You can see whether your node is correctly publishing images over ROS using either rostopic or by viewing the images using image_view. The image is then republished over ROS. For example, "/camera/rgb/image_color". find_package ( OpenCV REQUIRED ) include_directories ( $ {OpenCV_INCLUDE_DIRS} ) target_link_libraries ( your_target $ {OpenCV_LIBS} ) in the cmakelists, but I have those already. Image encodings can be any one of the following OpenCV image encodings: For popular image encodings, CvBridge will optionally do color or pixel depth conversions as necessary. Error: No code_block found Converting an image message pointer to an OpenCV message only requires a call to the function imgmsg_to_cv2(). CvImage class format: For an example of allocating one yourself please see the Publishing Images tutorial. } $(".versionshow").removeClass("versionshow").filter("div").show() The way you're passing the rotation part right now, as the output of quaternion_from_euler would cause a numpy array to be passed instead of a list. These basics will provide you with the foundation to add vision to your robotics applications. It acts as if the source of the image is placed on its virtual frame and renders all virtual objects in front of it. 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. It has 134 star(s) with 117 fork(s). Are you using ROS 2 (Dashing/Foxy/Rolling)? I used cudaMemcpy () directly into sensor_msgs::Image::data to avoid extra memory copy if (url_distro) Hence, the list of dependent packages in your manifest should be: Wiki: cv_bridge/Tutorials/ConvertingBetweenROSImagesAndOpenCVImagesPython (last edited 2020-06-12 21:57:44 by JohnStechschulte), Except where otherwise noted, the ROS wiki is licensed under the, ConvertingBetweenROSImagesAndOpenCVImagesPython, Converting ROS image messages to OpenCV images, Converting OpenCV images to ROS image messages, Converting between ROS images and OpenCV images (Android Java). return decodeURIComponent( About publishing ROS images: Get a processed NVMEDIA image from camera. OpenCV with ROS using Python Using OpenCV with ROS is possible using the CvBridge library. { CvBridge is a ROS library that provides an interface between ROS and OpenCV. Here is a node that listens to a ROS image message topic, converts the images into an cv::Mat, draws a circle on it and displays the image using OpenCV. }); Connect Your Built-in Webcam to Ubuntu 20.04 on a VirtualBox Create a New ROS Package Create the Image Publisher Node (Python) Create the Image Subscriber Node (Python) Build Both Nodes (Python) Launch Both Nodes (Python) Image encodings can be any one of the following OpenCV image encodings: For popular image encodings, CvBridge will optionally do color or pixel depth conversions as necessary. ROS opencv image publisher ROS1 ros2 rosbridge libopencv asked Jun 24 '21 sgttrieu 16 5 8 7 I run a simple program to continuously publish image to ROS server. It will not perform conversions to or from Bayer pattern; in a typical ROS system, this is done instead by image_proc. Once you run rospy.spin () the code doesn't go forward. For that. } Error: No code_block found We load a user-specified (on the command line) color image from disk using OpenCV, then convert it to the ROS type sensor_msgs/Image. As far as I know, we can get img data from CSI cameras to publish directly. Error: No code_block found Includes the header for CvBridge as well as some useful constants and functions related to image encodings. How can I set the footprint of my robot in nav2? # ROS Image message from sensor_msgs. function() { OpenCV with ROS using C++ Using OpenCV with ROS is possible using the CvBridge library. Toggle line numbers 24 The problem is solved. Toggle line numbers 22 ros::NodeHandle nh; 23 cv::namedWindow("view"); Create an OpenCV display window. On the turtlebot, run 3dsensor.launch: Now, let's break down the code piece by piece. Depending on whether more plugins are built, additional (per-plugin) topics derived from the base topic may also be advertised. Are you using ROS 2 (Dashing/Foxy/Rolling)? Toggle line numbers 26 image_transport::ImageTransport it(nh); 27 Opencv and ROS image data conversion is shown below: This function package not only needs to use [CvBridge], but also needs [Opencv] and [PCL], so we need to perform the following configuration. The image is then republished over ROS. The second argument is the size of our publishing queue. "+activesystem).hide(); On the turtlebot, run 3dsensor.launch: }) We can safely share the data owned by the ROS message instead of copying. In the complete example above, we explicitly copied the image data, but sharing (when possible) is equally easy: If the incoming message has "bgr8" encoding, cv_ptr will alias its data without making a copy. * OpenCV Example using ROS and