1. Chasing, Evading, Intercepting, Pattern Movements
Basic Movement AI
Chasing, Evading,
Intercepting, Pattern Movements

2. Movement AI Movement – Ubiquitous behavior found in many games
Chasing and Evading – the most basic movement AI that can be implemented on NPCs

3. 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

4. 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

5. 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

6. Basic Chasing Pseudocode for Chasing if (npcX > playerX) npcX--;
else if (npcX < playerX)
npcX++;
if (npcY > playerY)
npcY--;
else if (npcY < playerY) npcY++;

7. Basic Evading Moving farther, instead of moving closer
Pseudocode for Evading
if (npcX > playerX)
npcX++;
else if (npcX < playerX)
npcX--;
if (npcY > playerY)
npcY++;
else if (npcY < playerY) npcY--;

8. Tile-based environment – How?
8-way movement

9. 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++;

10. Tile-based Chase – Any problem?

11. 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

12. 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

13. 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

14. L-o-S Path in TBE

15. “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

16. 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)

17. 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

18. 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?

19. 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

20. 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

21. Further improvements?
Acceleration (and deceleration) for thrust force and steering force?
Any way to improve “intelligence” of chasing movement?

22. 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

23. 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

24. 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

25. 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

26. 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…

27. 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

28. Intercepting

29. 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

30. 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)

31. 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; };

32. 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

33. 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++; }

34. 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

35. 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?)

36. 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;

37. Patrolling: Cyclical & Ping PongB B C C D
ABCBABCBA
ABCDABCD

38. 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;

39. Pattern Movement in TBE
To prevent repetition and predictability
 Add a random factor for choosing alternative patrolling path
How to implement?

40. 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)

41. 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?

42. Pattern Movement in PSE
More implementation details can be found in textbook

43. 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

44. 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

45. 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

46. Other Useful Movements
Velocity Matching (for friendly NPCs, flocking)
Obstacle Avoidance
(can be found in Millington’s book)