sports games: artificial intelligence and physics
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Sports Games: Artificial Intelligence and Physics. Sports Game Design and Development Architectures Class Structures Agent Cooperation Team based cooperation and Optimization Physics Object physics Agent physics Dead Reckoning Strategy Development Artificial and Human Elements. Topics. - PowerPoint PPT PresentationTRANSCRIPT
Sports Games: Artificial Intelligence and
Physics
Topics
Sports Game Design and Development◦Architectures◦Class Structures
Agent Cooperation◦Team based cooperation and Optimization
Physics◦Object physics◦Agent physics◦Dead Reckoning
Strategy Development◦Artificial and Human Elements
Challenges
Simulating a natural, well-known environment◦Modeling fields, courts, rinks, etc.◦Modeling player personalities and animations◦Physics engine
Strategy◦Sports are a form of Real-Time strategy◦Strategies while confined to a set of rules and
the size of the field, are numerous and constantly changing
A Brief History
A well known fact: Video game detail has improved over the years
From Tecmo Bowl to Madden 2008◦Better player models◦Larger fields, increased atmosphere◦More intelligent AI◦More modes, styles of play
Arcade versus Simulation Longer play modes Online play
Building an Sports AI Architecture
Building a Sports AI Architecture
A Sports game has many CPU-Intensive tasks◦Player animation◦Route planning◦Physics modeling◦Strategy development
Breaking these down in a high level architecture confined by the rules of the game can help ease the complexity of the development
Building a Sports AI Architecture
Layering the architecture
Building a Sports AI Architecture
Plans◦ Offensive
Pass Shoot Drive to the basket
◦ Defensive Position the defender Double team the ball handler
◦ Shared Rebound Set up for a free throw
Plans enable the players to move freely and determine their own best routing methods
This level of abstraction provides a sense of realism to the game
Building a Sports AI Architecture
Building a Sports AI Architecture
Example planning classClass AgentPlan{
float EvaluateInitiation();float EvaluateContinuation();void Initiate();void Update();
}Initiation and continuation are very important concepts
because they determine if a plan is working and where to go next◦ Common return values are between -1.0 and 1.0
Update is called for every tick to update the AI
Building a Sports AI Architecture
High level strategy module sets a goal◦ An opportunity is seen to complete a through pass in front of
the goal for a scorePlan is created
◦ An opportunity is seen to complete a through pass in front of the goal for a score
Route finding plots course◦ Each player (opponent and team) is considered,
momentum(team and player), and even ball planningExecute plan
◦ Not all plans will work out◦ Depending on the skill level of the other team and unforeseen
occurrences, this leaves the game interesting
Building a Sports AI Architecture
States◦A game can be broken into states with
responsibilities and transition points. Moving from an offensive state to a defensive state
◦These states can govern the types of plans that will be used
◦In an offensive state you wouldn’t push all your defenders back Well you might But more often than not you will push defenders up so
that they can support an offense and keep momentum moving in the positive direction
Building a Sports AI Architecture
Building a Sports AI Architecture
Agent AI◦Well thought out collection of utility functions and
data used by the plans◦Determine a player’s chance of success◦Determine the likelihood of certain actions
Dunks, lay-ups, bicycle kicks, etc Based on real-world player abilities and conditions of
the game◦Perform the action and determine the type of
action that can be performed by providing a wrapper to the mechanics
Building a Sports AI Architecture
Agent Mechanics◦Considered to be low-level AI◦Manage and select animations to be used◦Some of these decisions are determined through the use
of a random number This gives the effect that the game is more real Humans do not always react to the same situation in the same
way◦This level also takes into consideration the kinds of
commands that users would input into the system Run Shoot Pass
Dead Reckoning
Dead Reckoning
Originally developed as a tool for navigators◦Determining the position of his ship given
parameters such as direction, intended course, and speed
Estimating position based on past position and trajectory◦This becomes more complicated when other
factors are taken into consideration Wind or current
Dead Reckoning
Inertia◦Dead Reckoning at its most basic level reduces to
Newton’s first law of motion Knowing an object’s position and speed, we can assume the
object will continue to travel in a straight line.◦Pt = P0 + vt
◦Px,t+1 = Px,t + vx
◦Py,t+1 = Py,t + vy
◦Pz,t+1 = Pz,t + vz
◦These calculations can sometimes prove to be “too good” Incorporating an error calculation might help Even the best quarterbacks have a bad game
Dead Reckoning
Pseudo- Brownian Motion◦An extremely maneuverable object is harder to
predict velocity vectors over lengths of time In the case of a UFO which can do whatever it pleases
(Or so we assume), knowing the velocity magnitude can only help us determine a spherical region of possible positioning
◦In sports games this can be observed as objects in motion which experience seemingly random movement which is an effect of outside forces As a pass is being completed, a strong wind may blow
and knock the ball off of its intended course.
