1. Artificial Intelligence in Game Design
Goal-Oriented Behavior

2. Goals Complex characters have goals Issue: Add to realism of character
Actions appear motivated instead or reflexive
Player must be aware of character motivations
Example: Napoleon “boss” NPC
Capture other cities
Protect own cities
Increase gold level
Issue:
What if goals conflict?
Get gold
Avoid player
???

3. Goals and the Sims Overall goals (player meets)
Positive and negative goals
Goals based on physical needs
Hunger
Energy
Fun
Etc.
Characters automatically choose actions to meet needs

4. Goals and Actions Goals determine actions than fulfill need
Game keeps list of actions for each need
Which actions should NPC take to fulfill goals?
Hunger
Eat snack
Cook dinner
Energy
Nap in chair
Sleep in bed
Fun
Go to concert
Paint
Read

5. Insistence of Needs Needs may have “degree of urgency” (insistence)
How critical is need?
How important is this need relative to others?
How much of an action is needed to fulfill need?
Represented as number
Higher = greater need
Zero = no need
Need
Insistence
Hunger 5
Energy
Fun 3

6. Actions and Goals Actions can permanently fulfill goals
Replaced with another related goal (Sims)
Ends game (Napoleon captures all cities in world)
Actions can temporarily lessen needs
Action: Need – n lessens that need by n
Hunger
Eat snack: Hunger – 1
Cook dinner: Hunger – 5
Energy
Nap in chair: Energy – 1
Sleep in bed: Energy – 5
Fun
Go to concert: Fun – 4
Paint: Fun – 2
Read: Fun – 1

7. Choosing Actions Simple approach: Choose goal with highest insistence
Choose action that gives largest decrease
Need
Insistence
Hunger 6
Energy
Fun 3
Greatest need
Best action
Eat
Eat snack: Hunger – 1
Cook dinner: Hunger – 5
Energy
Nap in chair: Energy – 1
Sleep in bed: Energy – 5
Fun
Go to concert: Fun – 4
Paint: Fun – 2

8. Side Effects Actions can effect multiple goals Side effect
Drink Coffee
Hunger – 1
Energy – 1
Eat at Restaurant
Hunger – 4
Fun – 3
Gook Gourmet Meal
Hunger – 5
Fun – 1
Energy + 1
Side effect
Fulfilling one goal increases other needs

9. Overall Utility Utility-based decision making
Determine effect of each action on total needs
Choose action that has best overall effect
One criteria: sum of all needs
Need
Current level
After Cook Gourmet Meal
After Drink Coffee
After Eat at Restaurant
After Sleep in Bed
Hunger 3 2
Energy 1
Fun 4
Total effect 6 7
Best action

10. Overall Utility Problem: This can ignore very severe needs
Behavior does not appear rational!
Need
Current level
After Eat at Restaurant
After Sleep in Bed
Hunger 3
Energy 10 5
Fun
Total effect 11
Will be chosen, even though Sim is very tired!

11. Overall Squared Utility
Common solution: Total discontentment = sum of square of needs
Need
Current level
After Eat at Restaurant
After Sleep in Bed
Hunger 3 02 32
Energy 10
102 52
Fun
Total effect
100 43

12. Planning and Preconditions
Actions may have preconditions before they can be taken
Multiple actions required to meet needs
Action
Preconditions
Postconditions
Preconditions which character meets
Action
Postconditions
meets
Action
Preconditions
Postconditions
meet need 12

13. Planning and Preconditions
Example: Microwaving food
Uncooked food in fridge
Get food
Have uncooked food
meets
Have uncooked food
Have microwave
Cook food in microwave
Have cooked food
meets
Have cooked food
Eat food
Hunger - 5

14. Planning Methods Scripted action sequences
Actually single action with what appears to be multiple steps
Long animated sequence
Very simple to implement (no planning needed!)
Weaknesses:
Cannot interrupt sequence without starting over entirely
Phone rings while cooking
Cannot start plan until all preconditions met
Get food
Cook food in microwave
Eat food
Have uncooked food
Have microwave
Hungry - 5