Question

I am trying to sort pixel values of an image (example 80×20) from lowest to highest.

Below is the some code:

bool sortPixel(int first, int second)
{
    return (first < second);
}

    vector<int>vect_sortPixel;
    for(int y=0; y<height; y++)
    {
        for(int x=0; x<width; x++)
        {
            vect_sortPixel.push_back(cvGetReal2D(srcImg, y, x));
            sort(vect_sortPixel.begin(), vect_sortPixel.end(), sortPixel);
        }
    }

But it takes quite long time to compute. Any suggestion to reduce the processing time?
Thank you.

Answer 1

Don’t use getReal2D. It’s quite slow.

Convert image to cv::Mat or Mat. Use its data pointer to get the pixel values. Mat.data() will give you pointer to the original matrix. Use that.

And as far as sorting is concerned, I would advise you to first make an array of all the pixels, then sort it using Merge sort (time complexity O(n log n))

#include<opencv2/highgui/highgui.hpp>
#include<stdio.h>
using namespace cv;
using namespace std;
int main()
{
    Mat img = imread("filename.jpg",CV_LOAD_IMAGE_COLOR);
    unsigned char *input = (unsigned char*)(img.data);
    int i,j,r,g,b;
    for(int i = 0;i < img.cols;i++){
            for(int j = 0;j < img.rows;j++){
                b = input[img.cols * j + i] ;
                g = input[img.cols * j+ i + 1];
                r = input[img.cols *j + i +2];
            }
        }
    return 0;
}

Using this you can access pixel values from the main matrix.

Warning: This is not how you compare it. I’m suggesting that by using something like this, you can access pixel values.

Mat.data() gives you pointer to the original matrix. This matrix is a 1 D matrix with all the given pixel values.

Image => (x,y,z),(x1,y1,z1), etc..

Mat(original matrix) => x,y,z,x1,y1,z1,…

If you still have some doubts regarding how to extract data from Mat, visit this link OpenCV get pixel channel value from Mat image

and here’s a link regarding Merge Sort http://www.cplusplus.happycodings.com/Algorithms/code17.html