How to write pointcloud in .vtk file using libpointmatcher?

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Hi I am new in pcl and libpointmatcher. In PCL ,1st i subscribe the PointCloud2 msg from kinect and in callback i covert it into PointCloud using fromRosMsg(), then i save this msg in .pcd file using savePCDFile(). Its working. But my question is simply how can i store pointcloud in .vtk file using libpointmatcher library. rosMsgToPointMatcherCloud(const sensor_msgs::PointCloud2& rosMsg) This is the API's to convert ros msg to Pointmatcher. but i am confused how can i use in callback function?