Following is the explanation of the complete code:

#include <image_transport/image_transport.h> 

We are using the image_transport package in this code for publishing and subscribing to image in ROS.

#include <cv_bridge/cv_bridge.h> 
#include <sensor_msgs/image_encodings.h> 

This header includes the CvBridge class and image encoding related functions in
the code.

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

These are main OpenCV image processing module and GUI modules which provide image processing and GUI APIs in our code.

  image_transport::ImageTransport it_; 
public: 
  Edge_Detector() 
    : it_(nh_) 
  { 
    // Subscribe to input video feed and publish output video feed 
    image_sub_ = it_.subscribe("/usb_cam/image_raw", 1,  
      &ImageConverter::imageCb, this); 
 
    image_pub_ = it_.advertise("/edge_detector/processed_image", 1); 

We will look in more detail at the line image_transport::ImageTransport it_ . This line creates an instance of ImageTransport which is used to publish and subscribe the ROS image messages. More information about the ImageTransport API is given next.