ros之message_filter

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 &) , 回调函数的输入类型必须是ConstPtr类型的共享指针，当年找了这个错误找了一下午，谨记。

boost::bind()

• 如果callback()函数是全局函数，如上例所示：

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

注： _1, _2为占位符， 代表接收的参数， 在函数真正调用的时候传入真正的参数。占位符的名字知识表示它在调用式中的顺序， 最多可以绑定9个占位符。

• 如果callback()函数是类成员函数：

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

注：第二个参数this指针指向当前对象， 从而可以调用当前对象的成员函数，因为this指针占用了一个占位符，所以最多绑定8个参数