# Chasing, Evading, Intercepting, Pattern Movements

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Basic Movement AI Chasing, 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

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 Chasing if (npcX > playerX) npcX--;
else if (npcX < playerX) npcX++; if (npcY > playerY) npcY--; else if (npcY < playerY) npcY++;

Pseudocode for Evading if (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 straight Note  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? Player Line-of-sight Chase NPC

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? Player Line-of-sight Chase NPC

Intercepting – In Principle
Predict some “future” position of the player and move towards that position, so that it reaches the same time as player… Player Line-of-sight Chase A “future” position of player NPC Intercepting Chase

Intercepting – A “Future” Position
“Future” can be safely predicted linearly How to compute this position? Player Line-of-sight Chase A “future” position of player NPC Intercepting Chase

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 Movement
void 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 initialization entityList[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 Pong
B B C C D ABCBABCBA ABCDABCD

Pattern Movement in TBE
Ping Pong Patrolling entityList[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 PSE
struct 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

Other Useful Movements
Velocity Matching (for friendly NPCs, flocking) Wander (roaming NPCs) (can be found in Millington’s book)