Motion Planning in Games Mark Overmars Utrecht University

2 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

3 Motion or Path Planning or … Many different terms: –Motion planning –Path planning –Navigation –Route planning –… Motion deals with –Interaction –Animation –Navigation –Simulation

4 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

5 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

6 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)

7 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

8 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

9 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!

10 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

11 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

12 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 C 1 -continuous

13 Basic Technique: PRM Problems: –Paths can take long detours –Paths can have a very low clearance – Paths will have C 1 -discontinuities at the nodes Remember, we cannot remedy this at the query time

14 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

15 Improving the Clearance Push nodes and edges away from the objects Remove or merge overlapping parts More difficult than it sounds when dimension > 2

16 Create C 1 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

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

18 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

19 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