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KIPA Game Engine Seminars

Jonathan BlowAjou University

December 6, 2002

Day 10

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Collision Detection Methods

• Early today I’ll overview some of the simpler methods and talk about real-world issues about integrating them in games– bounding spheres, boxes, bsp– find time of collision, prevent tunneling

• Later we’ll go to some of the more advanced collision detection schemes– GJK algorithm– Lin/Canny Closest Features

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Bounding Spheres

• Simplest, fastest bounding volume• Least information to store• Isotropic

– Means it may be a poor fit• Better fit: perhaps use the centroid, not the

origin (or find the two furthest points)– But this takes more information per sphere

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Bounding Boxes:Axis Aligned

• Don’t take much information to store• May be a poor fit for “diagonal” data• Easy to test them against each other for

intersection• Abbreviated “AABB”

– (“Axis-Aligned Bounding Box”)

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Bounding Boxes:Oriented

• Better fit than AABB– But a little more complicated to compute

• More complicated intersection detection– See Gottschalk’s 1998 paper

• I will include this in the course notes

• Unclear in the end whether it’s better than AABB or spheres– Probably depends on the nature of your data

• Abbreviated “OBB”– (“Oriented Bounding Box”)

• Software toolkit “RAPID”

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Bounding Volume Hierarchies

• Method of saving computation time– It costs memory, though

• Form volumes around pieces of a mesh so that you can “early-reject”

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BSP Tree

• Can be viewed as a hierarchical bounding volume system

• The split planes are spheres with infinite radius

• So an individual sphere has poor spatial locality, but the hierarchy makes up for that

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BSP Tree

• What might a BSP tree be like that didn’t use planes as its splitting surface?– Spheres of finite radius?– Other curved surfaces?

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Animated Meshes

• How are we to adapt hierarchical bounding volumes to animated meshes?– Fully recompute every time– Try to do an incremental update, but quality

will decay• Always slow!

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Animated Mesh Example

• In Barbaric Smackdown I made a set of spheres that track the skeleton (not the mesh)

• The spheres animate with the mesh, and we collide against the spheres

• This is faster than colliding against the mesh

• Also faster than building spheres from the mesh vertices each frame

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Intersection Primitives

• Box/box, sphere/sphere, etc• What kind of information do they return?• What kind of information do we get from a

triangle vs. triangle intersection routine?

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Finding the time of intersection

• Necessary for good physics?• How do you do it given a triangle/triangle

intersection style primitive?– Move through time in very small samples and

interpolate?– Binary search through time

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Preventing “tunneling”

• When long frame times happen, objects might pass through each other

• What do we do here?– Raycasting solution– Analytic method?

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Closed-form intersection testfor spheres

• Without acceleration• With acceleration (constant over time

period)

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Separating Axis Theorem

• Convex objects and separating planes• But we want non-convex objects!

– Model them as unions of convex objects and do separate collision tests

• Still very difficult (Maya is not a solid modeling program)– BSP the thing?

• This can be harder than you would think• Also it produces a lot of pieces

– Try to re-join the pieces?

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Algorithms that help usestimate collision times

• Maybe tell us how much two entities are colliding, so that we can better guide our search for the collision time

• Example of two spheres, or sphere vs plane

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GJK Collision DetectionAlgorithm

• “Gilbert-Johnson-Keerthi”• Operates on convex solids

– Again, that’s a problem!• Uses Minkowski sums

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Minkowski Sum / Minkowski Difference

• (examples on whiteboard)

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Lin-Canny Closest Feature Tracking

• Tells you the closest “features” between two objects– A feature can be a point, line segment, or face

• Relies on frame coherence• (example on whiteboard)

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Hierarchical Bounding VolumeQuantization/Packing

• Sphere tree, bounding box tree– Exploit multiscale nature of hierarchy

• BSP Tree– Exploit recurrence of plane equations

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AI, A* search,and “speculative collision

detection”

• (discussion)

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Higher-Level Scene Management

• Indoor shooter level– BSP– Portals

• Terrain• Flight / Space Sim / Heavily occupied

environment