open cv functions

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Hi one which is seen below is my code in this i have converted it into inverted image & now i need to convert it into grayscale image i have tried but i didn't get it so will u please help me

#include <ros/ros.h>
#include <image_transport/image_transport.h>
#include <cv_bridge/cv_bridge.h>
#include <sensor_msgs/image_encodings.h>
#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/highgui/highgui.hpp>
namespace enc = sensor_msgs::image_encodings;
static const char WINDOW[] = "Image Processed";
    image_transport::Publisher pub;
void imageCallback(const sensor_msgs::ImageConstPtr& original_image)
{
    cv_bridge::CvImagePtr cv_ptr;
    try
    {
        cv_ptr = cv_bridge::toCvCopy(original_image, enc::BGR8);
    }
    catch (cv_bridge::Exception& e)
    {
        ROS_ERROR("pub_node::main.cpp::cv_bridge exception: %s", e.what());
        return;
    }
    for(int i=0; i<cv_ptr->image.rows; i++)
    {
        for(int j=0; j<cv_ptr->image.cols; j++)
        {
          for(int k=0; k<cv_ptr->image.channels(); k++)
            {           
                cv_ptr->image.data[i*cv_ptr->image.rows*4+j*3 + k] = 255-cv_ptr->image.data[i*cv_ptr->image.rows*4+j*3 + k];
            }
        }
    }
        cv::imshow(WINDOW, cv_ptr->image);
            cv::waitKey(3);
            pub.publish(cv_ptr->toImageMsg());
}

int main(int argc, char **argv)
{
            ros::init(argc, argv, "image_processor");
                ros::NodeHandle nh;
            image_transport::ImageTransport it(nh);
         cv::namedWindow(WINDOW, CV_WINDOW_AUTOSIZE);
                image_transport::Subscriber sub = it.subscribe("camera/image_raw", 1, imageCallback);
        cv::destroyWindow(WINDOW);

        pub = it.advertise("camera/image_processed", 1);
            ros::spin();
        ROS_INFO("pub_node::main.cpp::No error.");

}