Collision processing

Transcription

1 Collision processing Different types of collisions Collision of a simulated deformable structure with a kinematic structure (easier) Collision with a rigid moving object, the ground, etc. Collision object can even be deforming as long as its deformation is kinematic (i.e. scripted), not simulated Self collision within a deformable structure (harder) Also includes collision between 2+ deformable structures CS838 Advanced Modeling and Simulation

2 Collision processing Different types of collisions Collision of a simulated deformable structure with a kinematic structure (easier) Collision with a rigid moving object, the ground, etc. Collision object can even be deforming as long as its deformation is kinematic (i.e. scripted), not simulated Self collision within a deformable structure (harder) Also includes collision between 2+ deformable structures CS838 Advanced Modeling and Simulation

3 Collision processing Typically partitioned into two tasks/phases Collision detection Detect if an interpenetration event occurred Localize such events, in space and time (If required) determine depth and direction of collision Collision response (or resolution) Attempt to resolve and fix all collisions, and/or Try to make collision less severe (but tolerate some), and/or Take steps to prevent collisions in the imminent future CS838 Advanced Modeling and Simulation

4 Collision processing Typically partitioned into two tasks/phases Collision detection Detect if an interpenetration event occurred Localize such events, in space and time (If required) determine depth and direction of collision The exact nature of collision detection depends on how we expect to use that information in the response stage! Collision response (or resolution) Attempt to resolve and fix all collisions, and/or Try to make collision less severe (but tolerate some), and/or Take steps to prevent collisions in the imminent future CS838 Advanced Modeling and Simulation

5 Collision response (general approaches) Penalty-based methods Detect proximity to collision objects and apply a repulsive penalty force when the distance to the collision target is small Increase strength of repulsion force as distance decreases (or as interpenetration starts to occur) Does not strictly enforce a collision-free state, but attempts to prevent it, and lessen the degree of collision CS838 Advanced Modeling and Simulation

6 Collision response (general approaches) Penalty-based methods Detect proximity to collision objects and apply a repulsive penalty force when the distance to the collision target is small Increase strength of repulsion force as distance decreases (or as interpenetration starts to occur) Does not strictly enforce a collision-free state, but attempts to prevent it, and lessen the degree of collision CS838 Advanced Modeling and Simulation

7 Collision response (general approaches) Penalty-based methods Detect proximity to collision objects and apply a repulsive penalty force when the distance to the collision target is small Increase strength of repulsion force as distance decreases (or as interpenetration starts to occur) Does not strictly enforce a collision-free state, but attempts to prevent it, and lessen the degree of collision CS838 Advanced Modeling and Simulation

8 Collision response (general approaches) Penalty-based methods Detect proximity to collision objects and apply a repulsive penalty force when the distance to the collision target is small Increase strength of repulsion force as distance decreases (or as interpenetration starts to occur) Does not strictly enforce a collision-free state, but attempts to prevent it, and lessen the degree of collision CS838 Advanced Modeling and Simulation

9 Collision response (general approaches) Penalty-based methods Detect proximity to collision objects and apply a repulsive penalty force when the distance to the collision target is small Increase strength of repulsion force as distance decreases (or as interpenetration starts to occur) Does not strictly enforce a collision-free state, but attempts to prevent it, and lessen the degree of collision CS838 Advanced Modeling and Simulation

10 Collision response (general approaches) Penalty-based methods Detect proximity to collision objects and apply a repulsive penalty force when the distance to the collision target is small Increase strength of repulsion force as distance decreases (or as interpenetration starts to occur) Does not strictly enforce a collision-free state, but attempts to prevent it, and lessen the degree of collision CS838 Advanced Modeling and Simulation

11 Collision response (general approaches) Penalty-based methods Detect proximity to collision objects and apply a repulsive penalty force when the distance to the collision target is small Increase strength of repulsion force as distance decreases (or as interpenetration starts to occur) Does not strictly enforce a collision-free state, but attempts to prevent it, and lessen the degree of collision CS838 Advanced Modeling and Simulation

12 Collision response (general approaches) Penalty-based methods Detect proximity to collision objects and apply a repulsive penalty force when the distance to the collision target is small Increase strength of repulsion force as distance decreases (or as interpenetration starts to occur) Does not strictly enforce a collision-free state, but attempts to prevent it, and lessen the degree of collision CS838 Advanced Modeling and Simulation

