How to pass arguments to/from subscriber callback functions

asked 2011-11-05 07:31:00 -0500

Paul0nc
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updated 2022-02-23 17:41:26 -0500

130s

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I'm currently subscribing to the depth image of a Kinect with the following code:

ros::Subscriber sub = nh.subscribe("/camera/depth/image", 3, myDepthViewer);

The callback function, myDepthViewer receives a sensor_msgs image:

void myDepthViewer( const sensor_msgs::ImageConstPtr& image )

So, my question is: how is myDepthViewer being passed the message if it does not appear in the call, and, how would I pass other arguments to it. I'd like to do some processing on the image and pass a result back to main.

Thanks for any advice. Paul.

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answered 2011-11-05 16:20:04 -0500

joq
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A boost shared pointer to the message does appear in the call. That is how you access the message. You can't pass other parameters, because this callback is invoked by ROS whenever a message arrives.

The best way to pass information back to main() is defining a C++ class in your program with the callback as a member function. It can then update class data as messages arrive.

Alternatively, you could update some global data structure. That is not generally a good practice unless your node is small and simple.

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Thanks joq. This was helpful.

Paul0nc ( 2011-11-06 09:31:59 -0500 )edit

joq or anyone: (pardon the basic nature of this question): is it possible to update a global variable in a callback function. When I try this, I get compilation errors that the variable is not defined in the scope of the function. Thanks.

Paul0nc ( 2011-11-06 22:56:15 -0500 )edit

That is a basic C++ language question, not appropriate for this forum. There are good C++ resources on-line: http://www.cplusplus.com/doc/tutorial/variables/

joq ( 2011-11-07 01:42:36 -0500 )edit

To repeat, I recommend using class variables rather than global variables.

joq ( 2011-11-07 01:43:51 -0500 )edit

3

Nice question. I'm having the same problems/questions. There should be some tutorial for this.

End-Effector ( 2015-03-23 09:59:28 -0500 )edit

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answered 2015-10-22 03:31:16 -0500

NickHock
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updated 2015-10-22 10:15:07 -0500

There are probably a fair number of ROS users who are learning C++, so the following links may be useful.

For why things are done this way: https://isocpp.org/wiki/faq/pointers-to-members

For the standard library way to solve the problem (note the differences between C++98 and C++11): http://www.cplusplus.com/reference/functional/bind/?kw=bind .

This code works, using boost::bind , though others may suggest better ways:

#include <string>  
#include <iomanip> 
#include <sstream> 
#include <fstream>
#include <functional>

// image subscriber
#include <ros/ros.h>
#include <image_transport/image_transport.h>
#include <opencv2/highgui/highgui.hpp>
#include <cv_bridge/cv_bridge.h>
#include <camera_info_manager/camera_info_manager.h>
#include <boost/assign/list_of.hpp>

// opencv2.4
#include <opencv2/core/core.hpp>
#include <opencv2/video/tracking.hpp>
#include <opencv2/imgproc/imgproc.hpp> 

using namespace cv;
using namespace std;
typedef const boost::function< void(const sensor_msgs::ImageConstPtr &)>  callback;

class myImages {
  public:
    cv::Mat myMatImage1;
    sensor_msgs::ImagePtr myMessagePtr;
    void imageCallback(const sensor_msgs::ImageConstPtr& msg );
};

static void wrapper_imageCallback(void* pt2Object, const sensor_msgs::ImageConstPtr& msg);

void myImages::imageCallback(const sensor_msgs::ImageConstPtr& msg )
{
  try
  {
    this->myMatImage1 = cv_bridge::toCvShare(msg, "bgr8")->image;
    cv::imshow("view", this->myMatImage1);
    cv::waitKey(30);
    this->myMessagePtr = cv_bridge::CvImage(std_msgs::Header(), "bgr8", this->myMatImage1).toImageMsg();  
  }
  catch (cv_bridge::Exception& e)
  {
    ROS_ERROR("Could not convert from '%s' to 'bgr8'.", msg->encoding.c_str());
  }
}

static void wrapper_imageCallback(void* pt2Object, const sensor_msgs::ImageConstPtr& msg)
   {
       myImages* mySelf = (myImages*) pt2Object; // explicitly cast to a pointer to class myImages
       mySelf->imageCallback(msg );          // call member
   }

int main(int argc, char **argv)
{
  ros::init(argc, argv, "flow_vision");
  ros::NodeHandle nh;
  cv::namedWindow("view");
  cv::startWindowThread();
  image_transport::ImageTransport it(nh);
  myImages MyImagesObj;
  callback boundImageCallback = boost::bind(&myImages::imageCallback, &MyImagesObj/*&images*/, _1); 
  // "_1" is a place holder for the first (and only) parameter of myImages::imageCallback(const sensor_msgs::ImageConstPtr& msg )
  /*subscribe (
   * const std::string &base_topic,
   * uint32_t queue_size,
   * const boost::function< void(const sensor_msgs::ImageConstPtr &)>       &callback,
   * const ros::VoidPtr &tracked_object=ros::VoidPtr(),
   * const TransportHints &transport_hints=TransportHints()) 
   */
  image_transport::Subscriber sub = it.subscribe("camera/image_raw", 1, boundImageCallback);
  image_transport::Publisher pub = it.advertise("out_flow_vision_image_base_topic", 1);
  ros::Rate loop_rate(15);  //need to increase for video
  while (nh.ok()) {
    pub.publish(MyImagesObj.myMessagePtr);
    ros::spinOnce();
    loop_rate.sleep();
  }
  cv::destroyWindow("view");
}