/*********************************************************/
/*   program 9: dither an image                          */
/*********************************************************/

#define  DITH   2
#define  XSIZE  640
#define  YSIZE  480

unsigned char *ir,*ig,*ib;
unsigned char *or,*og,*ob;

main()
  {
  long  int  x,y,i,j,d,v;
  long  int  dith[DITH][DITH] = {0,3,2,1};

  /* allocate input and output memory buffers */
  ir = (unsigned char *) malloc (XSIZE*YSIZE);
  ig = (unsigned char *) malloc (XSIZE*YSIZE);
  ib = (unsigned char *) malloc (XSIZE*YSIZE);
  or = (unsigned char *) malloc (XSIZE*YSIZE);
  og = (unsigned char *) malloc (XSIZE*YSIZE);
  ob = (unsigned char *) malloc (XSIZE*YSIZE);

  /* read the input image */
  read_image("input",ir,ig,ib);

  /* for each pixel, round according to dither matrix */
  for (y = 0; y < YSIZE; y++)
    {
    j = y % DITH;
    for (x = 0; x < XSIZE; x++)
      {
      i = x % DITH;
      d = dith[i][j] << 4;
      v = ((long)ir[(y*XSIZE)+x] + d);
       if (v > 255) v = 255;
       or[(y*XSIZE)+x] = (unsigned char)((v >> 7) << 7);
      v = ((long)ig[(y*XSIZE)+x] + d);
       if (v > 255) v = 255;
       og[(y*XSIZE)+x] = (unsigned char)((v >> 7) << 7);
      v = ((long)ib[(y*XSIZE)+x] + d);
       if (v > 255) v = 255;
       ob[(y*XSIZE)+x] = (unsigned char)((v >> 7) << 7);
      }
    }

  /* write the output image */
  write_image("output",or,og,ob);

  /* free memory buffers */
  free(ir);
  free(ig);
  free(ib);
  free(or);
  free(og);
  free(ob);
}