希望大家收藏：
本文更新地址：https://haoqchen.site/2018/05/07/understanding-of-message_filters/
左側專欄還在更新其他ROS實用技巧哦，關注一波？
0. 寫在最前面
因為日常看代碼經常能看到tf相關的一些函數，轉來轉去，繞得很暈，有不懂的就仔細查一下，將自己的理解整理出來，這篇是關於 tf::MessageFilter的。
message_filters，顧名思義是消息過濾器；tf::MessageFilter，顧名思義是tf下的消息過濾器。消息過濾器為什么要用tf呢？tf::MessageFilter可以訂閱任何的ROS消息，然后將其緩存，直到這些消息可以轉換到目標坐標系，然后進行相應的處理（一般在回調函數中處理）。說白了就是消息訂閱+坐標轉換。實際上，后者繼承於前者：
下面給出三個在ROS包中看到的例子：
示例一（amcl中激光雷達的回調）：
tf_ = new TransformListenerWrapper();
message_filters::Subscriber<sensor_msgs::LaserScan>* laser_scan_sub_;
tf::MessageFilter<sensor_msgs::LaserScan>* laser_scan_filter_;
laser_scan_sub_ = new message_filters::Subscriber<sensor_msgs::LaserScan>(nh_, scan_topic_, 100);
laser_scan_filter_ = new tf::MessageFilter<sensor_msgs::LaserScan>(*laser_scan_sub_,
*tf_,
odom_frame_id_,
100);
laser_scan_filter_->registerCallback(boost::bind(&AmclNode::laserReceived, this, _1));
void AmclNode::laserReceived(const sensor_msgs::LaserScanConstPtr& laser_scan){
this->tf_->transformPose(base_frame_id_, ident, laser_pose);//這個函數的意思是，ident在base_frame_id下的表示為laser_pose
}示例二（leg_detector中激光雷達的回調）：
TransformListener tfl_;
message_filters::Subscriber<sensor_msgs::LaserScan> laser_sub_;
tf::MessageFilter<sensor_msgs::LaserScan> laser_notifier_;
laser_sub_(nh_, "scan", 10)
laser_notifier_(laser_sub_, tfl_, fixed_frame, 10)
laser_notifier_.registerCallback(boost::bind(&LegDetector::laserCallback, this, _1))
laser_notifier_.setTolerance(ros::Duration(0.01));
void laserCallback(const sensor_msgs::LaserScan::ConstPtr& scan){
tfl_.transformPoint(fixed_frame, loc, loc);
}示例三（參考一中的示例）：
class PoseDrawer
{
public:
PoseDrawer() : tf_(), target_frame_("turtle1")
{
point_sub_.subscribe(n_, "turtle_point_stamped", 10);
tf_filter_ = new tf::MessageFilter<geometry_msgs::PointStamped>(point_sub_, tf_, target_frame_, 10);
tf_filter_->registerCallback( boost::bind(&PoseDrawer::msgCallback, this, _1) );
} ;
private:
message_filters::Subscriber<geometry_msgs::PointStamped> point_sub_;
tf::TransformListener tf_;
tf::MessageFilter<geometry_msgs::PointStamped> * tf_filter_;
ros::NodeHandle n_;
std::string target_frame_;
// Callback to register with tf::MessageFilter to be called when transforms are available
void msgCallback(const boost::shared_ptr<const geometry_msgs::PointStamped>& point_ptr)
{
geometry_msgs::PointStamped point_out;
try
{
tf_.transformPoint(target_frame_, *point_ptr, point_out);
}
catch (tf::TransformException &ex)
{
printf ("Failure %s\n", ex.what()); //Print exception which was caught
}
};
};
int main(int argc, char ** argv)
{
ros::init(argc, argv, "pose_drawer"); //Init ROS
PoseDrawer pd; //Construct class
ros::spin(); // Run until interupted
};以上的程序都需要添加以下頭文件：
#include "ros/ros.h"
#include "tf/transform_listener.h"
#include "tf/message_filter.h"
#include "message_filters/subscriber.h"可以看到示例一、二、三結構都是差不多：
- 定義數據：TransformListener、message_filters::Subscriber、tf::MessageFilter
- 用消息的名稱來初始化message_filters::Subscriber
- 用tf、message_filters::Subscriber、目標坐標系來初始化tf::MessageFilter
- 給tf::MessageFilter注冊callback。
- 編寫callback，並在回調中完成坐標轉換。至此完成消息訂閱+坐標轉換
在看message_filters的主頁過程中發現，它可以做的遠比以上說的多，比如：
An example is the time synchronizer, which takes in messages of different types from multiple sources, and outputs them only if it has received a message on each of those sources with the same timestamp.
有興趣的自己可以看看。