CPP Error: No code_block found Includes the headers for OpenCV's image processing and GUI modules. Also this QA is helpful. You should ensure that OpenCV 2 is installed in your system (libopencv-dev on Ubuntu 16.04). CvBridge is a ROS library that provides an interface between ROS and OpenCV. Remember to include opencv2 in your package.xml. $("div.version." In your manifest (alternatively when using roscreate-pkg or catkin_create_pkg), add the following dependencies: (Kinetic users, please see the compatibility section below.). The example above requires a path of an image file to be added as a command line parameter (cv::imread(argv[1],CV_LOAD_IMAGE_COLOR)). will insert information into the image message about the channel ordering. var url_distro = getURLParameter('buildsystem'); Example if we need to build sensors_msg/Image we should build all of the message dependencies. I think that your linker can't find the OpenCV library. More. // Show or hide according to tag Thanks for your info. Stream the image into CUDA to perform format conversions (for example, YUV420 -> RGB) if it is necessary (using dwImageFormatConverter) Stream the image into CPU and publish. I did not pay attention that I leave a " } " in the target_link_libraries configuration. $.each(sections.show, Note the ordering of x and y. Includes the header for CvBridge as well as some useful constants and functions related to image encodings. Note: the second overload of toCvShare is more convenient when you have a pointer to some other message type (e.g. image_transport/Tutorials/PublishingImages - ROS Wiki Note: This tutorial assumes that you have completed the previous tutorials: ROS tutorials, Using CvBridge to Convert Between ROS Images and OpenCV Images. Error: No code_block found In our subscriber callback, we first convert the ROS image message to a CvImage suitable for working with OpenCV. */, // Initialize the ROS Node "roscpp_example", // Instantiate the ROS Node Handler as nh, // Print "Hello ROS!" $("div" + dotversion + this).not(".versionshow,.versionhide").addClass("versionshow") } CvBridge defines a CvImage type containing an OpenCV image, its encoding and a ROS header. CvBridge can be found in the cv_bridge package in the vision_opencv stack. You will also learn how to convert OpenCV images to ROS format to be published over ROS. CvBridge can be found in the cv_bridge package in the vision_opencv stack. In your package.xml and CMakeLists.xml (or when you use catkin_create_pkg), add the following dependencies: Create a image_converter.cpp file in your /src folder and add the following: Using image_transport for publishing and subscribing to images in ROS allows you to subscribe to compressed image streams. It can perfectly convert and be converted image data format. when i try to publish the image it shows on ROS2 network using ros2 topic echo but after filling the data field of Image message with opencv MAT . sensor_msgs::image_encodings::BGR8 is simply a constant for "bgr8", but less susceptible to typos. Then you can run the example code above. ) It is open source and since 2012, the non profit organization OpenCV.org has taken over support. Note that mono8 and bgr8 are the two image encodings expected by most OpenCV functions. var dotversion = ".buildsystem." ** Visit my brand new portal at https://tiziano-school.thinkific.com/ where you can find this ROS series as a FREE course **ROS and OpenCV can work together . For lines not explained here, review Writing a Simple Publisher and Subscriber (C++). msg import Image # ROS Image message -> OpenCV2 image converter from cv_bridge import CvBridge, CvBridgeError # OpenCV2 for saving an image import cv2 # Instantiate CvBridge bridge = CvBridge () def image_callback ( msg ): print ( "Received an image!") try: # Convert your ROS Image message to OpenCV2 advertise() returns an image_transport::Publisher object, which serves two purposes: 1) it contains a publish() method that lets you publish images onto the base topic it was created with, and 2) when it goes out of scope, it will automatically unadvertise. A rospy.spin () essentially keeps the node alive so the callbacks and keep chugging. Remember to include image_transport in your manifest. The official tutorial is here at the ROS website, but we'll run through the steps of a basic example below. A SImilar issue was raised over here. Subscribe to an image topic "in" and advertise an image topic "out" using image_transport. 