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

asked 2018-06-04 12:17:43 -0500

Laurentiu gravatar image

updated 2018-06-04 12:29:32 -0500

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 ...
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Comments

don't write per-pixel code in the 1st place.

don't use at.

don't reinvent the wheel.

use cv::Sobel, and enable TBB and IPP support at opencv build time.

you're reinventing the "flat tyre", and now you try to optimize that. seems somewhat silly.

have a look, how the opencv devs did it !

berak gravatar imageberak ( 2018-06-04 12:25:16 -0500 )edit

I don't want to reinvent the wheel. I just want to do a college study about the OpenMP influence in Image Processing algorithms. But, Thank you for your answer!

Laurentiu gravatar imageLaurentiu ( 2018-06-04 12:31:14 -0500 )edit

thing is, as long as your code looks like that, ANY other optimization (using pointers, loop unrolling, vector intrinsics) might beat your code without openmp, so it's all rigged.

berak gravatar imageberak ( 2018-06-04 12:34:34 -0500 )edit

I need just an optimization for my algorithm using OpenMP, I don't need the best Sobel algorithm that exists. But thank you for your time and implication.

Laurentiu gravatar imageLaurentiu ( 2018-06-04 12:42:05 -0500 )edit

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answered 2018-06-04 21:45:29 -0500

Tetragramm gravatar image

The only thing I can see that would improve your OpenMP time alone is to parallelize along rows, not columns. IE: the outer for loop. It leads to more efficient memory access.

Really though, just like berak said, all the real improvement here would come from the base algorithm. Look up separable filters, look at THIS tutorial, and as berak said, take a look at how OpenCV does it and time yourself against it. Not all of it will make sense for your project, but you can at least make sure you're not doing silly things, and get an idea of how well optimized your code is.

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Thank you :D I will modify my code and I will watch the tutorial.

Laurentiu gravatar imageLaurentiu ( 2018-06-04 22:31:14 -0500 )edit
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Asked: 2018-06-04 12:17:43 -0500

Seen: 87 times

Last updated: Jun 04 '18