Motion Planning in Games

Mark OvermarsUtrecht University

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Goals and Contents

• Try to formulate the requirements for motion planning in games– Rather different from other applications

• Investigate what techniques are suitable– Almost none of the existing techniques are– They still use scripting and A* for a reason

• Indicate some of the open issues

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Motion or Path Planning or …

• Many different terms:– Motion planning– Path planning– Navigation– Route planning– …

• Motion deals with– Interaction– Animation– Navigation– Simulation

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First or Third Person Games

• Environment– Complex:

• Rooms, corridors, narrow passages• Stairways, ramps• Sometimes outdoors• Objects like trees, furniture, etc.

– Created offline• Takes huge amount of time

– Small changes occur, often at known places

• Doors, bridges that are blown up, etc.• Sometimes user can make further changes

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First or Third Person Games

• Moving entities– In general alone or in very small groups– Number of different types– Varying number

• Motion– Locally 2-dimensional, but 3-D obstacles

• Often orientation is not important for planning– Some special moves possible

• Jumping, climbing ladders– Path must be natural

• On a local level• But only within the (small) visible range

– Goals not precise • Goals change often, like when tracking an enemy

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Strategy Games

• Environment– Simple:

• Basically planar, or terrain• Simple obstacles (can be modeled in 2D)• Few narrow passages

– Created offline or during “loading”• Takes not much time to create

– Environment changes continuously• Forrest grows and is cut away• Buildings are constructed• Fences can be made (reducing possibilities)• Bridges can be made (extending

possibilities)

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Strategy Games

• Moving entities– Many different types of units of vary different

shapes– Often moving in groups– Varying numbers– Non-holonomic constraints

• Motion– 2-dimensional

• Some can be seen as cylinders but for others orientation matters

– Path must be natural• On a global level• Both for the entities and for the groups

– Goals not precise • Goals change often, like when tracking an enemy

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Time Issues: Preprocessing

• Offline– During scene modeling– Time can be as large as we want (hours, days)

• There should also be a faster technique for testing– Memory can be an issue

• During initialization– When a scene is “loaded” or created– Time can typically be a few seconds

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Time Issues: Runtime

• Calculation frame rate: 20 – 100 frames per second

• Leaves something like 0.02 seconds per step• This includes

– Updating game status– Handling user input– Some graphics processing– Physics computations– Strategic AI– Path Planning

• This leaves something like a millisecond per step for path planning!

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Time Issues: Runtime

• Path calculation can be distributed over a number of steps

• But visible units better start moving soon• Queuing of path planning requests can easily

occur– Many moving entities– Replanning requests– There must be a fast backup plan

• The Game Must Go On

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The Approach

• Do everything you can offline– Alternative routes– Preparation for changes– Dealing with unknown numbers of entities– Dealing with groups– Quality of results

• Preprocessing time is not very important for this– Unless during initialization

• Build efficient data structures– Query time should be close to 0– Even A* on a big graph can be too much– Avoid collision checking at runtime

• Plan while execute

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A High Quality Roadmap

• Given an environment, construct a roadmap that can directly be used for motion planning queries, without the need for improving the path

• Every path must be– Short– Has some preferred minimum clearance– Must be C1-continuous

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Basic Technique: PRM

• Problems:– Paths can take long detours– Paths can have a very low

clearance– Paths will have C1-discontinuities

at the nodes

• Remember, we cannot remedy this at the query time

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Reducing Path Length

• Add cycles– Add an edge when the edge is considerably shorter

than the current path through the graph– Can be done efficiently, avoiding expensive graph

search

– We do not want the graph to get too large by adding all edges, as this will slow down the query phase

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Improving the Clearance

• Push nodes and edges away from the objects

• Remove or merge overlapping parts

• More difficult than it sounds when dimension > 2

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Create C1 Continuity

• Replace nodes by circle arcs– Centers of the edges become the nodes– Connect by line-arc-line in a single plane– This is always possible

• The longer the edges the better the result– Reduce the number of nodes

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Result

• Smooth roadmap, resulting in smooth paths• Preprocessing time is still reasonable• Query is instantaneous

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Additions

• Dealing with known changes, like doors

• Coordinate multiple entities using these roadmaps

• Adapting to additional obstacles

• Use them as the basis for group motion

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Research Questions

• What is natural motion– How do people walk through buildings– How does the path change depending on the speed

• Preprocessing versus real-time– How can we do more work during preprocessing– Calculation paths while moving– Locally adapting to changes

• Dealing with groups– Multiple groups– Strategic behavior

• Goals– Dealing with imprecise goals– Motion interruption and changing goals