the current camera position.
                osg::Vec3 eye, center, up;
                renderInfo.getCurrentCamera()->getViewMatrixAsLookAt( eye, center, up);

// Get the max distance we need the far plane to be at,
                // which is the distance between the eye and the origin
                // plus the distant from the origin to the object (star sphere
                // radius, sun distance etc), and then some.
                double distance = eye.length() + _radius*2;

// Save old values.
                osg::ref_ptr<osg::RefMatrixd> oldProjectionMatrix = new osg::RefMatrix;
                oldProjectionMatrix->set( renderInfo.getState()->getProjectionMatrix());

// Get the individual values
                double left, right, bottom, top, zNear, zFar;
                oldProjectionMatrix->getFrustum( left, right, bottom, top, zNear, zFar);

// Build a new projection matrix with a modified far plane
                osg::ref_ptr<osg::RefMatrixd> projectionMatrix = new osg::RefMatrix;
                projectionMatrix->makeFrustum( left, right, bottom, top, zNear, distance );

renderInfo.getState()->applyProjectionMatrix( projectionMatrix.get());

// Draw the drawable
                drawable->drawImplementation(renderInfo);

// Reset the far plane to the old value.
                renderInfo.getState()->applyProjectionMatrix( oldProjectionMatrix.get() );
            }
            else
            {
                drawable->drawImplementation(renderInfo);
            }
        }

double _radius;
    };

struct AddCallbackToDrawablesVisitor : public osg::NodeVisitor
    {
        AddCallbackToDrawablesVisitor(double radius)
            : osg::NodeVisitor(osg::NodeVisitor::TRAVERSE_ALL_CHILDREN),
            _radius(radius) {}

virtual void apply(osg::Geode& node)
        {
            for (unsigned int i = 0; i < node.getNumDrawables(); i++)
            {
                node.getDrawable(i)->setDrawCallback( new OverrideNearFarValuesCallback(_radius) );

// Do not use display lists otherwise the callback will only
                // be called once on initial compile.
                node.getDrawable(i)->setUseDisplayList(false);
            }
        }

double _radius;
    };

调用

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AddCallbackToDrawablesVisitor visitor( mpr_innerRadius );
 geode->accept( visitor );

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