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From: Florent Monnier <fmonnier@linux-nantes.fr.eu.org>
To: caml-list@yquem.inria.fr
Subject: Re: [Caml-list] Portable PNG exporter
Date: Mon, 22 Sep 2008 16:47:21 +0200
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X-Spam: no; 0.00; ocaml:01 ocaml:01 bigarray:01 bigarray:01 printf:01 fprintf:01 pred:01 pred:01 byte:01 printf:01 sprintf:01 sprintf:01 allways:01 height:98 height:98 

> For reference, I've found the easiest way to export PNGs (in any
> language, not just OCaml) is to use netpbm.

Probably with any language, but perhaps not any OS ?

>                                              Simply fork pnmtopng
> using Unix.open_process_out and write a PPM file. =A0A PPM file has such
> a simple format that you can write it directly from just about any
> language, even a shell script.
>
> =A0 P3 <width> <height> 255
> =A0 followed by <width>x<height>x3 RGB triplets (in decimal, separated
> =A0 by writespace)

You can reduce the amount of exchanged datas using binary ppm instead of as=
cii=20
ppm. In such case the format is:

  P6\n<width in ascii> <height in ascii>\n255\n
  followed by <width>x<height>x3 RGB triplets of octect (in binary)

Here is the OCaml code I use, as well for jpeg:

let output_ppm ~oc ~img:(_, r_channel, g_channel, b_channel) =3D
  let width =3D Bigarray.Array2.dim1 r_channel
  and height =3D Bigarray.Array2.dim2 r_channel in
  Printf.fprintf oc "P6\n%d %d\n255\n" width height;
  for y =3D 0 to pred height do
    for x =3D 0 to pred width do
      (* output_byte doesn't raise any exception about the range *)
      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
;;

(* you can use different conversion commands (convert is from ImageMagick) =
*)
let print_jpeg ~img ?(quality=3D96) () =3D
  let cmd =3D Printf.sprintf "cjpeg -quality %d" quality in
  (*
  let cmd =3D Printf.sprintf "ppmtojpeg -quality %d" quality in
  let cmd =3D Printf.sprintf "convert ppm:- -quality %d jpg:-" quality in
  *)
  let ic, oc =3D Unix.open_process cmd in
  output_ppm ~img ~oc;
  try
    while true do
      let c =3D input_char ic in
      print_char c
    done
  with End_of_file -> ()
;;

(* output any of the hundred formats ImageMagick knows *)
let print_file ~img ~format =3D
  let cmd =3D Printf.sprintf "convert ppm:- %s:-" format in
  let ic, oc =3D Unix.open_process cmd in
  output_ppm ~img ~oc;
  try
    while true do
      let c =3D input_char ic in
      print_char c
    done
  with End_of_file -> ()
;;

let new_img ~width ~height =3D
  let all_channels =3D
    let kind =3D Bigarray.int8_unsigned
    and layout =3D Bigarray.c_layout
    in
    Bigarray.Array3.create kind layout 3 width height
  in
  let r_channel =3D Bigarray.Array3.slice_left_2 all_channels 0
  and g_channel =3D Bigarray.Array3.slice_left_2 all_channels 1
  and b_channel =3D Bigarray.Array3.slice_left_2 all_channels 2
  in
  (all_channels,
   r_channel,
   g_channel,
   b_channel)
;;
_________________________________________

Anyway I'm very pleased to see this png exporter, while it's allways useful=
l=20
to remove an external dependency !
Thanks for this piece of code !

=2D-=20
=46lorent