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. The ROS Wiki is for ROS 1. However, since OpenCV2 is not a ROS package anymore, you can't add it to catkin dependence. $("div.buildsystem").not(". Finally, CvBridge will recognize Bayer pattern encodings as having OpenCV type 8UC1 (8-bit unsigned, one channel). If you have successfully converted images to OpenCV format, you will see a HighGui window with the name "Image window" and your image+circle displayed. Converting between ROS images and OpenCV images (C++) Writing a Simple Image Publisher (C++) Writing a Simple Image Subscriber (C++) Running the Simple Image Publisher and Subscriber with Different Transports; ROS . You can see whether your node is correctly publishing images over ros using either rostopic or by viewing the images using image_view. $("div" + dotversion + this).not(".versionshow,.versionhide").addClass("versionhide") Here is a node that listens to a ROS image message topic, converts the image into a cv::Mat, draws a circle on it and displays the image using OpenCV. If the default value of "passthrough" is given, the destination image encoding will be the same as the image message encoding. video frames) to a topic, and we'll create an image subscriber node that subscribes to that topic. CvBridge is a ROS library that provides an interface between ROS and OpenCV. Error: No code_block found CvBridge also lives in cv_bridge. var bg = $(this).attr("value").split(":"); ) || null; It does, as before, refer to the cv::Mat. In this video we look at how to read images in python, publish them as a topic via a launch file, followed by visualization. We won't modify the data. OpenCV was initially started at Intel and later also developed by Willow Garage (that's where ROS was invented). Writing a Simple Image Publisher The Code The Code Explained Building your node Writing a Simple Image Publisher Here we'll create the publisher node which will continually publish an image. Prequisites This example requires an image stream on the /camera/rgb/image_raw topic. This section requires the catkin_ws to be initialized and the turtlebot_dabit package created. Error: No code_block found All of OpenCV is included by importing cv. changed topicname to rt/topicname and added the DS_ namespace. If the encodings do not match, it will allocate a new buffer and perform the conversion. See this tutorial for more on ROS-OpenCV image conversion. To convert a ROS image message into an cv::Mat, module cv_bridge.CvBridge provides the following function: The input is the image message, as well as an optional encoding. $.each(sections.hide, @sgttrieu: please post your comment as an answer, and then accept your own answer. The usage of ROS and OpenCV libr. If you have successfully converted images to OpenCV format, you will see a HighGui window with the name "Image window" and your image+circle displayed. In this case we want to use color if available, otherwise falling back to monochrome. Remember to include cv_bridge in your package.xml. See this tutorial for more on ROS-OpenCV image conversion. } Publishers which are created by this method. Error: No code_block found The edited image is converted back to ROS image message format using cv2_to_imgmsg() with the encoding "bgr8", so future subscribers will know the color order. You should always wrap your call to imgmsg_to_cv2() to catch conversion errors. To run the node, you will need an image stream. It also supports flipping of images and fps throttling. Draw a red circle on the image, then show it in the display window. To use this feature, specify the encoding to be one of the following strings: bgr8: CV_8UC3, color image with blue-green-red color order, rgb8: CV_8UC3, color image with red-green-blue color order, bgra8: CV_8UC4, BGR color image with an alpha channel, rgba8: CV_8UC4, RGB color image with an alpha channel. Now you can run this node, remapping the image stream topic to the "image_topic". Toggle line numbers 24 cvNamedWindow("view"); 25 cvStartWindowThread(); 26 Create an OpenCV display window. // --> Check out the ROS 2 Documentation. The ROS Wiki is for ROS 1. However, the special commonly used image formats above (bgr8, rgb8, etc.) Prequisites This example requires an image stream on the /camera/rgb/image_raw topic. Error: No code_block found image_transport/image_transport.h includes everything we need to publish and subscribe to images. Included is a sample node that can be used as a template for your own node. In this way, future consumers will know whether the image they receive is RGB or BGR. 1) load two usb cameras /dev/video0 amd /dev/video1, and . Here we'll create the publisher node which will continually publish an image. In your manifest (alternatively when using roscreate-pkg or catkin_create_pkg ), add the following dependencies: sensor_msgs opencv2 cv_bridge rospy std_msgs The Camera plugin allows you to visualize an image from a topic of type sensor_msgs/Image. Remember to include image_transport in your package.xml. catkin Please ask about problems and questions regarding this tutorial on answers.ros.org. When converting a ROS sensor_msgs/Image message