Need Help: Circular Platform: Pass Thru then land on

dotsonj23

In my game, my actor needs to be able pass through an circle platform when it approaches the platform from below but once it completely passes through the platform from below (but only if it completely passes through) and is on its way back down collide with (i.e., land on) the circular platform. I can't key collide of the relationship between: (a) the actor's Y-(1/2*Actor's Height); and (b) the circle platforms Y+(1/2*Circle's Height), because that would only work if the actor was passing through at exactly the Circle's X position (i.e, when the actor passes through to the left or right of the Circle's X position it completely passes through the Circle before it would pass through if it had been passing though exactly at the Circle's X position). I hope that make sense. Easy to see the problem at work but hard to explain in words. In any event, if anybody know how to do this or has a suggestion, I would much appreciate it.