KIPA Game Engine Seminars

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KIPA Game Engine Seminars
Day 10

• Jonathan Blow
• Ajou University
• December 6, 2002

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Collision Detection Methods

• Early today Ill 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 well 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 BoxesAxis Aligned

• Dont 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 BoxesOriented

• Better fit than AABB
• But a little more complicated to compute
• More complicated intersection detection
• See Gottschalks 1998 paper
• I will include this in the course notes
• Unclear in the end whether its 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 didnt 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, thats 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