into a CvImage, CvBridge recognizes two distinct use cases: CvBridge provides the following functions for converting to CvImage: The input is the image message pointer, as well as an optional encoding argument. You want to achieve the linkers that can find the OpenCV library. If no encoding (or rather, the empty string) is given, the destination image encoding will be the same as the image message encoding. A package to view video streams based on the OpenCV VideoCapture module, easy way to publish on a ROS Image topic (including camera info) usb cams, ethernet cameras, video streams or video files. to the terminal and ROS log file, /* Error: No code_block found Advertise that we are going to be publishing images on the base topic "camera/image". Are you using ROS 2 (Dashing/Foxy/Rolling)? Error: No code_block found We broadcast the image to anyone connected to one of our topics, exactly as we would have using a ros::Publisher. This takes in the image message, as well as the encoding of the destination OpenCV image. As long as you hold a copy of the returned CvImage, the ROS message data will not be freed. )(&|#|;|$)' C++ version only: intensity.val[0] contains a value from 0 to 255. Remember to include opencv2 in your manifest. In order to run this tutorial on Kinetic, you should find_package the version 2 in your CMakeLists.txt (see here). The image is then republished over ROS. activesystem = url_distro; */, // Include CvBridge, Image Transport, Image msg, #include , /* We use methods of ImageTransport to create image publishers and subscribers, much as we use methods of NodeHandle to create generic ROS publishers and subscribers. here is an example for anyone wondering how to do it. The use of "encoding" is slightly more complicated in this case. image_transport::Publisher advertiseImage (ros::NodeHandle &nh, const std::string &topic, int queue_size) Advertise an image topic and watch the publisher. Using OpenCV with ROS is possible using the CvBridge library. new RegExp( '[?|&]' + name + '=' + '([^&;]+? If you're interested in integrating OpenCV with ROS 2 Foxy, check out this tutorial. To convert an IplImage into a ROS image message, CvBridge provides the following function: Toggle line numbers. Note that OpenCV expects color images to use BGR channel order. Multimaster w/ Multicast UDP on ROS1 using ROS2. Now let's create a package named cv_basics. Wiki: image_transport/Tutorials/PublishingImages (last edited 2015-05-24 18:51:48 by Diego Alejandro Gomez), Except where otherwise noted, the ROS wiki is licensed under the, // for converting the command line parameter to integer, // Check if video source has been passed as a parameter, // Convert the passed as command line parameter index for the video device to an integer, // Check if video device can be opened with the given index, // Check if grabbed frame is actually full with some content, Using CvBridge to Convert Between ROS Images and OpenCV Images, Writing a Simple Publisher and Subscriber (C++). Remember to include cv_bridge in your manifest. See this tutorial for more on ROS-OpenCV image conversion. For example, if the incoming image is from the image_raw topic for a Bayer pattern camera, CvBridge will throw an exception because it (intentionally) does not support automatic Bayer-to-color conversion. Error: No code_block found Subscribe to an image topic "in" and advertise an image topic "out" using image_transport. sendTransform on the other hand, expects a list of values for the rotation argument. I run a simple program to continuously publish image to ROS server. Wiki: cv_bridge/Tutorials/UsingCvBridgeToConvertBetweenROSImagesAndOpenCVImages (last edited 2017-04-20 22:38:09 by AdamAllevato), Except where otherwise noted, the ROS wiki is licensed under the, // Case 1: Always copy, returning a mutable CvImage, // Case 2: Share if possible, returning a const CvImage, // Overload mainly intended for aggregate messages that contain, // Subscrive to input video feed and publish output video feed, // Draw an example circle on the video stream, UsingCvBridgeToConvertBetweenROSImagesAndOpenCVImages, Converting ROS image messages to OpenCV images, Converting OpenCV images to ROS image messages, Migration from codes written in C-Turtle or earlier, Converting between ROS images and OpenCV images (Python). The ROS image message must always have the same number of channels and pixel depth as the cv::Mat. I cannot get rid of below error, anyone know the solution please help. Error: No code_block found OpenCV HighGUI calls to create/destroy a display window on start-up/shutdown. Converting OpenCV images to ROS image messages. We want to modify the data in-place. The encoding refers to the destination CvImage. To run the node, you will need an image stream. Jetson Nano ROS2 publisher and