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Get the Mouse!

May 18, 1998
by Pat McClellan

Dear Multimedia Handyman,

I'm in need of a little help. I am currently working on a project involving multiple parent/child objects moving around the screen towards varies targets, ie the mouse. I have seen some similar projects done which can be seen at

My problem is that the child objects tend to overlap each other when moving toward the same target. What I want them to do ultimately, is to bump off each other when they collide, or to steer away from each other to avoid collisions. Is there any way that these child objects can detect each other or know the position of other child objects, and change their behaviour occordingly, all using the same parent script?

Clint Hannaford
Atomic Media

Dear Clint,

My friend Ed Bean is currently working on just such a program, so I thought I'd let him have a crack at this question. Ed writes...

In the case of non-colliding pathfinding objects, you might approach the problem like this...

Each of your objects needs to be able to test whether it is colliding with any other object. To do this, it needs access to a list of objects it can collide with.

At each frame, each of the objects needs to see if its next calculated position will collide with any of the other objects, by iterating through the object list, and calling a test handler on each of it's peers e.g., collideTest(the prospectRect of sprite mySprite, the calcd rect of otherObj) (returns boolean) If so, it's necessary to calculate a new prospective position for the colliding sprite, and test against the target object until on collideTest returns false. The method for calculating the new position might be to calculate the slope of the line formed by the center points of the two sprites, then march the test object away from the colliding object along that line until there is no collision returned. Obviously, it's possible to have resolution of one collision actually cause another. It may be necessary to iteratively resolve the object collisions until there are none. A Manager object would be best for overseeing this sort of issue.

My approach would be to place a manager object in the actorlist. Each time it receives a stepFrame message, it should in turn cause the objects to calculate new positions. Then it should iterate through the object list, requesting each object to resolve any resulting collisions. Finally, the objects should be drawn in their new positions.

(for the manager obj)

on stepframe me
  updateEnemies me
  resolveCollisions me 
  drawEnemies me

on updateEnemies me
  repeat with whichObj in p_AttackerList
    updatePosition (whichObj)
  end repeat

on resolveCollisions me
  set collisionsExist = true
  repeat while (collisionsExist = true)
    set collisionsExist = false
    repeat with whichObj in p_AttackerList
      -- call handler on each attacker, and pass
      -- in list of objects to test for collision
      -- recurse until no collisions are reported
      set enemycollision = resolveEnemycollisions( ¬
      if (enemycollision = true) then set ¬
            collisionsExist = true
    end repeat
  end repeat

on drawEnemies me
  repeat with whichObj in p_AttackerList
    draw (whichObj)
    -- tell all objects to draw themselves
    -- in their now collision-free positions
  end repeat

The other half of the equation is detecting the collision themselves, and having the enemies respond appropriately. I've included a sample movie which demonstrates the technique, and which you can examine for more in-depth information.

Realistic bouncing (ala a billiards game) is a little more involved, and perhaps a different issue...

Hope this helps!

And soon I'll get Ed to write a more detailed article on bouncing. For now, this should get you started. Thanks for your question and special thanks to Ed for his insight.

The source code for this article (with additional code) is also available in Mac or PC format.

Patrick McClellan is Director Online's co-founder. Pat is Vice President, Managing Director for Jack Morton Worldwide, a global experiential marketing company. He is responsible for the San Francisco office, which helps major technology clients to develop marketing communications programs to reach enterprise and consumer audiences.

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