ROS message_filters

ROS message_filters

参考地址：http://wiki.ros.org/message_filters

1.概述

message_filter是roscpp和rospy的一个应用库，他集中了一些滤波算法中常用的一些消息。当一个消息到来，在之后的一个时间点，该消息可能被返回或者不返回，将这样的一个过程或者容器理解为消息滤波器。

一个典型的例子就是时间同步器，他从多个数据源采集不同类型的数据，只有每个消息源的消息具体有相同的时间戳时候，他才将信息发布出去。

message_filters is a utility library for use with roscpp and rospy. It collects commonly used message "filtering" algorithms into a common space. 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.

2. 滤波模式

所有的消息滤波器服从相同的模式，用于连接输入和输出。输入通过滤波器的构造器或者connectInput()的方法连接。输出通过registerCallback()方法连接。

需要注意的是，每个滤波器各自定义输入输出的类型，所以并不是所有的滤波器可以直接互联。

For example, given two filters FooFilter and BarFilter where FooFilter's output is compatible with BarFilter's input, connecting foo to bar could be (in C++):

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   1 FooFilter foo;
   2 BarFilter bar(foo);

or:

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   1 FooFilter foo;
   2 BarFilter bar;
   3 bar.connectInput(foo);

2.1 registerCallback()

你可以采用registerCallback()方法，注册多个回调函数。他们将按照注册顺序进行函数的回调。

3 订阅器Subscriber

The  Subscriber  filter is simply a wrapper around a ROS subscription that provides a source for other filters. The Subscriber  filter cannot connect to another filter's output, instead it uses a ROS topic as its input.

3.1 连接

输入：没有输入连接

输出：void callback(const boost::shared_ptr<M const>&)

例子：

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   1 message_filters::Subscriber<std_msgs::UInt32> sub(nh, "my_topic", 1);
   2 sub.registerCallback(myCallback);

is the equivalent of:

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   1 ros::Subscriber sub = nh.subscribe("my_topic", 1, myCallback);

4. 时间一致器

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.

Connections

Input

• C++: Up to 9 separate filters, each of which is of the form void callback(const boost::shared_ptr<M const>&). The number of filters supported is determined by the number of template arguments the class was created with.

  Python: N separate filters, each of which has signature callback(msg).

Output

• C++: For message types M0..M8,void callback(const boost::shared_ptr<M0 const>&, ..., const boost::shared_ptr<M8 const>&). The number of parameters is determined by the number of template arguments the class was created with.

  Python: callback(msg0.. msgN). The number of parameters is determined by the number of template arguments the class was created with.

Example (C++)

Suppose you are writing a ROS node that needs to process data from two time synchronized topics. Your program will probably look something like this:

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   1 #include <message_filters/subscriber.h>
   2 #include <message_filters/time_synchronizer.h>
   3 #include <sensor_msgs/Image.h>
   4 #include <sensor_msgs/CameraInfo.h>
   5 
   6 using namespace sensor_msgs;
   7 using namespace message_filters;
   8 
   9 void callback(const ImageConstPtr& image, const CameraInfoConstPtr& cam_info)
  10 {
  11   // Solve all of perception here...
  12 }
  13 
  14 int main(int argc, char** argv)
  15 {
  16   ros::init(argc, argv, "vision_node");
  17 
  18   ros::NodeHandle nh;
  19 
  20   message_filters::Subscriber<Image> image_sub(nh, "image", 1);
  21   message_filters::Subscriber<CameraInfo> info_sub(nh, "camera_info", 1);
  22   TimeSynchronizer<Image, CameraInfo> sync(image_sub, info_sub, 10);
  23   sync.registerCallback(boost::bind(&callback, _1, _2));
  24 
  25   ros::spin();
  26 
  27   return 0;
  28 }

(Note: In this particular case you could use the CameraSubscriber class from image_transport, which essentially wraps the filtering code above.)