How to obtain a better time for execution for Sobel algorithm using OpenMP?

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Hello everyone,

I tried to parallelize the Sobel algorithm using OpenMP. I have good results but I want to improve the results.

Initial time for algorithm (sequential code): 1.49 s With OpenMP: 0.523 s

Can you tell me what improvements can I make to the code to get a better time? Thank you in advance and have a nice day!

This is my code:

    // -------  C/C++ includes ------
#include <iostream>
#include <stdio.h>

#include <omp.h>
#include <time.h>

// ------ OpenCV includes ------
#include <opencv2/core.hpp>
#include <opencv2/imgproc.hpp>
#include <opencv2/highgui.hpp>
#include <opencv2/opencv.hpp>

using namespace std;
using namespace cv;

// dimension of kernel
int x[3][3];
int y[3][3];

/*----- OpenMP -----*/
int num_of_threads, i ,j;
double start, end;

int main( int argc, char** argv )
{

    Mat initialImage = imread(argv[1], 0); // imread gray-scale image
    Mat finalImage = Mat::zeros(initialImage.size(), initialImage.type());

    if (finalImage.type() == initialImage.type() )
    {
        cout << "YES" << endl;
    }

    if(argc != 2 || !initialImage.data)
    {
        cout << "No image data or Usage: ./sobel imagePath" << endl;
        return -1;
    }
    else
      cout << "Image OK!" << endl; 

    //x direction
    x[0][0] = -1;  x[0][1] = 0;  x[0][2] = 1;
    x[1][0] = -2;  x[1][1] = 0;  x[1][2] = 2;
    x[2][0] = -1;  x[2][1] = 0;  x[2][2] = 1;

    //y direction
    y[0][0] = -1;  y[0][1] = -2;  y[0][2] = -1;
    y[1][0] =  0;  y[1][1] =  0;  y[1][2] =  0;
    y[2][0] =  1;  y[2][1] =  2;  y[2][2] =  1;

    num_of_threads = 8;//omp_get_num_procs();
    omp_set_num_threads(num_of_threads);

    start = omp_get_wtime();
    for(j = 0; j < initialImage.rows - 2; j++ ){
      #pragma omp parallel for private(i)
      for(i = 0; i < initialImage.cols -2; i++ ){
        // applay karnel in x direction
        int xValOfPixel = 
         (x[0][0] * (int)initialImage.at<uchar>(j, i    )) + (x[0][1] * (int)initialImage.at<uchar>(j + 1, i    )) + (x[0][2] * (int)initialImage.at<uchar>(j + 2, i    )) +
         (x[1][0] * (int)initialImage.at<uchar>(j, i + 1)) + (x[1][1] * (int)initialImage.at<uchar>(j + 1, i + 1)) + (x[1][2] * (int)initialImage.at<uchar>(j + 2, i + 1)) +
         (x[2][0] * (int)initialImage.at<uchar>(j, i + 2)) + (x[2][1] * (int)initialImage.at<uchar>(j + 1, i + 2)) + (x[2][2] * (int)initialImage.at<uchar>(j + 2, i + 2));

        // apply karnel in y direction
        int yValOfPixel =
         (y[0][0] * (int)finalImage.at<uchar>(j, i    )) + (y[0][1] * (int)finalImage.at<uchar>(j + 1, i    )) + (y[0][2] * (int)finalImage.at<uchar>(j + 2, i    )) +
         (y[1][0] * (int)finalImage.at<uchar>(j, i + 1)) + (y[1][1] * (int)finalImage.at<uchar>(j + 1, i + 1)) + (y[1][2] * (int)finalImage.at<uchar>(j + 2, i + 1)) +
         (y[2][0] * (int)finalImage.at<uchar>(j, i + 2)) + (y[2][1] * (int)finalImage.at<uchar>(j + 1, i + 2)) + (y[2][2] * (int)finalImage.at<uchar>(j + 2, i + 2));


         int sum = abs(xValOfPixel) + abs(yValOfPixel);
         if(sum > 255)
          sum = 255;

        finalImage.at<uchar>(j, i) = (uchar)sum;
      }
    }
    end = omp_get_wtime();

    cout << "Time: " << end - start << endl;

    // display the ...