Cse 380 computer game programming pathfinding ai
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CSE 380 – Computer Game Programming Pathfinding AI. Dig Dug, by Namco. Pathfinding. Computation and execution of a path from point p1 to p2 Perhaps most common AI problem in games Can be very frustrating to implement well Algorithms must be tailored to fit games

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Cse 380 computer game programming pathfinding ai l.jpg

CSE 380 – Computer Game ProgrammingPathfinding AI

Dig Dug, by Namco

Pathfinding l.jpg

  • Computation and execution of a path from point p1 to p2

  • Perhaps most common AI problem in games

  • Can be very frustrating to implement well

    • Algorithms must be tailored to fit games

  • For an object at point p1, suggest an algorithm to get to point p2

Simple 2d vectoring solution l.jpg
Simple 2D Vectoring Solution

  • Determine vector v to head towards p2

    v.x = p2.x – p1.x

    v.y = p2.y – p1.y

  • Now, scale v according to speed (use simple geometry)

  • What’s the problem?

    • we are assuming there are no obstacles

Simple trial and error l.jpg
Simple Trial and Error

  • For simple obstacles that aren’t large

  • Algorithm:

    • Go towards target destination

    • When you hit an obstacle:

      • Back it up

      • Randomly turn it right or left 45-90 degrees

      • Move it forward again for some random amount in a range

    • Go back to step 1

  • Easy & fast

  • Not good for fixed obstacles of any size

Contour tracing l.jpg
Contour Tracing

  • Algorithm:

    • Go towards target destination

    • If you hit an obstacle:

      • trace the contour of the obstacle blocking the path

        • move such that you are a uniform distance from edge

      • periodically test if a line to your destination intersects the obstacle anymore

        • if yes, stop tracing and head towards destination

        • if no, go back to tracing

Real time pathfinding l.jpg
Real Time Pathfinding

  • Modern strategy games use lots of units simultaneously

  • Dynamically computing paths can be expensive

  • Solution: precompute

Waypoint pathfinding l.jpg
Waypoint Pathfinding

  • Setup a network of paths

    • connect all points of interest in the game via a connected network of nodes

    • each node represents a waypoint

    • edges in the network represent:

      • vector direction

      • vector length

Key for precomputed paths l.jpg
Key for precomputed paths

  • Paths should avoid obstacles

What are precomputed paths l.jpg
What are precomputed paths?

  • Nodes:

    • locations on map (x, y)

  • Edges

    • direct paths to other nodes (x, y)

    • distance

  • Vectors

    • computed from node to node along path

  • Options:

    • pre-compute and store entire paths (only viable for small data sets)

    • dynamically calculate paths from pre-computed graph data

How do we improve finding a path l.jpg
How do we improve finding a path

  • Game objects don’t necessarily start at nodes

    • they shouldn’t have to go to nodes to pick up a path either

  • Made easier by path coverage

    • making sure that everywhere on the board can reach a path quickly

Dynamic pathfinding algorithms using graphs l.jpg
Dynamic Pathfinding Algorithms using Graphs

  • Breadth-first search

  • Depth-first search

  • Dijkstra’s algorithm

  • A*

  • Premise:

    • you have a graph of nodes

    • you are at one node & you want to get to another

    • What combination of edges will get you there?

Breadth first search l.jpg
Breadth-first search

  • Fan out in all directions at the same time

    • visit each node one unit away

    • then two units away

    • then three

    • etc.

  • Like a growing circle

  • What’s bi-directional breadth-first search?

Depth first search l.jpg
Depth-first search

  • Searches one way all the way until:

    • it finds the goal


    • it runs out of space

Dijkstra s algorithm l.jpg
Dijkstra’s Algorithm

  • Similar to minimum spanning tree algorithm

  • O(V2), where V is the # of vertices

  • Premise:

    • find the shortest path from starting node to other nodes along the way

    • use those shortest path to determine your ultimate shortest path

  • A nice Dijkstra’s Applet:

    • http://carbon.cudenver.edu/~hgreenbe/sessions/dijkstra/DijkstraApplet.html

A algorithm l.jpg
A* Algorithm

  • Solves shortest path problem for a directed graph

  • All nodes have a G & H value

    • G: min distance from origin node (A) to the given node

    • H: estimated distance to goal

  • Key to determining which nodes to use:

    • Minimize G + H

How does it work l.jpg
How does it work

  • Place origin node A in open list

  • Look at all adjacent nodes for A

    • Add them to open list with A as parent

    • Remove A from open list & add to closed list

    • Which node (B) is the minimum distance (G + H)?

    • Remove minimum from open list and add to closed list

  • Look at all adjacent nodes for B not on the closed list

    • Add them to open list with B as parent

    • Which node (C) is the minimum distance (G + H)?

    • Is that node (C) already on the open list?

      • If yes, just ignore B in path

  • Continue in this manner until destination is reached

A and grids l.jpg
A* and Grids

  • Strategy games are typically played on grids

  • A grid can be easily divided into large nodes

    • store the center of the node

    • nodes must not include impassible terrain (e.g. water)

  • The smaller the nodes, the more processing

  • Example assumptions:

    • horizontal/vertical movements cost 10

    • diagonal movements cost 14

    • no diagonal movements through obstacles

References l.jpg

  • A* Pathfinding for beginners

    • http://www.policyalmanac.org/games/aStarTutorial.htm