While working on a graphics related problem, I needed to save the
contents of a Graphics framebuffer to a .png file.  Quick googling for
"ocaml png" didn't bring up any results for libraries that would be
easy to install via GODI.  I am aware of CamlImages but unfortunately
I have never been successful at installing it due its heavy dependence
on external libraries.  I also often work on Windows and installing
external OCaml libraries on Windows is usually a major PITA (if
possible at all).

I decided to write a simple self-contained .png exporter that wouldn't
depend on any external libraries (not even libpng or zlib).  I thought
someone else might benefit from this piece of code and I thus made it
freely available here:

http://code.google.com/p/aihiot/source/browse/trunk/gfx/save_bitmap/o...

Janne

For reference, I've found the easiest way to export PNGs (in any
language, not just OCaml) is to use netpbm.  Simply fork pnmtopng
using Unix.open_process_out and write a PPM file.  A PPM file has such
a simple format that you can write it directly from just about any
language, even a shell script.

  P3 <width> <height> 255
  followed by <width>x<height>x3 RGB triplets (in decimal, separated
  by writespace)

As a concrete example in OCaml:

  open Printf

  let () =
    let chan = Unix.open_process_out "pnmtopng > /tmp/image.png" in
    fprintf chan "P3 128 128 255\n";
    for i = 0 to 127 do
      for j = 0 to 127 do
        let r = float (i+j) /. 256. in
        let g = float (i*2) /. 256. in
        let b = float j /. 128. in
        fprintf chan "%d %d %d\n"
          (int_of_float (r*.255.))
          (int_of_float (g*.255.))
          (int_of_float (b*.255.))
      done
    done;
    ignore (Unix.close_process_out chan)

Rich.

--
Richard Jones
Red Hat

--
Dr Jon Harrop, Flying Frog Consultancy Ltd.
http://www.ffconsultancy.com/?e

Int32.of_string "[number]" is unnecessary, OCaml(not sure which version
this was introduced in though) can read various specific size ints
natively:

# 0n, 0l, 0L;;
- : nativeint * int32 * int64 = (0n, 0l, 0L)

Secondly `Printf.fprintf chnl "string"' looks rather expensive compared
to `output_string chnl "string"'.

--
mailto:av1...@comtv.ru

Replacing the functional style of accumulating CRC and Adler to use
imperative style would allow for a clean rewrite that would be much
faster.

Janne

- My OCaml code is based on a C function written by Tero Karras.  I
  merely rewrote his masterpiece in OCaml.  The C version is available
  here:

  http://code.google.com/p/aihiot/source/browse/trunk/gfx/save_bitmap/c...

- The trick is to save out uncompressed PNGs.  This gets rid of the
  zlib dependency.  Although the files get bigger, uncompressed .png
  is still very useful.

If you meant GODI on Windows, well..

I like GODI and am a long-time user of it.  However, installation on
Windows is not very easy and (at least last time I tried) requires
Cygwin.  I prefer to work natively on Windows and I really dislike
emulating Unix with Cygwin (slow, broken, you name it).

Now, if I was the only one hacking on my code on Cygwin+GODI, things
would be pretty OK and I could live with the occasional "I can't get
GODI to install on my broken Cygwin installation/this package doesn't
compile on Cygwin/...".  The problem is that all my collegues use
Windows and aren't familiar with OCaml, GODI or Cygwin.  If something
goes wrong during GODI or a particular package installation, they
won't be able to solve the issue without my help.  This has pretty
effectively ruined my attempts to use OCaml for internal tools.

My OCaml environment nowadays consists of the following:

- OCaml MSVC version (Win32 installer, no Cygwin required)

- OMake (Win32 installer, no Cygwin required during installation or use)

- No external libraries (with the exception of ExtLib which is just a
  bunch of .ml files and thus easy to build)

This obviously limits what I can do with OCaml.  In fact, I have often
needed to resort to using Python (the horror!) for some of my scripts.
Everyone's already got Python installed and so my scripts work without
any installation.

It's too easy to dismiss Windows installation problems by suggesting
everyone to just adopt Linux.  Unfortunately that's not an option for
me.  I see value in being able to port from Unix to Windows and vice
versa.

Updating Adler and CRC in functional style was probably not a good
idea, an imperative version would be easier to read and maintain.  Not
to mention feeling a bit naughty when doing I/O inside a fold_left. :)

Saving a .tga is quite easy, copy&pasting from
http://code.google.com/p/aihiot/source/browse/trunk/gfx/save_bitmap/o...

(** Save a .tga file to chnl. *)
let write_tga_chnl chnl pixels w h =
  let header =
    [0; 0; 2; 0; 0; 0; 0; 0; 0; 0; 0; 0;
     w land 255; w lsr 8;
     h land 255; h lsr 8; 32; 8] in
  List.iter (fun e -> output_byte chnl e) header;
  for y = 0 to h-1 do
    for x = 0 to w-1 do
      let c = pixels.(x+(h-1-y)*w) in (* h-1-y = Flip image *)
      output_byte chnl (c land 255);
      output_byte chnl ((c lsr 8) land 255);
      output_byte chnl ((c lsr 16) land 255);
      output_byte chnl 255;
    done
  done

Janne

Le 22 sept. 08 à 16:59, Janne Hellsten a écrit :

Daniel

[1] http://en.wikipedia.org/wiki/Pngcrush

  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) =
  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_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=96) () =
  let cmd = Printf.sprintf "cjpeg -quality %d" quality in
  (*
  let cmd = Printf.sprintf "ppmtojpeg -quality %d" quality in
  let cmd = Printf.sprintf "convert ppm:- -quality %d jpg:-" quality in
  *)
  let ic, oc = Unix.open_process cmd in
  output_ppm ~img ~oc;
  try
    while true do
      let c = 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 =
  let cmd = Printf.sprintf "convert ppm:- %s:-" format in
  let ic, oc = Unix.open_process cmd in
  output_ppm ~img ~oc;
  try
    while true do
      let c = input_char ic in
      print_char c
    done
  with End_of_file -> ()
;;

let new_img ~width ~height =
  let all_channels =
    let kind = Bigarray.int8_unsigned
    and layout = Bigarray.c_layout
    in
    Bigarray.Array3.create kind layout 3 width height
  in
  let r_channel = Bigarray.Array3.slice_left_2 all_channels 0
  and g_channel = Bigarray.Array3.slice_left_2 all_channels 1
  and b_channel = 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 usefull
to remove an external dependency !
Thanks for this piece of code !

--
Florent