1 / 22

CS361

Week 14 - Monday. CS361. Last time. What did we talk about last time? Bounding volume/bounding volume intersections. Questions?. Assignment 5. Project 4. Student Lecture: Collision Detection. Collision Detection. Collision detection.

janae
Download Presentation

CS361

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Week 14 - Monday CS361

  2. Last time • What did we talk about last time? • Bounding volume/bounding volume intersections

  3. Questions?

  4. Assignment 5

  5. Project 4

  6. Student Lecture:Collision Detection

  7. Collision Detection

  8. Collision detection • There are three important pieces to collision handling: • Collision detection • Did these objects collide? • Collision determination • When and where did these objects collide exactly? • Collision response • What happens as a result of the collision?

  9. Collision detection expectations • Achieve interactive rates with complex models when those models are near and far from each other • Handle polygon soups (no guarantees about convexity or adjacency) • Assume that models can undergo rigid body transformations • Use efficient bounding volumes

  10. Collision detection with rays • Rather than try to test complex models for collision with an environment, we can use representative rays instead • Perhaps one ray for each wheel of a car • A positive ray distance means space between the objects • A negative ray distance means collision • A zero ray distance means the objects are merely touching • It's essentially a ray tracing problem

  11. BSP trees • Binary space partitioning trees (BSP trees) are a common way of dividing space hierarchically • For axis-aligned BSPs, one axis is chosen and a perpendicular plane is generated to divide the box • This process is repeatedly recursively until some criteria (like 3 or fewer objects per division) is reached • BSPs can also be split by choosing polygons to divide the world (usually done so as to make a perfectly balanced tree) • BSPs are good for static scenes (moving objects can cause huge portions of the tree to be recreated)

  12. Dynamic CD with BSP trees • We look at a movement of a time step going from point p0 to p1 • We then just need to see if the line connecting those points intersects any objects (easy to do in a BSP) • We have to temporarily alter each plane's location based on the size of the bounding volume (e.g. move the plane closer by a value of r to test against a sphere with radius r) • Note: characters in games are often represented with cylinders

  13. Hierarchical CD • We can build hierarchies using one of the three following approaches: • Bottom-up: Find nearby BVs and combine them, doing so recursively • Incremental tree-insertion:Start with an empty tree and add BVs according to what is going to add the least total volume to the tree • Top-down (most common): Find a BV for the entire model, then divide the BV into k or fewer parts, recursively • Finding a good split point is key

  14. Multiple objects CD • Hierarchies are generally made for static scenes • Then we test against them for collisions with dynamic objects • What about when there are multiple moving objects that might interact with each other? • We work in two phases • Broad phase collision detection • Exact collision detection among candidates

  15. Sweep-and-prune • Assume everything has an AABB or a bounding sphere • Assume temporal coherence (stuff doesn't move that much over a small amount of time) • On one dimension, we can sort the endpoints of the AABBs • We can quickly throw out objects that cannot possibly intersect • Bubble sort or insertion sort to the rescue! • We could sort everything in O(n log n) every time • Because of temporal coherence, not many end points change order and adaptive sorts work in around O(n)

  16. Grids • Another possibility is keeping large grid cells that keep track of which objects or BVs are inside them • Objects that do not share grid cells do not need to be checked for collision • Finding the right grid cell size can be difficult • Spatial hashing can be used as well (mapping to a hash table based on location)

  17. Putting it together • Here is an outline of a frame in a typical two-phase CD system

  18. Time critical CD • For games and other time critical problems, it may be necessary to restrict the amount of time available for CD to a fixed value (or whatever is left of the allotted time after rendering) • When we don't know if we are going to have time to visit the entire tree hierarchy, we may want to visit the tree breadth-first

  19. Collision response • Collision response means whatever action is done to prevent abnormal object interpenetration • Usually a bounce or something like that • It is important to find the exact time of the collision (might be in the middle of a frame) to get the correct bounce • Let velocity v = vn + vp where vn is the velocity parallel to the normal of the surface • For a perfectly elastic bounce, the new velocity is • v' = vp – vn • In real collisions, some energy is lost, described by the coefficient of restitution k, making • v' = vp – kvn

  20. Upcoming

  21. Next time… • Non-photorealistic rendering

  22. Reminders • Finish Assignment 5 • Due on Friday • Keep working on Project 4 • Due next Friday • Read Chapter 11

More Related