13 Collision response (general approaches) Penalty-based methods Detect proximity to collision objects and apply a repulsive penalty force when the distance Requirements to the collision for the detection stage: target Detection is small of static proximity (not just collision) Increase Estimation strength of of proximity repulsion or collision distance force as (such distance that forces decreases can be accordingly scaled) (or Estimation of collision direction as interpenetration starts (such that forces can be accordingly oriented) to occur) Does not strictly enforce a collision-free state, but attempts to prevent it, and lessen the degree of collision CS838 Advanced Modeling and Simulation

14 Collision response (general approaches) Impulse-based methods Usually attempt to guarantee that no collision is produced or left untreated, at any time Starting from a collision-free state at time t*, the system is advanced to time t*+dt Collisions that occurred in the interval [t*,t*+dt] are localized (in space and time) An impulse is applied to instantaneously correct the object trajectory and prevent (or fix) any collision events CS838 Advanced Modeling and Simulation

15 Collision response (general approaches) Impulse-based methods Usually attempt to guarantee that no collision is produced or left untreated, at any time Starting from a collision-free state at time t*, the system is advanced to time t*+dt Collisions that occurred in the interval [t*,t*+dt] are localized (in space and time) An impulse is applied to instantaneously correct the object trajectory and prevent (or fix) any collision events CS838 Advanced Modeling and Simulation

16 Collision response (general approaches) Impulse-based methods Usually attempt to guarantee that no collision is produced or left untreated, at any time Starting from a collision-free state at time t*, the system is advanced to time t*+dt Collisions that occurred in the interval [t*,t*+dt] are localized (in space and time) An impulse is applied to instantaneously correct the object trajectory and prevent (or fix) any collision events CS838 Advanced Modeling and Simulation

17 Collision response (general approaches) Impulse-based methods Can be structured to provide guarantees of noninterpenetration (makes other parts of the simulation simpler and easier) Capable of enforcing tight contact, instead of modeling a large, artificial thickness for the collision object Not guaranteed to succeed, especially with conflicting nonphysical constraints Relatively slow and expensive CS838 Advanced Modeling and Simulation

18 Collision response (general approaches) Impulse-based methods Can be structured to provide guarantees of noninterpenetration (makes other parts of the simulation simpler and easier) Capable of enforcing tight Requirements for the detection stage: contact, Detection instead of of modeling moving collisions a (not just static) large, artificial Estimation thickness of the exact for time/location of every the collision impact object event Not guaranteed to succeed, especially with conflicting nonphysical constraints Relatively slow and expensive CS838 Advanced Modeling and Simulation

19 Collision response (general approaches) Continuous time collisions Most physically justified technique : handle one collision event at a time, in the order they occur Can be structured to pursue full avoidance of collisions, while not requiring collision objects to be thickened Response can be formulated in terms of simple and intuitive penalty forces Disadvantage : May lead to very small time steps CS838 Advanced Modeling and Simulation

20 Collision response (general approaches) Continuous time collisions Most physically justified technique : handle one collision event at a time, in the order they occur Can be structured to pursue full avoidance of collisions, while not requiring collision objects to be thickened Response can be formulated in terms of simple and intuitive penalty forces Disadvantage : May lead to very small time steps CS838 Advanced Modeling and Simulation

21 Collision response (general approaches) Continuous time collisions Most physically justified technique : handle one collision event at a time, in the order they occur Can be structured to pursue full avoidance of collisions, while not requiring collision objects to be thickened Response can be formulated in terms of simple and intuitive penalty forces Disadvantage : May lead to very small time steps CS838 Advanced Modeling and Simulation

22 Collision response (general approaches) Continuous time collisions Most physically justified technique : handle one collision event at a time, in the order they occur Can be structured to pursue full avoidance of collisions, while not requiring collision objects to be thickened Response can be formulated in terms of simple and intuitive penalty forces Disadvantage : May lead to very small time steps CS838 Advanced Modeling and Simulation

23 Collision response (general approaches) Continuous time collisions Most physically justified technique : handle one collision event at a time, in the order they occur Can be structured to pursue full avoidance of collisions, while not requiring collision objects to be thickened Response can be formulated in terms of simple and intuitive penalty forces Disadvantage : May lead to very small time steps CS838 Advanced Modeling and Simulation