image processing Autonomous Machines Jetson & Embedded Systems Jetson Nano camera, ros, jetson-inference muhammedsezer12 December 27, 2021, 2:15pm #1 Hi, I want to publish image via ros2 on jetson nano but I also would like to use it. However, you are free to modify the returned CvImage. Includes the headers for OpenCV's image processing and GUI modules. Share Follow $(".versionhide").removeClass("versionhide").filter("div").hide() Run a camera or play a bag file to generate the image stream. sensor_msgs/CvBridge. ROS passes around images in its own sensor_msgs/Image message format, but many users will want to use images in conjunction with OpenCV. toCvCopy creates a copy of the image data from the ROS message, even when the source and destination encodings match. Image -> Header -> Time we can get these messages from Create a Package. Add the following line to your CMakeLists.txt file: Now let's write a simple image subscriber. rosbuild. The encoding refers to the destination cv::Mat image. I want to convert the following C++ code into python: int main(int argc, char . Now you can run this node, remapping "in" to the actual image stream topic. package.xml I am using cv_bridge to convert images from ROS to openCV and vice versa so I looked at the code from the tutorials. I also install Here is a node that listens to a ROS image message topic, converts the images into an cv::Mat, draws a circle on it and displays the image using OpenCV. There was a major api change ROS Diamondback regarding OpenCV, of which backward compatibility has been maintained for a while but removed in newer distro (eg. Converting OpenCV images to ROS image messages An example ROS node Examples of sharing the image data Concepts ROS passes around images in its own sensor_msgs/Image message format, but many users will want to use images in conjunction with OpenCV. frame_id = img-> header. I cannot get rid of below error, anyone know the solution please help. var activesystem = "catkin"; Here you are using a while loop for keeping the node alive, so shouldn't need rospy.spin () . Since ROS Kinetic, the default supported version is OpenCV 3. function() { * Hello World Example using ROS and CPP Creative Commons Attribution Share Alike 3.0. $("#"+activesystem).click(); It also supports flipping of images and fps throttling. Check out the ROS 2 Documentation. Error: No code_block found Convert the CvImage to a ROS image message and publish it on the "out" topic. )[1].replace(/\+/g, '%20') The video_stream_opencv package contains a node to publish a video stream (the protocols that opencv supports are supported, including rtsp, webcams on /dev/video and video files) in ROS image topics, it supports camera info and basic image flipping (horizontal, vertical or both) capabilities, also adjusting publishing rate. You can modify the example quite easily to make it work with a video device supported by cv::VideoCapture (in case it is not you have to handle it accordingly): If you have a single device you do not need to do the whole routine with passing a command line argument (argv[1]) and parsing it at all. Hello FastRTPS team, I'm trying to publish video camera images from opencv using fastRTPS to ROS2 Node. stereo_msgs/DisparityImage) that contains a sensor_msgs/Image you want to convert. Run a camera or play a bag file to generate the image stream. In the manifest, add a dependency to opencv2 and cv_bridge. ).exec(location.search) || [,""] If the incoming image is either "bgr8" or "mono8", we avoid copying data. This example requires an image stream on the /camera/rgb/image_raw topic. hydro), e.g. Please click here to learn how to build turtlebot_dabit with roscpp, gedit ~/catkin_ws/src/turtlebot_dabit/src/roscpp_opencv.cpp. In this tutorial, you will learn how to write a node that uses CvBridge to convert ROS images into OpenCV cv::Mat format. A package to view video streams based on the OpenCV VideoCapture module, easy way to publish on a ROS Image topic (including camera info) usb cams, ethernet cameras, video streams or video files. Here is an example of one such solution - they all center around including. // Tag hides unless already tagged // Tag shows unless already tagged It automatically reads latch boolean parameter from nh and it and publish topic with appropriate latch parameter. CvBridgeis a ROS library, equivalent to a bridge between ROS and Opencv. header. $(document).ready(function() { function getURLParameter(name) { ( My C++ code: #include <ros/ros.h> #include <cv_bridge/cv_bridge.h> #include <image . You are not permitted to modify the returned CvImage, as it may share data with the ROS image message, which in turn may be shared with other callbacks. * OpenCV Example using ROS and CPP Open a new terminal window, and navigate to the src directory of your