first image message does not get published

Question

I have a node that among many other things, publishes images for debugging purposes. However, this does not work all the time, and some messages never get seen by other nodes.

The messages get published from a subscription callback, if that makes a difference.

Symptoms:

• The first message never gets published
• Other messages (PointCloud2) work fine
• Messages don't get published, even when callback is idle
• cv_bridge or manual message creation makes no difference
• No error messages at all

I have verified that the publish method does get called, and even stepped into it. There does not seem to be a problem there.

I am at a loss. What could be the problem?

Edit :

#include <ros/publisher.h>
#include <ros/subscriber.h>
#include <ros/node_handle.h>
#include <sensor_msgs/Image.h>
#include <cv_bridge/cv_bridge.h>
#include <opencv2/core/core.hpp>

std::map<std::string, ros::Publisher> mPublishers;
ros::Subscriber imgSub;

template <class msg_type>
ros::Publisher getPublisher(const std::string& topic)
{

  if (mPublishers.find(topic) == mPublishers.end())
  {
    std::ostringstream ostr;
    ostr << "/" << "dump" << "/" << topic;
    ros::NodeHandle mNh;
    mPublishers[topic] = mNh.advertise<msg_type> (ostr.str(), 0, true);
  }

  return mPublishers[topic];
}

void cb(const sensor_msgs::ImageConstPtr& image_msg)
{
  ros::Time time = image_msg->header.stamp;


  cv_bridge::CvImageConstPtr cvImage = cv_bridge::toCvShare(image_msg, "");
  const cv::Mat& image = cvImage->image;

  sensor_msgs::ImagePtr msg(new sensor_msgs::Image());

  int numBytes = image.step * image.rows;
  msg->data.resize(numBytes);
  for (int i = 0; i < numBytes; ++i)
  {
    msg->data[i] = image.data[i];
  }
  msg->encoding = "rgb8";
  msg->header.stamp = time;
  msg->header.frame_id = image_msg->header.frame_id;
  msg->height = image.rows;
  msg->width = image.cols;
  msg->step = image.step;

  ros::Publisher pub = getPublisher<sensor_msgs::Image>("cascade_faces");
  pub.publish(msg);
}


int main (int argc, char** argv)
{
    // Initialize ROS
  ros::init (argc, argv, "my_template_alignment");

  ros::NodeHandle nh;

  imgSub = nh.subscribe("/camera/rgb/image_rect_color", 1, cb);


  std::cout << "ready!" << std::endl;

    // Spin
  ros::spin ();

  return (0);
}

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Originally posted by JFReuter on ROS Answers with karma: 13 on 2011-10-21

Post score: 0

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Original comments

Comment by JFReuter on 2011-10-21:
I have added a small example.

Comment by Eric Perko on 2011-10-21:
It would be much easier to debug this if you could post your code (or a link to it).

Answer 1

After you create a ros publisher, it will take some time before the connections to all the subscribers are created. So when you publish a message right after you create the publisher, the message will not reach the subscribers. In your example, this is exactly what happens, when you create a new publisher in the first callback, and immediately publish a message. The second time you publish a message form the same callback, the connections are established and the messages will reach the subscribers.

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Originally posted by Wim with karma: 2915 on 2011-11-18

This answer was ACCEPTED on the original site

Post score: 5

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Original comments

Comment by rhololkeolke on 2013-03-25:
Is there a way to check if the connection has been set up?

Comment by mbinna on 2017-10-17:
One way is to wait until pub.get_num_connections() > 0.