24 Collision detection (for kinematic objects) Simplest case: Collision object is rigid We can pre-process the object into a levelset Query : Is a point x* colliding with the object? Yes, if and only if Query : Is a point x* within a distance D of the object? Yes, if and only if Query : What is the direction of the collision (i.e. what is the shortest way out?) Given by the vector Query : What point on the surface of the object is closest to x*? Given as CS838 Advanced Modeling and Simulation

25 Represent a curve in 2D (or, a surface in 3D) as the zero isocontour of a (continuous) function, i.e. e.g. C = {(x.y) R 2 : φ(x, y) =0} circle x 2 + y 2 = R 2 {(x, y) :φ(x, y) =0} where φ(x, y) =x 2 + y 2 R 2 z =0 CS838 Advanced Modeling and Simulation

26 φ(x, y) < 0, if (x, y) isinsidec φ(x, y) > 0, if (x, y) is outside C φ(x, y) =0, if (x, y) is on C and φ(x, y) = distance of (x, y) from C CS838 Advanced Modeling and Simulation

27 Collision detection (for kinematic objects) Simplest case: Collision object is rigid We can pre-process the object into a levelset Query : Is a point x* colliding with the object? Yes, if and only if Query : Is a point x* within a distance D of the object? Yes, if and only if Query : What is the direction of the collision (i.e. what is the shortest way out?) Given by the vector Query : What point on the surface of the object is closest to x*? Given as φ(x ) < 0 CS838 Advanced Modeling and Simulation

28 Collision detection (for kinematic objects) Simplest case: Collision object is rigid We can pre-process the object into a levelset Query : Is a point x* colliding with the object? Yes, if and only if Query : Is a point x* within a distance D of the object? Yes, if and only if Query : What is the direction of the collision (i.e. what is the shortest way out?) Given by the vector Query : What point on the surface of the object is closest to x*? Given as φ(x ) < 0 φ(x ) <D CS838 Advanced Modeling and Simulation

29 Collision detection (for kinematic objects) Simplest case: Collision object is rigid We can pre-process the object into a levelset Query : Is a point x* colliding with the object? Yes, if and only if Query : Is a point x* within a distance D of the object? Yes, if and only if Query : What is the direction of the collision (i.e. what is the shortest way out?) Given by the vector Query : What point on the surface of the object is closest to x*? Given as φ(x ) < 0 φ(x ) <D φ(x ) CS838 Advanced Modeling and Simulation

30 Collision detection (for kinematic objects) Simplest case: Collision object is rigid We can pre-process the object into a levelset Query : Is a point x* colliding with the object? Yes, if and only if Query : Is a point x* within a distance D of the object? Yes, if and only if Query : What is the direction of the collision (i.e. what is the shortest way out?) Given by the vector φ(x ) < 0 φ(x ) <D φ(x ) Query : What point on the surface of the object is closest to x*? Given as x surface = x φ(x ) φ(x ) CS838 Advanced Modeling and Simulation

31 Collision detection (for simulated objects) Cannot (easily and efficiently) convert into levelsets to facilitate O(1) collision queries Sometimes we seek collisions between open surfaces, which do not have an interior to describe as a levelset If simulation contains N primitives (particles, segments, triangles, etc) there is a potential for O(N^2) candidate intersection pairs Brute force check would require O(N^2) cost Every simulation step ideally requires O(N) effort (e.g. with Forward Euler, or BE with fixed CG iterations) Ideally the detection cost should not exceed O(N) by much Popular approach : Using axis-aligned bounding box (AABB) queries to accelerate collision detection CS838 Advanced Modeling and Simulation

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72 Collision detection (for simulated objects) Popular approach : Using axis-aligned bounding box (AABB) queries to accelerate collision detection Prunes away most of the faraway collisions Cost to check one primitive, against a box B-tree hierarchy with k leaves : O(logk) in the best case Cost will increase if the box hierarchy is not optimally constructed (i.e. if we chose to merge faraway boxes) Quality of hierarchy will degrade as object moves : May choose to re-build the hierarchy from scratch every few time steps KD-Tree or Quad-/Oct-trees can be used to generate box hierarchies CS838 Advanced Modeling and Simulation