workspace: cd ~/dev_ws/src. $("input.version:hidden").each(function() { If the retrieved frame from the video device is not empty it will be then converted to a ROS message, which will be published by the publisher. In this tutorial, you will learn how to write a node that uses CvBridge to convert ROS images into OpenCV cv::Mat format. We convert the ROS image message to an OpenCV image with BGR pixel encoding, then show it in a display window. Replace find_package(catkin REQUIRED COMPONENTS) with: Add your build target for roscpp_opencv.cpp: Replace the Hello ROS code with the following OpenCV code: ~/catkin_ws/src/turtlebot_dabit/src/opencv_example.cpp, /* The problem is solved. In addition multiple checks are also included here to make sure that the publisher does not brake if in case the camera is shut downs or similar issue). Change to the directory you've created for these tutorials: $ roscd learning_image_transport The Code Convert the CvImage to a ROS image message and publish it on the "out" topic. Error: No code_block found These headers will allow us to load an image using OpenCV and convert it to the ROS message format. You should always wrap your calls to toCvCopy() / toCvShared() to catch conversion errors as those functions will not check for the validity of your data. However, cv2_to_imgmsg() does not do any conversions for you (use CvtColor and ConvertScale instead). You will also learn how to convert OpenCV images to ROS format to be published over ROS. See the design decision. Error: No code_block found We create an ImageTransport instance, initializing it with our NodeHandle. I think the best shot for you is to read the ROS tutorial Converting between ROS images and OpenCV images (C++): This tutorial describes how to interface ROS and OpenCV by converting ROS images into OpenCV images, and vice versa, using cv_bridge. Please start posting anonymously - your entry will be published after you log in or create a new account. Key parameters: Image topic: Selects the image topic to visualize from the list of available images in the combo box 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. Without the exception handling this would only be one line of code, but then an incoming message with a malformed (or unsupported) encoding would bring down the node. OpenCV HighGUI calls to create/destroy a display window on start-up/shutdown. Since we're going to draw on the image, we need a mutable copy of it, so we use toCvCopy(). add image_publisher opencv 2 compatibility; Contributors: hannometer. https://github.com/HemaZ/FastRTPS-ROS2 First we need to get all the messages dependencies from ROS2 installation. You'd probably have to load the numpy array contents manually into a list and pass it to sendTransform. CvBridge defines a CvImage type containing an OpenCV image, its encoding and a ROS header. In rospy as soon as you have the rospy.Subsriber () line it spins off another thread for the callback. We'll create an image publisher node to publish webcam data (i.e. Change to the directory you've created for these tutorials: (Assuming you have created your package in ~/image_transport_ws). // @@ Buildsystem macro The solution there was to make a link for the library to a known lib location: Assuming that OpenCV library located in /usr/lib/x86_64-linux-gnu/ you can also add the library folder to the Libraries path: You may need to update the "Dynamic Linker": Hi gvdhoorm & Janjit, Example usages in launch folder (only the argument video_stream_provider is mandatory): ROS passes around images in its own sensor_msgs/Image message format, but many users will want to use images in conjunction with OpenCV. 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. To convert an cv::Mat into a ROS image message, CvBridge provides the following function: The use of "encoding" is slightly more complicated in this case. The dependencies necessary for using OpenCV with image_pipeline are as follows: opencv3 cv_bridge image_transport stereo_image_proc image_view nodelet cv_camera camera_calibration These should be installed with rosdep: >$ rosdep install package_name where package_name is the name of the package. In this tutorial, you will learn how to write a node that uses CvBridge to convert ROS images into OpenCVcv::Mat format. We convert the ROS image message to an OpenCV image with BGR pixel encoding, then show it in a display window. ros::Timer publish_timer; sensor_msgs::CameraInfo cam_info_msg; // Based on the ros tutorial on transforming opencv images to Image messages virtual sensor_msgs::CameraInfo get_default_camera_info_from_image (sensor_msgs::ImagePtr img) { sensor_msgs::CameraInfo cam_info_msg; cam_info_msg. Since we're going to draw on the image, we need a mutable copy of it, so we use toCvCopy(). Check out the ROS 2 Documentation,