Basic Movement AIChasing, Evading,Intercepting, Pattern Movements

Movement AI

•Movement – Ubiquitous behavior found in many games

•Chasing and Evading – the most basic movement AI that can be implemented on NPCs

Chasing & Evading

•2 parts▫Decision to initiate a chase or evade▫Effecting the chase or evade

•Sometimes, a 3rd part, to avoid obstacles while chasing and evading is required, but that can be considered separately

•1st part – decision making•Focus on 2nd part – concerns the action

itself, to chase and evade

Basic method

•Simplest method: Update NPC coordinates through each game loop to ▫Decrease the distance between NPC and

player (chase)▫Increase the distance between NPC and

player (evade)•Pays no attention to NPC or player

headings (direction of travel) or speeds

More complex methods

•Positions and velocities (direction of travel) of both NPC and player are considered so that NPC can be moved to intercept the player instead of relentless chasing

•In short: Predicting where the player will likely to be at in the future, and moving towards that as target

Basic Chasing

•Pseudocode for Chasingif (npcX > playerX)npcX--;

else if (npcX < playerX) npcX++;

if (npcY > playerY) npcY--;

else if (npcY < playerY) npcY++;

Basic Evading

•Moving farther, instead of moving closer•Pseudocode for Evadingif (npcX > playerX)npcX++;

else if (npcX < playerX) npcX--;

if (npcY > playerY) npcY++;

else if (npcY < playerY) npcY--;

Tile-based environment – How?

•8-way movement

Tile-based Chase movement• Updating difference

▫Continuous environment: x, y coordinates in Cartesian coordinate system (float)

▫Tile-based environment: x, y in tile coordinate system (int.) are columns and rows

if (npcCol > playerCol)npcCol--;

else if (npcCol < playerCol) npcCol++;

if (npcRow > playerRow) npcRow--;

else if (npcRow < playerRow) npcRow++;

Tile-based Chase – Any problem?

Line-of-sight Chasing

•Make NPC take a direct straight-line path towards player

•Stationary player – straight path•Moving player – path is not necessarily

straightNote may not look too bad if game loop constantly updates position, path should appear curved and natural

•Movements in TBE will appear less smoother than CE due to coarse movement units

L-o-S Chasing in TBE

• Aesthetic disadvantage of using simple basic chase – although no. of steps are the same

• Limitation of 8-way movement, no other possible angles

• Will look really BAD if many NPCs are chasing this way

L-o-S Chasing in TBE•Solution: To calculate an intermediate path

(closest to a straight line) for the NPC to chase•Use a standard line-drawing algorithm –

Bresenham’s algorithm▫Nice thing: Does not draw two adjacent pixels

along a line’s shortest axis•Path calculation function

▫Calculate and store a series of tiles to be moved to▫Needs to be called every time player changes

position

L-o-S Path in TBE

“Looking At” Rotation• Without “Looking At”

Rotation

• Only x and/or y are updated at each time (8-direction)

• With “Looking At” Rotation

• Additional rotation steers NPC to look at target and chase in that direction

(x1, y1)

(x2, y2)

(x1, y1)

(x2, y2)

θ

Thrust and Steering forces

•Thrust force: pushes object to the front (velocity)

•Steering force: pushes the nose of object to the right or left to rotate (angular velocity)

L-o-S Chasing in CE

•No need to determine path like in TBE, movement updates are in floating-point (rounding to nearest pixel integer gives good approximation)

•Similarly, rotation angle (“looking at” direction) should be re-calculated every time player changes position

L-o-S Chasing in CE

• Curved path taken by predator when chasing prey

• What happens if the speed of the predator is set too fast?

Preventing overshooting

•Implement speed control logic to allow predator to slow down as it gets closer to prey▫ Calculate distance between both, if distance <

some predefined distance, reduce thrust velocity to slow down

L-o-S Evading•Since we now understand L-o-S Chasing, L-o-S

Evading is just doing everything the reverse

•TBE▫Update movement in opposite direction (good

enough!)•CE

▫“Looking at” rotation should steer to opposite face

▫Update movement in opposite direction

Further improvements?

•Acceleration (and deceleration) for thrust force and steering force?

•Any way to improve “intelligence” of chasing movement?

Intercepting• Line-of-Sight Chase: NPC will always head

directly towards player • In the case of a moving target, what happens?

NPC

Player

Line-of-sight Chase

Intercepting• Movement can be more “intelligent”, if it knows

how to intercept the player somewhere along the player’s “trajectory”.

• How do we work this out? What information do we need?

NPC

Player

Line-of-sight Chase

Intercepting – In Principle

•Predict some “future” position of the player and move towards that position, so that it reaches the same time as player…

NPC

Player

Line-of-sight Chase

Intercepting Chase

A “future” position of player

Intercepting – A “Future” Position•“Future” can be safely predicted linearly•How to compute this position?

