1. Racing Vehicle Control
Adam Balgach

2. Alex Darby FreeStyleGames Ltd (www.freestylegames.com) Downforce PS2
Moves Track Not Car

3. Track Space
Track modeled as long strip with fixed width=1 and an infinite length
2 Major pieces of knowledge about car
Horizontal position on the track (fWidthPos)
Location from some “start” point (fLengthPos)
This location is course and track dependent
fLengthPos = X.yy
X = position on track
Yy = %done with the current position

4. Track Space
For car not seen:
fWidthPos=.75
fLengthPos=42.333

5. Downforce AI System
Note Layers, all take same input

6. Layers in AI Operate in Psudeo-Parallel Behaviors
Code executed serially
Congitative System not used until all layers are done processing
Behaviors
Each layer composed of behaviors
Behaviors are independent but operate much the way the overall layers work
Layer Interaction – Layers can
Override the results of other layers
Layer mode switiching – defesive to aggressive

7. Path Following Layer
Correct controller inputs so a vehicle can follow a specific path at max speed.
Two behaviors
Steering Control
Corner Speed Regulation
Steering to follow vehicle racing line
Steering to converge back onto racing line if lost
Since working from same input data, subtasks can be independent
Saves in computational time
Less code

8. Path Following Cont... Corner Speed Regulation
Only following race line, cares about nothing else (ie obstacles, track conditions, etc)
Dead-Reckoning Approach
FSM based around corners
Corner Identification – including corner apex
Braking distance is calculated based on current speed
Braking For Corner – break until the current speed is the same as the necessary speed to clear the corners apex
Cruse To Apex – controls throttle so that it does not go too below corner breaking speed, and then accelerates once the apex of the corner is reached.
Corner – a section of track which the vehicle is unable to ravel through at its current speed without losing control

9. Fine Grain Avoidance Layer
Deals with potential collisions the other layers have not
Two behaviors
Race Priority
Collision Avoidance (side-to-side)
Race Priority selects which AI will back off incase of collision
Player has significantly higher priority
“Inside” car around a corner gets priority

10. Avoidance Cont… Side-to-Side collisions Front-to-Back collisions
Overrides steering/speed from Path Following Layer
Dependent on Race Priority
If “outer” car will collide with inner car, limits the speed and steering to run parallel slightly behind the inner car
Front-to-Back collisions
Rear car must make sure not to hit car in front of it.
Override Breaking/Throttle from P.F.L.
Avoids collisions that have not been done by the Optimum Road Position Layer
Side-to-side: Only does this if does not have race priority
Front-to-Back: only occurs when cars are very close, to encourgae high speed passing/drafting.

11. Tactical Racing Layer
Assists in change of speed & racing line in a pack of cars to gain an overall advantage
Three behaviors
Optimum Road Position
New Race Line
Collision Avoidance Speed Control

12. Optimum Road Position
Each Car has 4 simple “eyes” to find relative speed of other cars, and their proximity
Each eye stores a 1D that represents the entire track width.
The 4 eyes give a low resolution image of what is ahead, next to and behind the car

13. Determining Optimal Line
4 “Eye” images combined, current racing line added in.
Determine best place to “be” based on the locations of others and racing line
Simple mathematical average of current positions
Must be fine tuned for each track

14. Path Generation
Once a new position have been identified to give vehicle an “advantage” a new race line must be constructed
Old racing line is buffered, and simply interpolated in track space toward the new optimal position
Sometimes this new line may go off the track, in which case other layers have to deal with it

15. Collision Avoidance Handles most of front-to-back collisions
Only occurs if vehicle in front is currently going slower.
Ignores curves in track, simply brakes to avoid a car in front based on current speeds and fWidthPos of both cars
Not super efficient, but cheap, reliable and sufficiently accurate

16. Driving Assist Layer
Controls obscure events such as brake lock, wheel spins and washing out
Two behaviors
Traction Control
Anti-Lock Braking
Traction Control minimizes slippage
Once the slip in the lengthwise axis > threshold value, actual slip is calculated

17. Slip Cont…
Also a min throttle value to start slipping to force high power-weight cars wouldn’t slip a greater amount then expected when at a standstill.
Type of car (4wd, fwd, rwd) must be taken into account, as to which wheels to measure slip from and how to determine curvature of car from slippage. (direction of the skid)

18. Anti-Lock Breaking
Same Code as Traction Control System, but by scaling brake value under breaking, instead of throttle under acceleration
In straight line, forces wheels to lock up, but breaking efficiency is less then with a drum break wheel lockup. This is not how the real world works, but was deemed acceptable in the game.

19. Demo 2
Show the Demo…

20. AI System Wrap Up Modular Independent Scalable
Representational abstractions
Sensor “eyes”
Track Space

21. Optimal Racing Lines
Not easy task, math/physics intensive and track dependent
How to overcome
Limit custom built tracks, only allow users to use predefined segments
Boring, lack of variety for users
Just because pieces have racing lines, doesn’t mean an optimal race line will occur from sticking pieces together
Predetermined segments, have predetermined racing lines

22. Race Lines Cont… Search for line of minimum curvature
Does not account for car (width, weight), or speed. Is simply the most effective path along a track
Very quick, efficient and highly reliable.

23. How to do it…
Initially put points equally spaced in the center of the track from start to finish
In sets of three points, gradually reduce the curvature by using the sum of the squares of the angles between them.

24. Points, and Vectors… Joined by vector AB, BC
These vectors make up angle alpha
Applying a small force to f proportional to alpha:a,b,c the points move in such a way
That alpha is reduced. This process if repeated over and over, until an optimal path is produced

25. Anyone Remember Physics 21?
Now show demo…

26. Model App
Show the App

27. Surviving a Simulation
Simulation AI very important
Controls all the other things in a single player game
Is used as to show how the game development is “progressing”
Overall quality of game play is based on how good the AI is

28. Design & Implementation
Code well
Create a Prototype
Test
Force the AI to use the same inputs a player would use
Make the AI do real-world logical things.
Layer the AI Design
No mistakes, logical names, comments, etc.
Layering – as seen in the previous AI

29. Layered Example

30. What else can be done?
Include a debugger
Make AI visible to players

31. Summary A Good AI System is made of Layers Mathematics Influenced
Layers operate in parallel
Goal To generate lifelike racing AI with ability to pass and force the player to react more
Mathematics Influenced
Optimizing Best Race Line