ROS knowledge point - message_filters

Reference resources ：
1） Time synchronizer
https://blog.csdn.net/u012700322/article/details/51784412
https://blog.csdn.net/fana8010/article/details/23880067（ This article is the original of the previous one ）
2） Policy based synchronizer
https://blog.csdn.net/muyiyushan/article/details/118418040（ Official translation , The first 7 part ）
http://wiki.ros.org/message_filters#Policy-Based_Synchronizer_.5BROS_1.1.2B-.5D（ Official website , The first 7 part ）

The following is for Reference resources 1） The first article in ：

bug1：3.1 Connect

Two sentences are equivalent to one , But why separate it into two sentences ？ Because only one sentence can read one input and send it to the callback function , So you need to use two sentences , First, encapsulate multiple data with a separate sentence , Then use one sentence to pass multiple data to the callback function .
boost bind

bug2：4 Time synchronizer

The TimeSynchronizer filter synchronizes incoming channels by the timestamps contained in their headers

PointCloud2ROS Point cloud data message structure ：header There's a stamp, Compare different data sources stamp For time synchronization .

The following is for Reference resources 2） Explanation in ：

bug3： Yes “ own ” Code understanding

typedef message_filters::sync_policies::ApproximateTime<sensor_msgs::PointCloud2, sensor_msgs::PointCloud2> SyncPolicyT;
// Define strategy ,message_filters::sync_policies::ApproximateTime The strategy uses an adaptive algorithm based on timestamp to match messages , Match two point clouds 
void DualLidarCalib::start()
{
    
	  //  Subscriber  sub and pub 
	  parent_cloud_sub_ = new message_filters::Subscriber<sensor_msgs::PointCloud2>(nh_, parent_cloud_topic_, 10);
	  child_cloud_sub_ = new message_filters::Subscriber<sensor_msgs::PointCloud2>(nh_, child_cloud_topic_, 10);
	
	  // Point cloud synchronizer  ApproximateTime takes a queue size as its constructor argument, hence MySyncPolicy(100)
	  cloud_synchronizer_ = new message_filters::Synchronizer<SyncPolicyT>(SyncPolicyT(100), *parent_cloud_sub_, *child_cloud_sub_);
	  // Call callback function 
	  cloud_synchronizer_->registerCallback(boost::bind(&DualLidarCalib::cloudCallback, this, _1, _2));
	//registerCallback() Input is const boost::shared_ptr<M const>&
}
void DualLidarCalib::cloudCallback(const sensor_msgs::PointCloud2::ConstPtr& in_parent_cloud_msg,
                                   const sensor_msgs::PointCloud2::ConstPtr& in_child_cloud_msg)
{
    
	...
}

boost::bind() understand ：
1）https://blog.csdn.net/holyjw/article/details/89883363（ Just look “1. adopt functions and function pointers Use bind”）
2）https://blog.csdn.net/bj318318/article/details/116200356（ Just look “1 Introduce ”）