/*********************************************************/
/*   program 8: halftone an image                        */
/*********************************************************/

#define  HTONE  4
#define  XSIZE  640
#define  YSIZE  480

unsigned char *in;
unsigned char *out;

main()
  {
  long  int  x,y,h,hx,hy;
  char       mask;
  unsigned char p;

  /* initialize halftone group patterns */
  unsigned char htone[HTONE][HTONE*HTONE] =
   {0x0,0x0,0x0,0x0,0x0,0x4,0x4,0x4,0x4,0x4,0x6,0x6,0x6,0x7,0x7,0xf,
    0x0,0x4,0x4,0x6,0x6,0x6,0x7,0x7,0x7,0xf,0x7,0xf,0xf,0xf,0xf,0xf,
    0x0,0x0,0x2,0x2,0x6,0x6,0x6,0xe,0xf,0x7,0xf,0xf,0xf,0xf,0xf,0xf,
    0x0,0x0,0x0,0x0,0x0,0x0,0x0,0x0,0x0,0x2,0x2,0x2,0x6,0x6,0xe,0xf};

  /* allocate input and output memory buffers */
  in = (unsigned char *)malloc(XSIZE*YSIZE);
  out= (unsigned char *)malloc(XSIZE*YSIZE*HTONE*HTONE);

  /* read the grey scale input image */
  read_image_gs("input",in);

  /* halftone each pixel in the input image */
  for (y = 0; y < YSIZE; y++)
    {
    for (x = 0; x < XSIZE; x++)
      {

      /* get the input pixel to halftone
           note: for odd lines, offset the pattern by 1/2 the group size */
      if ((y % 2) == 0)
        h = (long)in[(y*XSIZE)+x] >> 4;
      else
        h = ((long)in[(y*XSIZE)+x] + (long)in[(y*XSIZE)+x+1]) >> 5;

      /* generate the halftone pattern for this input pixel */
      for (hy = 0; hy < HTONE; hy++)
        {
        mask = htone[hy][h] << 4;
        for (hx = 0; hx < HTONE; hx++)
          {
          if ((mask & 0x80) != 0)
            p = 0xff;
          else
            p = 0x00;

          /* for odd lines, offset the pattern by 1/2 the group size */
          if ((y % 2) == 0)
            out[(((y*HTONE)+hy)*XSIZE*HTONE)+(x*HTONE)+hx] = p;
          else
            out[(((y*HTONE)+hy)*XSIZE*HTONE)+(x*HTONE)+hx+(HTONE/2)] = p;

          mask = mask << 1;
          }
        }
      }
    }

  /* write the output image */
  write_image_gs_ht("output",out,HTONE);

  /* free memory buffers */
  free(in);
  free(out);
}