ros之message_filter

A message filter is defined as something which a message arrives into and may or may not be spit back out of at a later point in time. 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.

Time Synchronizer

The TimeSynchronizer filter synchronizes incoming channels by the timestamps contained in their headers, and outputs them in the form of a single callback that takes the same number of channels. The C++ implementation can synchronize up to 9 channels.

示例：

#include <message_filters/subscriber.h>
#include <message_filters/time_synchronizer.h>

#include <sensor_msgs/Image.h>
#include <sensor_msgs/CameraInfo.h>

using namespace message_filters;

void callback(const sensor_msgs::ImageConstPtr& image, const sensor_msgs::CameraInfoConstPtr& cam_info)
{
  // Solve all of perception here...
}

int main(int argc, char** argv)
{
  ros::init(argc, argv, "vision_node");

  ros::NodeHandle nh;

  message_filters::Subscriber<sensor_msgs::Image> image_sub(nh, "image", 1);
  message_filters::Subscriber<sensor_msgs::CameraInfo> info_sub(nh, "camera_info", 1);
  TimeSynchronizer<sensor_msgs::Image, sensor_msgs::CameraInfo> sync(image_sub, info_sub, 10);
  sync.registerCallback(boost::bind(&callback, _1, _2));

  ros::spin();
  return 0;
}
 

ExactTime Policy

The message_filters::sync_policies::ExactTime policy requires messages to have exactly the same timestamp in order to match. Your callback is only called if a message has been received on all specified channels with the same exact timestamp. The timestamp is read from the header field of all messages (which is required for this policy).

#include <message_filters/subscriber.h>
#include <message_filters/synchronizer.h>
#include <message_filters/sync_policies/exact_time.h>
#include <sensor_msgs/Image.h>
#include <sensor_msgs/CameraInfo.h>

using namespace sensor_msgs;
using namespace message_filters;

void callback(const ImageConstPtr& image, const CameraInfoConstPtr& cam_info)
{
  // Solve all of perception here...
}

int main(int argc, char** argv)
{
  ros::init(argc, argv, "vision_node");

  ros::NodeHandle nh;
  message_filters::Subscriber<Image> image_sub(nh, "image", 1);
  message_filters::Subscriber<CameraInfo> info_sub(nh, "camera_info", 1);

  typedef sync_policies::ExactTime<Image, CameraInfo> MySyncPolicy;
  // ExactTime takes a queue size as its constructor argument, hence MySyncPolicy(10)
  Synchronizer<MySyncPolicy> sync(MySyncPolicy(10), image_sub, info_sub);
  sync.registerCallback(boost::bind(&callback, _1, _2));

  ros::spin();
  return 0;
}

ApproximateTime Policy

The message_filters::sync_policies::ApproximateTime policy uses an adaptive algorithm to match messages based on their timestamp.

#include <message_filters/subscriber.h>
#include <message_filters/synchronizer.h>
#include <message_filters/sync_policies/approximate_time.h>

#include <sensor_msgs/Image.h>

void callback(const sensor_msgs::ImageConstPtr& image1, const sensor_msgs::ImageConstPtr& image2)
{
  // Solve all of perception here...
}

int main(int argc, char** argv)
{
  ros::init(argc, argv, "vision_node");

  ros::NodeHandle nh;
  message_filters::Subscriber<sensor_msgs::Image> image1_sub(nh, "image1", 1);
  message_filters::Subscriber<sensor_msgs::Image> image2_sub(nh, "image2", 1);

  typedef message_filters::sync_policies::ApproximateTime<sensor_msgs::Image, sensor_msgs::Image> MySyncPolicy;
  // ApproximateTime takes a queue size as its constructor argument, hence MySyncPolicy(10)
  message_filters::Synchronizer<MySyncPolicy> sync(MySyncPolicy(10), image1_sub, image2_sub);
  sync.registerCallback(boost::bind(&callback, _1, _2));

  ros::spin();
  return 0;
}

注： void callback(const boost::shared_ptr &, ..., const boost::shared_ptr &) , 回撥函式的輸入型別必須是ConstPtr型別的共享指標，當年找了這個錯誤找了一下午，謹記。

boost::bind()

• 如果callback()函式是全域性函式，如上例所示：

boost::bind(&callback, _1, _2)  

注： _1, _2為佔位符， 代表接收的引數， 在函式真正呼叫的時候傳入真正的引數。佔位符的名字知識表示它在呼叫式中的順序， 最多可以繫結9個佔位符。

• 如果callback()函式是類成員函式：

boost::bind(&X::back, this, _1, _2)

注：第二個引數this指標指向當前物件， 從而可以呼叫當前物件的成員函式，因為this指標佔用了一個佔位符，所以最多繫結8個引數