Convert RGB image to gray scale by looping through pixels (two for loops) using OpenCV

ImageOperator.cpp

void ImageOperator::to_gray_m3(const cv::Mat &input, cv::Mat &output) {

    unsigned char *data_in = (unsigned char*)(input.data);
    unsigned char *data_out = (unsigned char*)(output.data);

    int index = 0;
    for (int row = 0; row < input.rows; ++row) {
        for (int col = 0; col < input.cols*input.channels(); col+=input.channels()) {
            data_out[index]= 0.11*data_in[row*input.step+col]+
                             0.59*data_in[row*input.step+col+1]+
                             0.3*data_in[row*input.step+col+2];
            index++;
        }
    }
}