[Solved] C++: ApproximateTime with more than two topics?

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Hi @all,

short version:

Could you please provide me with a full version for synchronizing up to eight image topics with ROS?

long version:

I am currently trying to get my Synchronizer to work. Thanks to this forum I was able to get a Synchronizer to work with two topics (for interested users, see: http://wiki.ros.org/message_filters?d... ).

Now, all I have found so far deals with two topics. In my case, there are more than two topics (namely, four, six or eight image topics). I need to synchronize them too. And using ApproximateTime for that looks like a good option to me. However, I am getting errors doing so.

Essential code for two topics (Working):

#include <sensor_msgs/Image.h>

#include <image_transport/subscriber_filter.h>

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

class My_Synchronizer {

private:
 ros::NodeHandle nh_;
 image_transport::ImageTransport it_;

 typedef image_transport::SubscriberFilter ImageSubscriber;
 typedef ros::Publisher                    ImagePublisher;

 ImageSubscriber img1_sub_;
 ImageSubscriber img2_sub_;

 ImagePublisher img1_pub_;
 ImagePublisher img2_pub_;

 typedef message_filters::sync_policies::ApproximateTime<sensor_msgs::Image, sensor_msgs::Image> ImageSyncPolicy_1; //two topics

 message_filters::Synchronizer< ImageSyncPolicy_1 > sync_1;

//Constructor for two topics:
My_Synchronizer(
std::string t1,
std::string t2
) :
it_(nh_),
img1_sub_( it_, t1, 100),
img2_sub_( it_, t2, 100),
sync_1( ImageSyncPolicy_1( 10 ), img1_sub_, img2_sub_)
{


  img1_pub_ = nh_.advertise<sensor_msgs::Image>("/synchronized/" + t1, 1000);
  img2_pub_ = nh_.advertise<sensor_msgs::Image>("/synchronized/" + t2, 1000);


  std::cout << "Image Synchronizer on two topics started" << std::endl;
  sync_1.registerCallback( boost::bind( &My_Synchronizer::Sync1_Callback, this, _1, _2) );
};

void Sync1_Callback(
/*
  Callback with two synchronized topics:
*/
  const sensor_msgs::ImageConstPtr& image1,
  const sensor_msgs::ImageConstPtr& image2
 );
};
//Implementation of Sync1_Callback in separate file - not important for this question

Changes in Code for eight topics (Not Working!):

typedef message_filters::sync_policies::ApproximateTime<sensor_msgs::Image, sensor_msgs::Image, sensor_msgs::Image, sensor_msgs::Image, sensor_msgs::Image, sensor_msgs::Image, sensor_msgs::Image, sensor_msgs::Image> ImageSyncPolicy_4; //eight topics

message_filters::Synchronizer< ImageSyncPolicy_4 > sync_4; //Does not work, but [...] sync_4(); works - still problems below

 //Constructor for eight topics:
 My_Synchronizer(
 std::string t1,
 std::string t2,
 [...]
 ) :
 it_(nh_),
 img1_sub_( it_, t1, 100),
 img2_sub_( it_, t2, 100),
 [...]
 sync_4( ImageSyncPolicy_1( 10 ), img1_sub_, img2_sub_, img3_sub_, img4_sub_, img5_sub_, img6_sub_, img7_sub_, img8_sub_)
 {

  img1_pub_ = nh_.advertise<sensor_msgs::Image>("/synchronized/" + t1, 1000);
  img2_pub_ = nh_.advertise<sensor_msgs::Image>("/synchronized/" + t2, 1000);
  [...]

  std::cout << "Image Synchronizer on eight topics started" << std::endl;
  sync_4.registerCallback( boost::bind( &My_Synchronizer::Sync4_Callback, this, _1, _2, _3, _4, _5, _6, _7, _8) );
};

 void Sync4_Callback(
 /*
   Callback with eight synchronized topics:
 */
  const sensor_msgs::ImageConstPtr& image1,
  const sensor_msgs::ImageConstPtr& image2,
  const sensor_msgs::ImageConstPtr& image3,
  const sensor_msgs::ImageConstPtr& image4,
  const sensor_msgs::ImageConstPtr& image5,
  const sensor_msgs::ImageConstPtr& image6,
  const sensor_msgs::ImageConstPtr& image7,
  const sensor_msgs::ImageConstPtr& image8
 );

The above code throws many boost related errors, about missing arguments or wrong function calls. I cannot really find out where the necessary change is to be made. Trying to use a SyncPolicy for four topics already throws this error for example:

error: no matching function for call to ‘message_filters::sync_policies::ApproximateTime<sensor_msgs::Image_<std::allocator<void> >, sensor_msgs::Image_<std::allocator<void> >, sensor_msgs::Image_<std::allocator<void> >, sensor_msgs::Image_<std::allocator<void> > >::ApproximateTime()’

Could you please tell me ... (more)