_ATTACHED SPRITES_
by Diana Gruber

Listing One

/********************  sprite declarations *************************/
int nsprites;
typedef struct _sprite
{
   char *bitmap;
   int width;
   int height;
   int xoffset;
   int yoffset;

}  SPRITE;
SPRITE *sprite[40];

/* forward declarations */
struct OBJstruct;
typedef struct OBJstruct OBJ, near *OBJp;

/* pointer to object action function */
typedef void near ACTION (OBJp objp);
typedef ACTION *ACTIONp; 

/* data structure for objects */
typedef struct OBJstruct 
{
  OBJp  next; 
  OBJp  prev; 
  int x;
  int y;
  int xspeed;
  int max_xspeed;
  int yspeed;
  int direction;
  int frame;
  int tile_xmin;
  int tile_xmax;
  int tile_ymin;
  int tile_ymax;

  SPRITE *image;

  ACTIONp action; 
  OBJp attached_sprite;
};
SPRITE *explosion[11];

/**********************************************************************/
void near start_explosion(OBJp objp)
{
   OBJp node;

   /* allocate space for the object */
   node = (OBJp)malloc(sizeof(OBJ));
   if (node == (OBJp)NULL) return;

   /* assign values to the structure members */

   /* after the plane has been killed, the explosion moves at a slower
      speed because smoke drifts slower than metal */
   node->xspeed = objp->xspeed/2;
   node->yspeed = objp->yspeed/2;

   /* tile extents */
   node->tile_xmin = 2;
   node->tile_xmax = 21;
   node->tile_ymin = 0;
   node->tile_ymax = 14;

   /* the sprite will be the first frame explosion bitmap */
   node->image = explosion[0];
   node->x = objp->x+16;
   node->y = objp->y-4;
   node->frame = -1;

   /* insert at the top of the linked list */
   node->prev = top_node;
   node->prev->next = node;
   top_node = node;
   node->next = (OBJp)NULL;

   /* set up the links between the explosion and the enemy plane */
   node->attached_sprite = objp;
   objp->attached_sprite = node;

   /* assign the action function */
   node->action = do_explosion;
}
/**********************************************************************/
void near do_explosion(OBJp objp)
{
   /* If the explosion has reached the frame 3 state, at which point
      the bitmap is bigger than the airplane, it is time to kill the
      airplane. */

   if (objp->frame > 3)
   {

      /* if the attached sprite is NULL that means the airplane was
         already killed */
      if (objp->attached_sprite != (OBJp)NULL)
         objp->attached_sprite->action = &kill_enemy;

      objp->x += objp->xspeed;
      objp->y += objp->yspeed;
   }
   else
   {
      /* The position of the explosion depends on the position of
         the airplane */
      if (objp->attached_sprite != (OBJp)NULL)
      {
         objp->x = objp->attached_sprite->x+16;
         objp->y = objp->attached_sprite->y-4;
      }

      /* it is possible for the explosion to be at less than frame 3
         but there is no attached sprite.  That happens when the enemy
         plane has drifted off the edge of the screen. */
      else
      {
         objp->x += objp->xspeed;
         objp->y += objp->yspeed;
      }
   }

   /* Increment the explosion frame */
      objp->frame++;

   /* define which sprite will be displayed this frame */
   objp->image = explosion[objp->frame];

   /* We have 10 frames for the explosion */
   if (objp->frame > 10)
   {
      objp->image = explosion[10];
      objp->action = kill_explosion;
   }
}


Example 1:

typedef struct OBJstruct 
{
  OBJp  next; 
  OBJp  prev; 
  int x;
  int y;
  int xspeed;
  int max_xspeed;
  int yspeed;
  int direction;
  int frame;
  int tile_xmin;
  int tile_xmax;
  int tile_ymin;
  int tile_ymax;

  SPRITE *image;
  ACTIONp action; 
  OBJp attached_sprite;
};


Example 2:

typedef struct _sprite
{
   char *bitmap;
   int width;
   int height;
   int xoffset;
   int yoffset;

}  SPRITE;



Example 3:

/* set up the links between the explosion and the enemy plane */
node->attached_sprite = objp;
objp->attached_sprite = node;


Example 4:

if (objp->attached_sprite != (OBJp)NULL)
   objp->attached_sprite->action = &kill_enemy;


Example 5:

/* The position of the explosion depends on the position of the airplane */
if (objp->attached_sprite != (OBJp)NULL)
{
   objp->x = objp->attached_sprite->x+16;
   objp->y = objp->attached_sprite->y-4;
}