struct img_t {
    char *pixels;
    int width;
    int height;
};

int img_invert(struct img_t *img)
{
    for(int i = 0; i < img->width; i++)
    {
        for(int j = 0; j < img->height; j++)
        {
            char tmp = img->pixels[i*j];
            img->pixels[i*j] = 255 - tmp;
        }
    }
    return 0;
}