Artificial Intelligence

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What is Skeletal Animation?

  • A rig made of bones is used
  • Each bone associates with the vertices of the mesh around it with a certain weight
  • Keyframes are made with the bones in different positions
  • Animation is done by interpolationg between the keyframes

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What is Sprite Sheet Animation?

  • Similar versions of the same sprite are displayed one after the other creating motion

Review

How does the Unity Animator work?

  • Operates like a finite state automaton
  • Each state is an animation
  • Edges between states describe under what condition the animation change will occur and how

AI Definition

An agent that can perceives its environment and takes actions based on its perception to successfully achieving its goals

Regular AI vs Game AI

  • Regular AI aims to maximize its chance of success
  • Game AI aims to maximize the fun factor of a game
    • Playing agains a bot that does perfect moves every turn isn't fun

Uses of AI in games

Pathfinding

  • You have a character
  • It needs to go from point A to point B
  • There may be obstacles
  • The character may not fit through gaps

Waypoint Graph

  • You have a level
  • You break it up into a graph of waypoints
  • Each waypoint marks a walkable location
  • Edges denote that the area near them has no obstacles

Pathfinding Algorithms

DFS (Depth first search)

  • Useful for traversing graphs with a large branching factor and low height
  • Inefficient for finding shortest path
  • Goes as deep as possible before backtracking (returning to parent nodes)

BFS (Breath first search)

  • Can find the shortest path in unweighted graphs
  • Traverses the graph by levels
  • May use way too much memory

Dijkstra

  • Finds the shortest path from one point to every other point in a weighted graph
  • Every point is initialy infinitely far from the starting point
  • At every step we add the closest to the start unvisited point

A*

  • At every step we choose the node with smallest value f(n) = g(n) + h(n)
  • g(n) is the distance from the starting node to the current node n
  • h(n) is the heurestic that gives a distance from n to the goal node
  • Heurestic means that it is good enough but not always guranteed to be correct
    • Used when the original function is difficult to compute

A* Details

  • Much faster than Dijkstra for finding the shortest path between exactly two points
  • Is always correct if the heurestic function underestimates the real value
  • Example heurestic functions:
    • Euclidian distance
    • Manhattan distance (if we can't move diagonally)

A* Demo

A* Code

						
openSet = empty set of nodes
add startNode to openSet
closedSet = empty set of nodes

while openSet is not empty
  current = node in openSet with minimum fCost
  remove current from openSet

  if current is the final node
    return

  foreach neighbour in neighbours of current
    if (neighbour is not traversable
        or neighbour is in closedSet)
      continue
    
    if (new path to neighbour has a lower fCost
        or neighbour is not in openSet)
      set improved fCost of neighbour
      set parent of neighbour to current
      if neighbour is not in openSet
        add neighbour to openSet

Navigation Meshes (NavMesh)

  • А collection of 2D convex polygons that define which areas of an environment are traversable by an agent
  • The polygons are the graphs vertices
  • An edge between two polygons exists if they are adjacent

NavMesh vs Waypoint Graph

NavМeshes in Unity

  • Agents are represented as cylinders
    • Found in Window/AI/Navigation/Agents
  • Most components are located in a separate repo

NavМeshe Live Demo

State Machine AI

  • Each state represents a behaviour for the AI
  • Each edge represents a change of world state

Unity State Machine AI

  • Integrated with the Animator
  • Each state can have several attached StateMachineBehaviours
  • A StateMachineBehaviours has:
    • OnStateEnter
    • OnStateUpdate
    • OnStateExit

Unity State Machine AI Live Demo

Extra Resources

Questions?

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