Racing Vehicle Control

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Racing Vehicle Control

• Adam Balgach
• asb7_at_cse.lehigh.edu

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Alex Darby

• FreeStyleGames Ltd (www.freestylegames.com)
• Downforce
• PS2
• Moves Track Not Car

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Track Space

• Track modeled as long strip with fixed width1
  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

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Track Space
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Downforce AI System
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Layers in AI

• Operate in Psudeo-Parallel
• 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

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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 Control
• Steering to follow vehicle racing line
• Steering to converge back onto racing line if lost

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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.

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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

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Avoidance Cont

• Side-to-Side 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.

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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

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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

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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

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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

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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

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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 gt threshold
  value, actual slip is calculated

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Slip Cont

• Also a min throttle value to start slipping to
  force high power-weight cars wouldnt 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)

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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.

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Demo 2

• Show the Demo

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AI System Wrap Up

• Modular
• Independent
• Scalable
• Representational abstractions
• Sensor eyes
• Track Space

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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, doesnt
  mean an optimal race line will occur from
  sticking pieces together

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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.

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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.

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Points, and Vectors
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Anyone Remember Physics 21?
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Model App

• Show the App

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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

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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

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Layered Example
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What else can be done?

• Include a debugger
• Make AI visible to players

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Summary

• A Good AI System is made of Layers
• 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