Image doesn't get modified using PNG++

Question

I was trying to implement a floyd-steinberg dithering program by using png++ but the only thing that happens to output.png is that it becomes grayscale rather than applying dithering

#include <iostream>
#include "lib/png.hpp"
#include <cmath>

png::image< png::gray_pixel > image("input.png");
int h = image.get_height();
int w = image.get_width();

int main() {

    for (int y = 0; y < h; ++y) {
        for (int x = 0; x < w; ++x) {
            double o = image[y][x];
            int n = std::round(o);
            image[y][x] = n;
            double error = o - n;

            if (x+1 < w)             {image[y  ][x+1] += error * 7/16.0;}
            if (y+1 < h && x-1 >= 0) {image[y+1][x-1] += error * 3/16.0;}
            if (y+1 < h)             {image[y+1][x  ] += error * 5/16.0;}
            if (x+1 < w && y+1 < h)  {image[y+1][x+1] += error * 1/16.0;}
        }
    }

    image.write("output.png");
    return 0;
}

And I'm very confused I don't know what to do...

I tried to make a separate image variable, but didn't work either