The Obstacle Problem in Tweening
As one might expect, there are things to keep in mind when creating Tweens. One of these is "the obstacle problem."
Let’s say you place a ramp exactly halfway between the position of the car in Key Frame A and the position of the car in Key Frame B. Now you create a single Tween frame, A/B, as previously described. What’s going to happen? The car and ramp are going to be forced to occupy the same place at the same time, that's what. In the real world, that can mean only one thing: the car hits the ramp, angles up, flies through the air, then angles down and lands.
But here’s what happens in FrameForge:
FIG V: Bad Tween 1
Actually, FrameForge made a valiant effort to hop the car over the ramp via Collision Checking, but even so, the physics are all wrong. The car isn’t slanting up the ramp like it would in the real world--instead, it’s simply taken up a position exactly 50 percent of the way between where it was in Key Frame A and where it ends up in Key Frame B.
The point being this: When it comes to Tweens, FrameForge doesn’t know what you want it to do, only what you tell it to do.
Question: Okay, then, how do I avoid the Obstacle Problem?
Basically, by creating as many Key Frames as necessary to teach FrameForge how you want moving objects to negotiate any obstacles on the set. For example, what if I set up not two but three Key Frames for the car-jump shot, as follows:
FIG VI: Car going up ramp
In my new Key Frame B, I have positioned the car so it’s beginning to fly off the ramp, just the way I want it to. So now I generate a Tween between A and B, and...
FIG VII: Car taking off too soon
Huh? It’s still not right. Why not? That's discussed in the next topic, "The Relative Object Position Problem."