NPC

Player

Line-of-sight Chase

Intercepting Chase

A “future” position of player

Intercepting

•Important to consider relative velocities (direction and magnitude) and distance instead of just their current positions

•Let’s work out the math to determine what is the predicted future position…

Intercepting - Code

•Recall: You would have implemented the Enemy class already, and have it to chase the player. Target Player position

•For Intercept, Target Player’s predicted future position▫Easy to modify and adapt code from Chase to

Intercept•Function to be called through the game

loop to constantly update interception point

Intercepting

Intercepting

•There are scenarios where intercepting may not be possible or unrealistic▫NPC moving at much slower velocity▫NPC ends up chasing from behind a player

moving in straight line ▫NPC gets ahead of player and moving at a

faster speed

Pattern Movement

•Simple way of giving the illusion of intelligent behavior

•“Choreographed” movements of enemy NPCs making organized maneuvers, loops

•Can be used effectively for ▫Enemy NPCs (patrol, attack)▫Friendly NPCs (to exhibit or act

intelligently to cooperate with player)▫Secondary NPCs (doing all kinds of random

patterned actions)

Standard Pattern Movement

•Uses lists or arrays of encoded instructions to tell NPC how and where to move each step▫Easy to loop through again

•Sample control data

ControlData { double turnRight; double turnLeft; double stepForward; double stepBackward;

};

Standard Pattern Movement

•Can include other kinds of actions:▫Fire weapon, release chaff, drop bomb, do

nothing, speed up, slow down, etc.•Pattern initialization

▫Hardcoded in game▫Loaded from a data file (text, XML)

•Process pattern▫Maintain and increment an index to the

pattern array through the game loop

Standard Pattern Movementvoid GameLoop(void) { . . .

Object.orientation += Pattern[CurrentIndex].turnRight;

Object.orientation -= Pattern[CurrentIndex].turnLeft;

Object.x += Pattern[CurrentIndex].stepForward; Object.x -= Pattern[CurrentIndex].stepBackward;CurrentIndex++;

. . . }

Pattern Movement in TBE

•Bresenham’s line drawing algorithm used again to pre-calculate path steps

•More complex paths can be created by joining up line segments▫Each new segment will begin once a

previous one ends▫Line-of-sight function End of segment

now assigned as the target

Pattern Movement in TBE

• Instead of resetting L-o-S path array, new line segment is appended to previous path (Why do we do this?)

Pattern Movement in TBE

•High-level code for pattern initializationentityList[1].InitializePathArrays(); entityList[1].BuildPathSegment(10, 3, 18, 3); entityList[1].BuildPathSegment(18, 3, 18, 12); entityList[1].BuildPathSegment(18, 12, 10, 12); entityList[1].BuildPathSegment(10, 12, 10, 3); entityList[1].NormalizePattern(); entityList[1].patternRowOffset = 5; entityList[1].patternColOffset = 2;

Patrolling: Cyclical & Ping PongA B

C

A B

CD

ABCBABCBA ABCDABCD

Pattern Movement in TBE

•Ping Pong PatrollingentityList[1].InitializePathArrays(); entityList[1].BuildPathSegment(10, 3, 18, 3); entityList[1].BuildPathSegment(18, 3, 10, 3); entityList[1].NormalizePattern(); entityList[1].patternRowOffset = 5; entityList[1].patternColOffset = 2;

Pattern Movement in TBE

•To prevent repetition and predictability Add a random factor for choosing alternative patrolling path

How to implement?

Pattern Movement in PSE

•PSE – Physically Simulated Environments (or Continuous Environments with Physics)▫You may specify positions for NPC to

traverse, but that defeats the purpose of using CE. Bad realism!

•Control structure needs to be designed to accommodate forces (thrust, steering)

Pattern Movement in PSEstruct ControlData {

bool PThrusterActive; bool SThrusterActive; double dHeadingLimit; double dPositionLimit; bool LimitHeadingChange; bool LimitPositionChange;

};

• Changes in heading and position are relative in patrolling

• Based on this control data, how do you initialize a square pattern patrol?

Pattern Movement in PSE• More implementation details can be found in

textbook

Wander• Useful movement for creating “random” walks

through the environment• Naïve approach:

▫ Calculate a random steering force each time step▫ But, produces jittery behavior with no long

persistent turns• Craig Reynolds’ solution:

▫ Project a circle in front of the NPC▫ Steer towards a target that is constrained to move

along the perimeter (of the circle)▫ Each time step, a random displacement is added to

this target move back and forth the perimeter

Wander

• Jitter-free wander movement• Of course, you can always implement the naïve

version but increase the time delay required to trigger the next steering change still turn abruptly, but wandering improves

Wander• From previous target

position, add a small random displacement to target

• Project target back onto wander circle (normalize to unit circle and multiply radius)

• The new target position is moved in front of the NPC by the wander distance set

Other Useful Movements

•Velocity Matching (for friendly NPCs, flocking)

•Obstacle Avoidance

(can be found in Millington’s book)