Messages.md

There are some standard messages in ROS. Here are its definitions and structure

geometry_msgs/msg/Twist Message

C++ declaration

#include "geometry_msgs/msg/twist.hpp"
rclcpp::Publisher<geometry_msgs::msg::Twist>::SharedPtr publisher_;
auto velocityMsg = geometry_msgs::msg::Twist();
velocityMsg.linear.x = 0.1;
velocityMsg.linear.y = 0.1;
velocityMsg.linear.z = 0.1;
velocityMsg.angular.x = 0.1;
velocityMsg.angular.y = 0.1;
velocityMsg.angular.z = 0.1;

Python declaration

from geometry_msgs.msg import Twist
self.cmd_pub = self.create_publisher(Twist, 'cmd_vel', 10)
self.cmd.linear.x = 0.0
self.cmd.linear.y = 0.0
self.cmd.linear.z = 0.0

self.cmd.angular.x = 0.0 
self.cmd.angular.y = 0.0
self.cmd.angular.z = 0.0

self.cmd_pub.publish(self.cmd)

sensor_msgs/Image Message

P.S. almost always goes with CvBridge

C++ declaration

#include "cv_bridge/cv_bridge.h"
#include "opencv2/opencv.hpp"
#include "sensor_msgs/msg/image.hpp"

auto velocityMsg = geometry_msgs::msg::Twist();
velocityMsg.linear.x = 0.1;
velocityMsg.linear.y = 0.1;
velocityMsg.linear.z = 0.1;
velocityMsg.angular.x = 0.1;
velocityMsg.angular.y = 0.1;
velocityMsg.angular.z = 0.1;

rclcpp::Subscription<sensor_msgs::msg::Image>::SharedPtr subscription_;
virtual void cameraCallback(const sensor_msgs::msg::Image::SharedPtr cameraMsg);
static cv_bridge::CvImagePtr cvPtr; //Create OpenCv Image pointer
cvPtr = cv_bridge::toCvCopy(cameraMsg, "bgr8"); //translate ROS cameraMSG to //OpenCv image

Python declaration

self.__bridge = CvBridge()
self.__camera_cb = self.create_subscription(Image,'/drone_vision/image_from_airsim', self.__camera_cb, 1)
self.__frame = self.__bridge.imgmsg_to_cv2(data, desired_encoding='bgr8')