Add a gradient to an image using c++ and opencv

add_gradient.cpp

/*
To compile:
$ g++ gradient_adder.cpp -lm -lopencv_core -lopencv_highgui -lopencv_imgproc -o gradient_adder
To run:
$ ./gradient_adder non_grad_image.png

This will take a black and white input image and seperately apply opposite up/down
gradients to the white and the black portions
*/

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

using namespace std;
using namespace cv;

#define FROM_GRAD_BLACK 10
#define TO_GRAD_BLACK 35
#define FROM_GRAD_WHITE 10
#define TO_GRAD_WHITE 60
#define BLACK_WHITE_CUTOFF 128

int main(int argc, char* argv[]) {

    Mat img = imread(argv[1], 0);

    for (int j=0; j<img.rows; j++) {
        int black_grad = FROM_GRAD_BLACK + ((TO_GRAD_BLACK - FROM_GRAD_BLACK) * j/img.rows);
        int white_grad = FROM_GRAD_WHITE + ((TO_GRAD_WHITE - FROM_GRAD_WHITE) * j/img.rows);

        for (int i=0; i<img.cols; i++) {
            int pixel_val = img.at<uchar>(j, i);
            if (pixel_val < BLACK_WHITE_CUTOFF) {
                img.at<uchar>(j, i) = pixel_val + black_grad;
            }
            else {
                img.at<uchar>(j, i) = pixel_val - white_grad;
            }
        }
    }

    string out_filename = "gradient.png";
    imwrite(out_filename, img);

    return 0;
}