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a sample C++ code maybe helpful ( i modified akarsakov's code from SO)

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


void RGB2Gray(const cv::Mat& src, cv::Mat& dst)
{
    CV_Assert(src.type() == CV_8UC3);
    int rows = src.rows, cols = src.cols;

    dst.create(src.size(), CV_8UC1);

    if (src.isContinuous() && dst.isContinuous())
    {
        cols = rows * cols;
        rows = 1;
    }

    for (int row = 0; row < rows; row++)
    {
        const uchar* src_ptr = src.ptr<uchar>(row);
        uchar* dst_ptr = dst.ptr<uchar>(row);

        for (int col = 0; col < cols; col++)
        {
            dst_ptr[col] = (uchar)(src_ptr[0] * 0.114f + src_ptr[1] * 0.587f + src_ptr[2] * 0.299f);
            src_ptr += 3;
        }
    }
}

int main()
{
    cv::Mat SrcImg = cv::imread("../data/lena.jpg");
    cv::Mat DstImg;
    RGB2Gray(SrcImg, DstImg);
    imshow("gray", DstImg);
    cv::waitKey();
    return 0;
}