let output_ppm ~oc ~img:(_, r_channel, g_channel, b_channel) =
  let width = Bigarray.Array2.dim1 r_channel
  and height = Bigarray.Array2.dim2 r_channel in
  Printf.fprintf oc "P6\n%d %d\n255\n" width height;
  for y = 0 to pred height do
    for x = 0 to pred width do
      output_char oc (char_of_int r_channel.{x,y});
      output_char oc (char_of_int g_channel.{x,y});
      output_char oc (char_of_int b_channel.{x,y});
    done;
  done;
  output_char oc '\n';
  flush oc;
;;