In this video we show how to publish a OpenCV Image on a ROS Topic and see the image on RViz by solving a real question: https://answers.ros.org/question/103911/how-can-i-publish-a-opecv-mat-as-a-topic-and-subscribe-by-rviz/

 

Step1. Create a project in Robot Ignite Academy(RIA)

We have the best online ROS course available in RIA. It helps you learn ROS in the easiest way without setting up ROS environment locally. The only thing you need is a browser! Create an account here and start to browse the trial course for free now! We’ll use the ROS Basic in 5 days course unit 3 as an example today.

Step2. Create package

Let’s create a package for the code with the following command

cd ~/catkin_ws/src
catkin_create_pkg publisher roscpp sensor_msgs image_transport cv_bridge

Let’s put the code from the question under the publisher/src folder with the name publisher_node.cpp

#include <cv_bridge/cv_bridge.h>
#include <opencv2/opencv.hpp>
#include <iostream>
#include <ros/ros.h>
#include <image_transport/image_transport.h>
#include <sensor_msgs/image_encodings.h>

namespace enc = sensor_msgs::image_encodings;

int main(int argc, char **argv)
{
    int trajectory[3] = {512,612,712};
    cv::Mat image(1024,1024,CV_8UC3);

    for(int i = 0;i<3;i++)
    {
        cv::circle(image, cv::Point(trajectory[i],trajectory[i]), 100, CV_RGB(255,0,0));
    }

#if 0
    cv::namedWindow("image", CV_WINDOW_NORMAL);
    cv::resizeWindow("image", 1024,1024);
    cv::imshow("image", image);   
    cv::waitKey(0);   
    cv::destroyWindow("image"); 
#endif

#if 1
    ros::init(argc, argv, "draw_circle");
    ros::NodeHandle n;
    image_transport::ImageTransport it_(n);
    image_transport::Publisher image_pub_ = it_.advertise("/traj_output", 1);

    cv_bridge::CvImagePtr cv_ptr(new cv_bridge::CvImage);

#if 1
    ros::Time time = ros::Time::now();
cv_ptr->encoding = "bgr8";
    cv_ptr->header.stamp = time;
    cv_ptr->header.frame_id = "/traj_output";
#endif

    cv_ptr->image = image;
    image_pub_.publish(cv_ptr->toImageMsg());

    ROS_INFO("ImageMsg Send.");
#endif
    ros::spin();
    return 0;

}

To compile it, we have to modify the CMakeLists.txt file in the build part. Please uncomment the add_executable and target_link_librarys part in the file. Then we can compile it with the following command

cd ~/catkin_ws
catkin_make

Then we can run the node with

rosrun publisher publisher_node

Step3. Visualize image in Rviz

Let’s open rviz with the following command

rosrun rviz rviz

By clicking add->Image and select the topic /traj_output, we still can’t see the image.

It turns out the image only publish once with the original code, to visualize it in rviz we have to keep publishing, so let’s change the code

#include <cv_bridge/cv_bridge.h>
#include <opencv2/opencv.hpp>
#include <iostream>
#include <ros/ros.h>
#include <image_transport/image_transport.h>
#include <sensor_msgs/image_encodings.h>

namespace enc = sensor_msgs::image_encodings;

int main(int argc, char **argv)
{
    int trajectory[3] = {512,612,712};
    cv::Mat image(1024,1024,CV_8UC3);

    for(int i = 0;i<3;i++)
    {
        cv::circle(image, cv::Point(trajectory[i],trajectory[i]), 100, CV_RGB(255,0,0));
    }

#if 0
    cv::namedWindow("image", CV_WINDOW_NORMAL);
    cv::resizeWindow("image", 1024,1024);
    cv::imshow("image", image);   
    cv::waitKey(0);   
    cv::destroyWindow("image"); 
#endif

        #if 1
        ros::init(argc, argv, "draw_circle");
        ros::NodeHandle n;
        image_transport::ImageTransport it_(n);
        image_transport::Publisher image_pub_ = it_.advertise("/traj_output", 1);

        cv_bridge::CvImagePtr cv_ptr(new cv_bridge::CvImage);

    ros::Rate loop_rate(10);
    while(ros::ok()) {


        #if 1
        ros::Time time = ros::Time::now();
        cv_ptr->encoding = "bgr8";
        cv_ptr->header.stamp = time;
        cv_ptr->header.frame_id = "/traj_output";
        #endif

        cv_ptr->image = image;
        image_pub_.publish(cv_ptr->toImageMsg());

        ROS_INFO("ImageMsg Send.");
        #endif
        ros::spinOnce();
        loop_rate.sleep();
    }
    ros::spin();
    return 0;

}

Then we have to compile and run the node again, you should see the image now.

Want to learn more?

If you are interested in this topic, please check our ROS perception in 5 days course.

 

Edit by: Tony Huang

---

Q: The user says that the code compiles and runs without any problem but the message can’t be shown on RViz.

A: The problem is because the user was publishing the message only once. When RViz is loaded, the message is not being published anymore. To solve this, the solution is keep publishing until we make sure RViz is subscribed. The code can be found on the answer (https://answers.ros.org/question/103911/how-can-i-publish-a-opecv-mat-as-a-topic-and-subscribe-by-rviz/?answer=278560#post-id-278560)