void operator()(osg::Node* node, osg::NodeVisitor* nv) {
if(!_motion.valid()) return;
osg::MatrixTransform* mt = dynamic_cast<osg::MatrixTransform*>(node);
if(!mt) return;
double t = nv->getFrameStamp()->getSimulationTime();
// This avoids a little glitch when the animation doesn't start right away
// when the application is launched.
if(_previous == 0.0f) _previous = t;
_motion->update(t - _previous);
_previous = t;
mt->setMatrix(osg::Matrix::translate(_pos * _motion->getValue()));
}
