Convert RGB image to gray scale by looping through pixels using char raw pointers C++

ImageOperator.cpp

void ImageOperator::to_gray(const unsigned char* bgr_input,
                            const int width,
                            const int height,
                            const int channel,
                            unsigned char* gray_output){
    int index = 0;
    int step = channel*width;
    for (int row = 0; row < height; ++row) {
        for (int col = 0; col < width*channel; col+=channel) {
            gray_output[index] = 0.11*bgr_input[row*step+col]+
                             0.59*bgr_input[row*step+col+1]+
                             0.3*bgr_input[row*step+col+2];
            index++;
        }
    }
}