Dead Reckoning
Kinematics◦If the object’s initial velocity is unknown, it can
be computed from observation by plotting the curve of its position for an arbitrary interval and computing speed as the first derivative of the position curve.
◦Adding an estimate of its acceleration vector can help in estimating the object’s future trajectory.
◦P = P0 + v0t + 0.5at2
Dead Reckoning
Uses in sports games◦Shooting a ball or puck◦Passing a ball or puck
Dead reckoning is useful for planning the trajectory of players to determine if a player can make it to an open position to complete the pass.
Good examples of this can be found in soccer games◦Players will often link passes together by passing to a
teammate when faced with a defender and running past the defender to accept a return pass up the pitch
Dead Reckoning
Online play◦Dead Reckoning can be used in games to minimize
the effects of network latency Each player periodically broadcasts a packet containing
his avatar’s location, velocity, and acceleration During the intervals between packets, each machine
runs a dead reckoning algorithm to compute the approximate positions and orientations of all other players
When a new incoming packet from another player is received, the local state of the world is updated accordingly, and the process starts anew
Dead Reckoning
Inferring goals◦Dead Reckoning can help to infer another
players goals and try to intercept them This can be seen as interceptions in football and
soccer
Interceptions
Interceptions
Interceptions occur in many sportsThe interesting aspect of calculating
interceptions is that the same theories apply as in Dead Reckoning, but the system is less planned◦Meaning that, interceptions are not intentional◦Games should include interceptions as
incidental and unplanned from an AI perspective in order to accurately model a sports game
Interceptions
Interceptions
An object is at a position PbIt travels in a straight line with velocity VbAnother object is at a Pp and wants to
intercept the first object◦The intercepting object has a set speed it can
move atA velocity to intercept, Vp, is calculated
This is a simplified model
Interceptions
For instance:◦A basketball bouncing off of a rim has a path
that is parabolic in shape◦Break the model down into two submodels
Altitude of the ball Motion in the ground plane
◦The ground plane motions are orthogonal to the altitude axis These motions can be considered in isolation
Interceptions
Another simplification occurs in the previously described model◦No turning radius◦Infinite Acceleration◦Indefinite travel at maximum velocity
Error isn’t always frowned upon…by developers◦Passes are missed. It happens.◦Other methods can be used to calculate for
heading changes
Interceptions
Interceptions
For an interception to occur, the position of the ball and the player must be the same at some time t
If Vp is known prior, then the function can look like◦ Pb + Vbt = Pp + Vpt
However Vp is the variable that needs to be solved for◦ Distance between the player’s initial position and the ball
at time t: |(Pb – Pp) + Vbt| If the player can move a distance equivalent to how distant the
ball is, the player can intercept the ball at time t |(Pb – Pp) + Vbt| = sit
Interceptions
|P + Vt| = st
(P +Vt)*(P+Vt)=(st)2
P*P + 2P * Vt + V*Vt2 = s2t2
(V*V – s2)t2 + (2P*V)t + (P*P) = 0Now the equation is a second-order
polynomial of tNow it’s time to use the quadratic equation
stVtPVtP )(*)(
Interceptions
Category of solution is determined by the expression in the radical◦b2 – 4ac
A better form to look at our equations from◦b2-4ac = (2P*V)2 – 4(V*V-s2)(P*P)◦…◦(P*V)2 + (s2 – V*V)(P*P)
Interceptions
No Real Roots◦The radicand(quantity with the radical) is negative
There are no real roots The ball cannot be intercepted
◦This occurs when the ball travels at a speed greater than the maximum speed of the player
◦S2 – V*V must be negative◦S<|V|◦The player has to be able to move faster than the
ball if he hopes to intercept it
Interceptions
One Real Root◦Border case between whether or not the player can
intercept the ball◦Only one point in time for interception◦The radicand must be zero◦Two Special Cases
(P*V)<0◦ The ball’s velocity is toward the interceptor and can be caught
(P*V)>0◦ The ball’s velocity is not toward the receiver
This happens because the interception theoretically happened in the past
Interceptions
Two Real Roots◦Does not require the speed of the player to be
greater◦Two positive roots
The player is close to the line of motion and able to catch the ball anytime
◦Two negative roots Impossible interception. Negative time.
◦One positive and one negative root The player is moving faster than the ball and can
meet at any time to meet it in the positive direction
Dead Reckoning
Dead Reckoning
Error correction◦The translational error between the real
position of an agent and the estimate provided by dead reckoning can become unbounded with time. The agent will compute its own short-term map of
its surroundings The short-term map is compared with the a priori
map using pattern recognition techniques Small, incremental corrections are applied on the
fly
Agent Cooperation
Agent Cooperation
Coordination and communication are key amongst human teams and the same goes for the PS3, Xbox360, and Commodore 64
The first step is to define behaviors for the agents.
