turtlebot2-tutorials

OpenCV with ROS using C++

Using OpenCV with ROS is possible using the CvBridge library. This tutorial will show you how to get a message from an Image topic in ROS, convert it to an OpenCV Image, and manipulate the image.

Prequisites

This example requires an image stream on the /camera/rgb/image_raw topic.

  1. On the turtlebot, run 3dsensor.launch:
    1. roslaunch turtlebot_bringup 3dsensor.launch

This section requires the catkin_ws to be initialized and the turtlebot_dabit package created.
Please click here to learn how to initialize the catkin workspace

This section requires the roscpp example to be built in the turtlebot_dabit package.
Please click here to learn how to build turtlebot_dabit with roscpp

Getting an Image from a ROS topic and using OpenCV

  1. Copy your example roscpp_hello_world.cpp code to roscpp_opencv.cpp:
    1. cp ~/catkin_ws/src/turtlebot_dabit/src/roscpp_hello_world.cpp ~/catkin_ws/src/turtlebot_dabit/src/roscpp_opencv.cpp

    2. gedit ~/catkin_ws/src/turtlebot_dabit/src/roscpp_opencv.cpp

       /*
  * Hello World Example using ROS and CPP
  */

 // Include the ROS library
 #include <ros/ros.h>

 // Main function
 int main(int argc, char** argv)
 {
   // Initialize the ROS Node "roscpp_example"
   ros::init(argc, argv, "roscpp_example");

   // Instantiate the ROS Node Handler as nh
   ros::NodeHandle nh;

   // Print "Hello ROS!" to the terminal and ROS log file
   ROS_INFO_STREAM("Hello from ROS node " << ros::this_node::getName());

   // Program succesful
   return 0;
 }
  2. Set up your package dependencies:
    1. gedit ~/CMakeLists.txt

      • Replace find_package(catkin REQUIRED COMPONENTS) with:

          find_package(catkin REQUIRED COMPONENTS
    roscpp
    sensor_msgs
    std_msgs
    cv_bridge
    image_transport
  )

  find_package(OpenCV 2 REQUIRED)

      • Replace catkin_package( with:

          catkin_package(
    INCLUDE_DIRS
    CATKIN_DEPENDS roscpp
                   sensor_msgs
                   std_msgs
                   cv_bridge
                   image_transport
  )

      • Replace your include_directories( with:

          include_directories(
    ${catkin_INCLUDE_DIRS}
    ${OpenCV_INCLUDE_DIRS}
  )

      • Add your build target for roscpp_opencv.cpp:

          add_executable(roscpp_opencv_example src/roscpp_opencv_example.cpp)
  target_link_libraries(roscpp_opencv_example ${catkin_LIBRARIES} ${OpenCV_INCLUDE_DIRS})
  3. Edit opencv_example.cpp in your src folder
    1. gedit ~/catkin_ws/src/turtlebot_dabit/src/opencv_example.cpp

    2. Replace the Hello ROS code with the following OpenCV code:

       /*
  * OpenCV Example using ROS and CPP
  */

 // Include the ROS library
 #include <ros/ros.h>

 // Include opencv2
 #include <opencv2/imgproc/imgproc.hpp>
 #include <opencv2/highgui/highgui.hpp>

 // Include CvBridge, Image Transport, Image msg
 #include <image_transport/image_transport.h>
 #include <cv_bridge/cv_bridge.h>
 #include <sensor_msgs/image_encodings.h>

 // OpenCV Window Name
 static const std::string OPENCV_WINDOW = "Image window";

 // Topics
 static const std::string IMAGE_TOPIC = "/camera/rgb/image_raw";
 static const std::string PUBLISH_TOPIC = "/image_converter/output_video";

 // Publisher
 ros::Publisher pub;

 void image_cb(const sensor_msgs::ImageConstPtr& msg)
 {
   std_msgs::Header msg_header = msg->header;
   std::string frame_id = msg_header.frame_id.c_str();
   ROS_INFO_STREAM("New Image from " << frame_id);

   cv_bridge::CvImagePtr cv_ptr;
   try
   {
     cv_ptr = cv_bridge::toCvCopy(msg, sensor_msgs::image_encodings::BGR8);
   }
   catch (cv_bridge::Exception& e)
   {
     ROS_ERROR("cv_bridge exception: %s", e.what());
     return;
   }

   // Draw an example crosshair
   cv::drawMarker(cv_ptr->image, cv::Point(cv_ptr->image.cols/2, cv_ptr->image.rows/2),  cv::Scalar(0, 0, 255), cv::MARKER_CROSS, 10, 1);

   // Update GUI Window
   cv::imshow(OPENCV_WINDOW, cv_ptr->image);
   cv::waitKey(3);

   // Output modified video stream
   pub.publish(cv_ptr->toImageMsg());
 }
    3. Save and exit
  4. Build and run your new code:
    1. catkin_make --directory ~/catkin_ws --pkg turtlebot_dabit
    2. source ~/devel/setup.sh
    3. rosrun turtlebot_dabit opencv_example

First Example Complete

Additional Resources

ROSCPP Tutorials
Cv_Bridge Tutorials

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