osg简单渐变动画物体的淡入淡出

#include <osg/BlendFunc>
#include <osg/Geode>
#include <osg/ShapeDrawable>
#include <osgAnimation/EaseMotion>
#include <osgDB/ReadFile>
#include <osgViewer/Viewer>
class FadeCallback:public osg::NodeCallback
{
public:
    FadeCallback()
    {
        _motion=new osgAnimation::InOutCubicMotion;
    }
    virtual void operator()(osg::Node* node,osg::NodeVisitor* nv)
    {
        osg::Geode* geode=dynamic_cast<osg::Geode*>(node);
        if (geode)
        {
            osg::ShapeDrawable* shape=dynamic_cast<osg::ShapeDrawable*>(geode->getDrawable(0));
            if (shape)
            {
                _motion->update(0.005);
                float alpha=_motion->getValue();
                if (alpha>1.0)
                {
                    alpha=2.0-alpha;
                }
                shape->setColor(osg::Vec4(1.0,1.0,1.0,alpha));
            }
        }
        traverse(node,nv);
    }
protected:
    osg::ref_ptr<osgAnimation::InOutCubicMotion> _motion;
};
int main()
{
    osg::ref_ptr<osg::ShapeDrawable> shape=new osg::ShapeDrawable(new osg::Box(osg::Vec3(0.0,0.0,0.0),1.0));

    shape->setColor(osg::Vec4(1.0,1.0,1.0,1.0));
    shape->setSupportsDisplayList(false);

    osg::ref_ptr<osg::Geode> geode=new osg::Geode;
    geode->addDrawable(shape.get());
    geode->setUpdateCallback(new FadeCallback);
    geode->getOrCreateStateSet()->setAttributeAndModes(new osg::BlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA));
    osgViewer::Viewer viewer;
    viewer.setSceneData(geode.get());

    return viewer.run(); 
}