In the case of baseball:◦Baserunning◦Fielding◦Hitting◦Pitching
Agent Cooperation
Hitting and Pitching◦ Not too complex
Very much animation based◦ The pitch is thrown◦ It is hit, or not hit◦ The ball will have a determined path◦ Once the ball is hit, its initial velocity an angle are the only real
factors of importance These two factors in addition to existing baserunners directly affect
how the ball is fielded◦ Ball hit and ball pitched act more like events
Prepare to field Field Prepare to run
Agent Cooperation
Fielding Behaviors◦Assignments happen off of two main triggers
(events) Ball Hit Ball Fielded
◦Players should react differently to these situations Only one player fields a ball, so the other players
watch, or prepare for other strategies
Agent Cooperation
Agent Cooperation
Class StructureClass BasicBehavior{…void Entry(Cplayer*);void Exit(Cplayer*);void Process(Cplayer*);…}
Each derived behavior has a transition table to switch between behaviors and actions◦ Many times this can result in a similar behavior being performed◦ Transition tables do not always lead to a distinctly different behavior
Agent Cooperation
Initial Behavior Assignments◦In the case of a ball hit event, once the initial
angle and velocity are known, it is easy to determine where the ball will land, and what actions and behaviors to disperse
◦Determining the Hit Type and Hit Zone can be done using physics and trigonometry The Hit Type and Hit Zone values can be used to
control values across the entire field
Agent Cooperation
Hit Types◦ Ground ball◦ Fly ball◦ Line drive◦ Popup◦ Deep drive
In the case of a hit type ground ball Zone 7 ◦ The first baseman and right fielder should motion to the
Behavior Field Ball◦ The rest of the infielders will move to Behavior Cover Base,
with exception to the second baseman who will assume Cutoff or Field Ball depending on velocity
◦ Left and Center Field are in Behavior Back Up
Agent Cooperation
Hit Zones
Agent Cooperation
Initial Behavior Assignments for Runners◦Hit Types
Ground Ball – Behavior Go or Behavior Go Back A runner on second, not forced will assume Behavior
Go on any ball hit in a right-hand zone Hit type fly ball will throw Behaviors Go Halfway, or Tag
Up◦The runners evaluate situations exactly like humans
do They check proximities of fielders to the ball, evaluate
the actions of other runners and make a best decision on what to do
Agent Cooperation
Agent Cooperation
Throw Determination◦Fly balls
95 percent of the balls either go to second or home
Or throw to the cutoff man and let him decide◦Rundowns
We have enough information to run the player down every time (100 percent success rate)
So the better approach to take is to make it interesting and allow the runner to fool with the window
Real Time Strategy Development
Real Time Strategy Development
Sports games are confined to sets of rules and the development of strategies both team-wide and personal that confine themselves to the rules of physics and the rules of the game
This does not limit however the amount of work and planning that is done by each individual player, who has to model the abilities and limitations of his or her real-life component.
Team dynamic is equally important◦ Some teams have characteristically strong defense◦ Some players communicate better than others, and are
more intelligent
Real Time Strategy Development
Soccer Games◦Real time strategy in the sense of team control◦Moving offense/defense forward and back◦Controlling player motions
Many of the players use advanced AI techniques to predict good strategies to use with you
Games can allow you to change offense and defensive strategies on the fly.
Different players have different abilities◦This skill set in some ways can become very detailed◦Some famous players are known for their speed, shooting,
defensive abilities and a knowledgeable player can exploit these to their own advantage
Real Time Strategy Development
Positions and zones are of extreme importance◦Players have roles◦Like in RTS games, some characters cannot perform
well on certain terrain◦The same goes for players playing in muddy
conditions or snowTeams have characteristics and players that
make them more powerful against other teams◦Not a division on race and inherent abilities◦A division based on skill and communication
There are optimal configurations
Real Time Strategy Development
A new trend is also to give you manager control◦This involves more long term planning
regarding player trading, development, and economics
◦Played over a long period of time, your decisions affect your win percentages and offers you receive. This kind of play rewards players who are very
tied to the individual sport
Real Time Strategy Development
Skills Vs. Strategy◦An interesting development in sports gaming is that of
providing you with additional skills◦Managing a team from the top level, while managing a
player’s actions on a lower level are becoming increasingly common
◦FIFA 2008 included a new mode titled “Be a Pro” which allows the user to master skills such as dribbling and tackling at a level that goes beyond simply pressing A or B
◦This type of a game feature can almost be compared to some aspects of RPG gaming, where a player levels up and is rewarded with experience points and abilities
Final Notes
Sports games have the difficult task of being true to the sport and physics◦Accurate player modeling◦Robust physics engines
Sports games incorporate features from many different genres◦Real-time strategy styles◦RPG-type player development
Through the years sports games have advanced significantly to allow a deeper level of play
Questions?