Bitmap/PPM conversion through a pipe

Bitmap/PPM conversion through a pipe
You are encouraged to solve this task according to the task description, using any language you may know.

Using the data storage type defined on this page for raster images, delegate writing a JPEG file through a pipe using the output_ppm function defined on this other page.

There are various utilities that can be used for this task, for example: cjpeg (package "jpeg-progs" on Linux), ppmtojpeg (package "netpbm" on Linux), convert (from ImageMagick, multi-platform).

C

This one uses the ImageMagick convert tool.

<lang c>/* interface */ void print_jpg(image img, int qual);</lang>

<lang c>#define MAXCMDBUF 100 void print_jpg(image img, int qual) {

  char buf[MAXCMDBUF];
  unsigned int n;
  FILE *pipe;
  
  snprintf(buf, MAXCMDBUF, "convert ppm:- -quality %d jpg:-", qual);
  pipe = popen(buf, "w");
  if ( pipe != NULL )
  {
          fprintf(pipe, "P6\n%d %d\n255\n", img->width, img->height);
          n = img->width * img->height;
          fwrite(img->buf, sizeof(pixel), n, pipe);
          pclose(pipe);
  }

}</lang>

The code that writes to the pipe is the same of output_ppm of course. A complete example is

<lang c>#include "imglib.h"

int main() {

     image img;
     
     img = alloc_img(100,100);
     fill_img(img, 50, 20, 200);
     draw_line(img, 0, 0, 80, 80, 255, 0, 0);
     print_jpg(img, 75);
     free_img(img);

} </lang>

In order to make it working, you must link it with the raster image functions given by the codes here and here

OCaml

<lang ocaml>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 -> ()

</lang>