1. Animation and Simulation Plus Interaction
Norm Badler
CSE 377
March 3, 2003

2. Discrete Simulation Event is primary entity.
Time jumps forward to next event.
Not good for animation per se as events may happen at arbitrary time points (e.g. customers arriving at a server)
Time line

3. Animation
Frames must be produced at fixed time intervals, whether or not anything is actually happening.
In general, this means that animated actions must be represented as functions of TIME.
Time line
Frame times

4. So what about the EVENTS?
Events are changes in the world environment caused by:
User actions
Objects in the world
Interactions between objects (and user)
E.g.,
Button push or mouse action
Object property (movements or other attribute changes)
Collisions or “in view”

5. Events Cause Branches in Action
Stuff changes because of events in the world or user actions.
HOW DO WE REPRESENT THE ACTION ALTERNATIVES?
Representational options:
Sequential logic (IF THEN ELSE)
Finite State Machines
Rule-based systems

6. Sequential Logic IF condition-a THEN action-1 ELSE IF condition-b
ELSE IF condition-c
THEN …
PROBLEMS: Awkward, brittle, hard to debug and modify.

7. Finite State Machines Flexible for small-ish action sets.
Simple semantics.
Can be hierarchical (can invoke sub-FSMs)
Allow clear think about the transitions in the action space.

8. Example Finite State Machine for a Game
Watt and Policarpo

9. FSM Generally nodes are processes.
Edges are transitions from one process to another: occur at events (as defined before)
Edge transitions may be deterministic or dependent on a statistical distribution of options.
Transitions may have side effects besides changing program state.

10. Edge Transitions
Happen if something is observed (visible to entity): “Looking for enemy” changes to “Tracking enemy”
Happen if objects interact: “Enemy destroyed” changes state to “Looking for [new] enemy” {note side effect!}
Happen if user interacts: Button push may fire missile that collides with enemy causing it to be destroyed.

11. Implementing a FSM
Determine process states and all possible transitions.
Determine all possible variables that need checking in a state.
Organize in a table. (May not be as efficient as sequential logic but easier to design and maintain!)

12. State
Var –1:
Enemy
in view
Var –2:
User fires
missile
(in flight)
Var –3
Collision
detected
Var –4
Missile
position
Var –5:
User position
Actions
Required or Allowed
Side
Effects T
(x,y,z)[t]
(x,y,z)[inter-face]
Run vaporize
graphic
Enemy deleted;
Missile deleted; User arsenal--
User score++ F
Enemy/user
moves
Missile deleted;
User arsenal-- -

13. Existing FSM Packages Game engines (MOTIVATE) PaT-Nets
KPL (Ken Perlin’s Language)
Write your own…