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Crowd Simulations

Learn the basic principles of crowd simulations, including animation basics, key framing, simulation loops, and collision avoidance. Explore different crowd models and techniques for creating realistic crowd movements.

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Crowd Simulations

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  1. Crowd Simulations Guest Instructor - Stephen J. Guy

  2. Outline • Animation basics • Key framing • Simulation Loop • How to move one man • Walk Cycle • IK • How to move one thousand • Crowd Models • Collision Avoidance • Data Structures • Rendering

  3. Outline • Animation basics • Key framing • Simulation Loop • How to move one man • Walk Cycle • IK • How to move one thousand • Crowd Models • Collision Avoidance • Rendering

  4. Animation - Basics • Comp 768 Preview… • Goal: Illusion of continuous motion • Divide into several small time-steps (length T) • Show new image at each time-step • Needs to happened at least ~12/second (more is better) Advance T Draw Picture Update State

  5. Outline • Animation basics • Key framing • Simulation Loop • How to move one man • Walk Cycle • IK • How to move one thousand • Crowd Models • Collision Avoidance • Data Structures • Rendering

  6. Walk Cycle • Simply Translating a character to its goal is unrealistic • Walk Cycle: A looping series of positionswhich represent a character walking (or running or galloping) • Shifting the animation provides the illusion of walking Inplace Shifted w/ Time

  7. Digression - Eadweard Muybridge • 19th Century English Photograyher • Used multiple cameras to capture motion • Invented Zoopraxiscope (spinning wheel of still images) to animate images

  8. Walk Cycle - Analysis • Pros: • Simple to implement • Captures the basics of human movement • Cons: • Walks must cycle • Can’t handle changes in stride length • Can’t handle jumps • Must be animated by hand

  9. Walk Cycle - Alternatives • Inverse Kinematics • Using math to figure out where to place the rest of the body to get the feet moving forward • Motion Capture • Record data of real humans walking • Motion Clips • FSM of different motions 

  10. Outline • Animation basics • Key framing • Simulation Loop • How to move one man • Walk Cycle • IK • How to move one thousand • Crowd Models • Collision Avoidance • Data Structures • Rendering

  11. Crowd Simulation Models • Simplest model – Agent Based: • Capture Global Behavior w/ many interacting autonomous agents • Each person is represented by one agent • Chooses next state based on goal and neighbors • Pioneered by Craig Reynolds • Won 1998 (Technical) Academy Award Advance T For Each Agent s Draw Agent Update State Gather Neighbors

  12. Agent Based Simulations • Flocking • Craig Reylonds • SIGGRAPH1987 • Social Forces Model • Dirk Helbing • Physics Review B 1995 • Nature 2000 • Reciprocal Velocity Obstacles • Van den Berg • I3D 2008

  13. Agent Based Simulations • Flocking • Craig Reylonds • SIGGRAPH1987 • Social Forces Model • Dirk Helbing • Physics Review B 1995 • Nature 2000 • Reciprocal Velocity Obstacles • Van den Berg • I3D 2008

  14. Flocking • Seminal work in multi-agent movement • Assign simple force to each agent • Used in • Lion King • Batman Returns Separation Alignment Cohesion

  15. Boids - Continued • New forces can be added to incorporate more behaviors • Avoiding Obstacles • Collision Avoidance • Be Creative!

  16. Boids Online • Visit: http://www.red3d.com/cwr/boids/ • And: http://www.red3d.com/cwr/steer/Unaligned.html

  17. Agent Based Simulations • Flocking • Craig Reylonds • SIGGRAPH1987 • Social Forces Model • Dirk Helbing • Physics Review B 1995 • Nature 2000 • Reciprocal Velocity Obstacles • Van den Berg • I3D 2008

  18. Helbing’s Social Force Model • Very similar to boid model • Treats all agents as physical obstacles • Solves a = F/m where F is “social force”: • Fij – Pedestrian Avoidance • FiW – Obstacle (Wall) Avoidance Desired Velocity Current Velocity Avoiding Other Pedestrians Avoiding Walls

  19. Social Force Model – Pedestrian Avoidance • rij – dijEdge-to-edge distance • nij – Vector pointing away from agent • Ai*e[(rij-dij)/Bi] Repulsive force which is exponential increasing with distance • g(x)  x if agents are colliding, 0 otherwise • tij – Vector pointing tangential to agent • Vtji – Tangential velocity difference • FiW is very similar Collision Avoidance Non-penetration Sliding Force

  20. Helbing - Continued • Noticed arching • Also observed in real crowds • Killed or injured people whoexperienced too much force (1,600 N/m) – became unresponsive obstacles • Noticed Faster-is-slower effect

  21. Agent Based Simulations • Flocking • Craig Reylonds • SIGGRAPH1987 • Social Forces Model • Dirk Helbing • Physics Review B 1995 • Nature 2000 • Reciprocal Velocity Obstacles • Van den Berg • I3D 2008

  22. Reciprocal Velocity Obstacles • Applied ideas from robotics to crowd simulations • Basic idea: • Given n agents with velocities, find velocities will cause collisions • Avoid them! • Planning is performed in velocity space • RVOAB(vB, vA) = {v’A | 2v’A – vA VOAB(vB)}

  23. RVO: Planning In Velocity Space ?

  24. RVO: Planning In Velocity Space ?

  25. RVO: Planning In Velocity Space RA+ RB ?

  26. RVO: Planning In Velocity Space (VA + VB)/2 ?

  27. RVO: Planning In Velocity Space ?

  28. RVO: Planning In Velocity Space

  29. RVO: Planning In Velocity Space

  30. RVO: Planning In Velocity Space

  31. RVO: Planning In Velocity Space

  32. RVO: Planning In Velocity Space

  33. Videos • 12 Agents in a Circle

  34. Videos • 1,000 agent’s in a circle

  35. Related data-structures • KD-trees • Allowing efficient gathering of nearby neighbors O(log n) • Roadmaps & A* • Allows global navigation around obstacles

  36. Roadmaps • Create roadmap in free space • Find visible source nodes • Graph Search to find path to Destination • A* is very popular graph search algorithm

  37. Video • 1,000 people leaving Sitterson Hall • Uses RVO, Roadmaps, A* and Kd-Trees

  38. Outline • Animation basics • Key framing • Simulation Loop • How to move one man • Walk Cycle • IK • How to move one thousand • Crowd Models • Collision Avoidance • Data Structures • Rendering

  39. Rendering Crowds • Traditional OpenGL pipeline can be too slow for 1000s of agents • View Culling helps, but often not enough • Need Level-of-Detail techniques • Use models with more polygons up close, less when far away

  40. Imposters • Replace Far off agents with an oriented texture • Several Issues • “Popping” • Uniformity • Lighting • Shadows • Many issues addressed in recent works

  41. Questions ?

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