osg不要让视角穿到地底代码

以下是osg中避免视角穿过地底的示例代码：

#include <osgViewer/Viewer>
#include <osg/Node>
#include <osg/Geode>
#include <osg/ShapeDrawable>
#include <osg/PositionAttitudeTransform>
#include <osgDB/ReadFile>
#include <osg/MatrixTransform>
#include <osgGA/TrackballManipulator>
#include <osgUtil/CullVisitor>

int main(int argc, char** argv)
{
    osg::ref_ptr<osg::Node> scene = osgDB::readNodeFile("cow.osg");

    // 设置相机位置和朝向
    osg::Vec3d eye(-100.0, 0.0, 100.0);
    osg::Vec3d center(0.0, 0.0, 0.0);
    osg::Vec3d up(0.0, 0.0, 1.0);
    osg::ref_ptr<osgGA::TrackballManipulator> manipulator = new osgGA::TrackballManipulator();
    manipulator->setHomePosition(eye, center, up);
    osg::ref_ptr<osgViewer::Viewer> viewer = new osgViewer::Viewer();
    viewer->setCameraManipulator(manipulator);

    // 设置透视投影
    double aspectRatio = static_cast<double>(viewer->getCamera()->getViewport()->width()) / static_cast<double>(viewer->getCamera()->getViewport()->height());
    double nearDist = 0.1;
    double farDist = 10000.0;
    double fovY = 30.0;
    viewer->getCamera()->setProjectionMatrixAsPerspective(fovY, aspectRatio, nearDist, farDist);

    // 设置遮挡剔除
    osg::ref_ptr<osgUtil::CullVisitor> cullVisitor = new osgUtil::CullVisitor();
    cullVisitor->setCullingMode(osgUtil::CullVisitor::VIEW_FRUSTUM_SIDES_ONLY);
    viewer->getCamera()->setCullingMode(osgUtil::CullVisitor::VIEW_FRUSTUM_SIDES_ONLY);

    // 加载地形模型
    osg::ref_ptr<osg::Node> terrain = osgDB::readNodeFile("terrain.osg");
    osg::ref_ptr<osg::MatrixTransform> terrainTransform = new osg::MatrixTransform();
    terrainTransform->addChild(terrain);

    // 创建根节点
    osg::ref_ptr<osg::Group> root = new osg::Group();
    root->addChild(terrainTransform);
    root->addChild(scene);

    viewer->setSceneData(root);

    return viewer->run();
}

在这个示例代码中，我们设置相机的位置和朝向，同时设置透视投影和遮挡剔除，以避免视角穿过地底。同时，我们加载了地形模型，并将地形模型作为根节点的子节点，以实现地形的显示。