static void
pixelize_pixbuf (GdkPixbuf *pixbuf,
		 gint       pixel_size)
{
  gint width, height, rowstride, n_channels;
  gint block_width, block_height;
  gint i, j;
  gint m, n;
  gint p;
  guchar *pixels;

  g_assert (pixel_size > 0);

  pixels = gdk_pixbuf_get_pixels (pixbuf);
  width = gdk_pixbuf_get_width (pixbuf);
  height = gdk_pixbuf_get_height (pixbuf);
  rowstride = gdk_pixbuf_get_rowstride (pixbuf);
  n_channels = gdk_pixbuf_get_n_channels (pixbuf);

  for (i = 0; i < width; i = i + pixel_size)
    {
      for (j = 0; j < height; j = j + pixel_size)
	{
	  block_width = MIN (width - i, pixel_size);
	  block_height = MIN (height - j, pixel_size);
	  for (m = 0; m < block_width; m ++)
	    {
	      for (n = 0; n < block_height; n++)
		{
		  if (m == 0 && n == 0)
		    continue;
		  p = 0;
		  *(pixels + (i + m )* n_channels + (j + n) * rowstride + p) =
		    *(pixels + j*rowstride + i*n_channels + p);
		  p++;
		  *(pixels + (i + m )* n_channels + (j + n) * rowstride + p) =
		    *(pixels + j*rowstride + i*n_channels + p);
		  p++;
		  *(pixels + (i + m )* n_channels + (j + n) * rowstride + p) =
		    *(pixels + j*rowstride + i*n_channels + p);
		}
	    }
	}
    }
}