/*********************************************************/
/*   program 13: blue screen composite                   */
/*********************************************************/

#define  XSIZE  640
#define  YSIZE  480
#define  LUTMIN 80
#define  LUTMAX 150

unsigned char *fgr,*fgg,*fgb;
unsigned char *bgr,*bgg,*bgb;
unsigned char *or,*og,*ob;
unsigned char *m;

main()
  {
  long  int  x,y;
  long  int  i;
  float val,incr;
  unsigned char lut[256];

  /* allocate input and output memory buffers */
  fgr = (unsigned char *) malloc (XSIZE*YSIZE);
  fgg = (unsigned char *) malloc (XSIZE*YSIZE);
  fgb = (unsigned char *) malloc (XSIZE*YSIZE);
  bgr = (unsigned char *) malloc (XSIZE*YSIZE);
  bgg = (unsigned char *) malloc (XSIZE*YSIZE);
  bgb = (unsigned char *) malloc (XSIZE*YSIZE);
  or  = (unsigned char *) malloc (XSIZE*YSIZE);
  og  = (unsigned char *) malloc (XSIZE*YSIZE);
  ob  = (unsigned char *) malloc (XSIZE*YSIZE);
  m   = (unsigned char *) malloc (XSIZE*YSIZE);

  /* read the foreground and background images */
  read_image("foreground",fgr,fgg,fgb);
  read_image("background",bgr,bgg,bgb);

  /* build the lut that will generate mask in step 3 */
  for (i = 0; i < LUTMIN; i++) lut[i] = 255;
  incr = 255.0 / (float)(LUTMAX - LUTMIN);
  val = 255;
  for (i = LUTMIN; i < LUTMAX; i++)
    {
    lut[i] = (unsigned char)(val + 0.5);
    val = val - incr;
    }
  for (i = LUTMAX; i < 256; i++) lut[i] = 0;

  /* perform composite for all pixels in image */
  for (y = 0; y < YSIZE; y = y++)
    {
    for (x = 0; x < XSIZE; x = x++)
      {

      /* step 1: r = -r */
      m[(y*XSIZE)+x] = -fgr[(y*XSIZE)+x];

      /* step 2: r = r * b / 256 */
      m[(y*XSIZE)+x] = (unsigned char)
        ((long)m[(y*XSIZE)+x] * (long)fgb[(y*XSIZE)+x] >> 8);

      /* step 3: invert/contrast enhance r */
      m[(y*XSIZE)+x] = lut[m[(y*XSIZE)+x]];

      /* step 4: generate synthetic blue (min g,b) */
      if (fgg[(y*XSIZE)+x] < fgb[(y*XSIZE)+x])
        fgb[(y*XSIZE)+x] = fgg[(y*XSIZE)+x];

      /* step 5: alpha filter */
      bgr[(y*XSIZE)+x] = (unsigned char)
        ((((long)fgr[(y*XSIZE)+x] * (long)m[(y*XSIZE)+x]) +
          ((long)bgr[(y*XSIZE)+x] * (255-(long)m[(y*XSIZE)+x]))) >> 8);
      bgg[(y*XSIZE)+x] = (unsigned char)
        ((((long)fgg[(y*XSIZE)+x] * (long)m[(y*XSIZE)+x]) +
          ((long)bgg[(y*XSIZE)+x] * (255-(long)m[(y*XSIZE)+x]))) >> 8);
      bgb[(y*XSIZE)+x] = (unsigned char)
        ((((long)fgb[(y*XSIZE)+x] * (long)m[(y*XSIZE)+x]) +
          ((long)bgb[(y*XSIZE)+x] * (255-(long)m[(y*XSIZE)+x]))) >> 8);
      }
    }

  /* write the output image */
  write_image("output",bgr,bgg,bgb);

  /* free memory buffers */
  free (fgr);
  free (fgg);
  free (fgb);
  free (bgr);
  free (bgg);
  free (bgb);
  free (or);
  free (og);
  free (ob);
  free (m);
}