void blur(int height, int width, RGBTRIPLE image[height][width])
{
RGBTRIPLE(*alt_img)[width] = calloc(height, width * sizeof(RGBTRIPLE));

//3x3 radius
/*       +-----------------------------------------+
         |             |             |             |
         | N[h-1][w-1] |  N[h-1][w]  | N[h-1][w+1] |
         |             |             |             |
         |-------------|-------------|-------------|
         |             |             |             |
         |  N[h][w-1]  |   N[h][w]   |  N[h][w+1]  |
         |             |             |             |
         |-------------|-------------|-------------|
         |             |             |             |
         | N[h+1][w-1] | N[h+1][w-1] | N[h+1][w+1] |
         |             |             |             |
         +-----------------------------------------+
*/
reduce(height, width, image, alt_img, 1);
reduce(height, width, image, alt_img, 0);
free(alt_img);
}

//loop over image, execute radius for each pixel, dumb idea but it look neat.
void reduce(int height, int width, RGBTRIPLE (*pixel)[width], RGBTRIPLE (*alt_pixel)[width], int alt)
{
for (int h = 0; h < height; h++)
    {
        for (int w = 0; w < width; w++)
        {
          if (alt)
          {
             radius(h, w, height, width, pixel);
          }
          else
          {
              pixel[h][w] = alt_pixel[h][w];
          }
        }
    }
}

void radius(int h, int w, int height, int width, RGBTRIPLE (*pixel)[width])
{
    int rad_h;
    int rad_w;

    int total[] = {pixel[h][w].rgbtRed,pixel[h][w].rgbtGreen,pixel[h][w].rgbtBlue};
    int n = 0;
    //select pixels in 3x3 area.
    for (int i = 0; i < 3; i++)
    {
        for (int j = 0; j < 3; j++)
        {
        rad_h = h + (i - 1);
        rad_w = w + (j - 1);
        //if rad_w,rad_h is not out of bound, increase total.
            if (rad_w >= 0 && rad_h >= 0)
            {
                if(rad_h < height && rad_w < width)
                {
                    n++;
                    total[0] += pixel[rad_h][rad_w].rgbtRed;
                    total[1] += pixel[rad_h][rad_w].rgbtGreen;
                    total[2] += pixel[rad_h][rad_w].rgbtBlue;
                }
            }
        }
    }
    //get avg value of each pixel's color and convert to it BYTE
    for (int k = 0; k < 3; k++)
    {
        total[k] = (BYTE) round(total[k]/n);
    }
    pixel[h][w].rgbtRed = total[0];
    pixel[h][w].rgbtGreen = total[1];
    pixel[h][w].rgbtBlue = total[2];
}