Convert RGB image to gray scale by looping through pixels using OpenCV

ImageOperator.cpp

void ImageOperator::to_gray_m2(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;
    int byte_size = input.channels()*input.rows*input.cols;
    while(index!=byte_size){
        data_out[index/input.channels()] = unsigned(0.11*data_in[index]+0.59*data_in[index+1]+0.3*data_in[index+2]);

        index+=3;
    }
}