RViz displays "No messages received", what could be the reason?

edit

Hi Kevin,

I think that the problem is that you are subscribing to the topic "output" and then you are publishing to the topic "output" again.

But, you publish to the topic "output" inside the callback of the subscriber to the topic "output" (cloud_cb). So, unless there is an external source pouring data into the topic "output", you will never trigger the callback (cloud_cb) and therefore, you will never publish anything. It is like a dog trying to bite its own tail :P

Even if you have an external source of content for the topic "output", I have never tried this kind of scenario, so I don't really know what behavior to expect. If you trigger the callback with a single external message to the topic "output" then you will get what you want, but maybe if there is an external continuous source of data for the topic "output" the amount of messages published and read again would grow exponentially? Not sure about this.

Probably this would work:

#include <ros/ros.h>
//#include <sensor_msgs/PointCloud2.h>
#include <pcl/ros/conversions.h>
#include <pcl_ros/point_cloud.h>
#include <pcl/point_types.h>
#include <iostream>
using namespace std;

ros::Publisher pub;

void cloud_cb (const sensor_msgs::PointCloud2ConstPtr& input){

  float x=0,y=0,z=0;
  pcl::PointCloud<pcl::PointXYZ>::Ptr msg (new pcl::PointCloud<pcl::PointXYZ>);
  msg->header.frame_id = "some_frame";
  msg->height = msg->width = 1;
  msg->points.push_back (pcl::PointXYZ(x, y, z));
  pub.publish (msg);
}

int main (int argc, char** argv){

   ros::init (argc, argv, "some_init");
   ros::NodeHandle nh;
   ros::Subscriber sub = nh.subscribe ("output", 100, cloud_cb);
   pub = nh.advertise<sensor_msgs::PointCloud2> ("output", 100);


   //Publish the first message so we can trigger cloud_cb for the first time
   float x=0,y=0,z=0;
   pcl::PointCloud<pcl::PointXYZ>::Ptr msg (new pcl::PointCloud<pcl::PointXYZ>);
   msg->header.frame_id = "some_frame";
   msg->height = msg->width = 1;
   msg->points.push_back (pcl::PointXYZ(x, y, z));
   pub.publish (msg);

   ros::spin ();

}