ONLINE CONFERENCE DESIGN.BUILD.DELIV ER with WINDOWS PHONE THURSDAY 24 MARCH 2011
Dec 13, 2015
Edward PowellVectric Ltd
@VeraShackle
XNA-UK Blog
3D Collisions
Assumptions
• You have reasonable understand basic 3D rendering with BasicEffect, Components, Services and the Content Pipeline.
• You have had a crack at some 2D games• You have some ideas about the
principles 2D collision detection.• You’re probably better at C# than me!
Content
• 15 mins - A bit of theory• 15 mins - Building a simple, but flexible
system • 5 mins - The Full Isaac Newton!
Elements of a collision system
• Collision detection– Have any objects collided?– Which object pairs have collided?– In what way have the objects collided?
• Collision Resolution– Simple event?– Complex interaction?– Correcting for detection lag?
Discrete time step between checks
• The actual moment of collision is never going to coincide with the checking algorithm.
• So Two Options:– Look forward to find collisions about to occur
• Complex interactions, complex algorithm• Never quite sure when the next check will be
– Look back at collisions that have already occurred• Simple algorithm (relatively!)• Know time since the last check• But, have to correct things that have already occurred
Looking back (hopefully not in anger)
• Things to watch out for:– Some overlap already at the point of
detection- ‘Drill through’...– Complete containment – boundaries no
longer intersect• Swept volumes?
– Multiple apparently ‘simultaneous’ collisions
Have objects collided – the lessons from 2D?
• Pixel perfect collision was too expensive– We checked for overlapping bounding
rectangle or circle – Multiple collision shapes per object– Use hierarchy of collision areas to reduce
the pixel perfect search to a manageable area
Same principles apply in 3D - Especially under WP7!
• triangle-triangle is the new pixel perfect!• Same solutions, but replace 2D areas
with 3D entities...
Collision Entities
• XNA Intrinsics– BoundingBox (AABB)– BoundingFrustum– BoundingSphere– Plane– Ray– Model (via spheres)
• Other common ones– Capsule – swept volumes– Oriented Bounding Box (OBB)– Height map– ‘Slabs’– Discrete Oriented Polytopes (DOP)!!– Triangle Mesh
Collision Primitives Summary...
• We don’t need a one-to-one mapping of collision primitive to drawable object
• Several simple volumes is generally cheaper than a single more complicated one.
• We can still do high-accuracy collision tests, but limit the volume with cheaper tests first.
(Image Source: http://sharky.bluecog.co.nz )
Pruning the Collision Search Space
• Brute force• Refining Bounding Volumes• Sweep & Prune (SAP)
– Insertion sort, relies on the fact that objects tend to be in a similar state between time steps.
• Grid Registration• BSP, kd-tree, quad-tree , oct-tree...
Approaches to Resolvers
• Distinct Resolver Class– Single location for all your resolution code– Needs to be modified as new entities are
added– What collision information is needed?
• Entities Resolve their own collisions– Collision entities are self-contained– Entities need to ‘know’ about at least some
others– Easily extensible
How much information do you need about the collision?
• When did the collision occur?• Which point (or points) contacted?• Other significant properties of the
object?– Velocity– Mass– Angular Momentum, coefficient of
restitution... etc. etc.!
Implementation
• Simple Collision Manager Component– Brute force check each update for...
• ICollidable Interface– Collision check– Collision resolution
• 2 Basic Classes:– CollidableSphere– CollidablePlane
Simple start, but...
• Bounding Spheres are calculated by the pipeline for meshes
• Discrete Oriented Polytope (DOP) – defining convex volumes using only planes.
(Image Source: epicgames.com)
Has a sphere collided with a plane?
Distance = Plane.DotCoordinate(sphere.Centre)
1. If the distance is positive, the sphere is in front of the plane
2. If the distance is negative, the sphere is behind the plane
3. If the distance from the plane is less than the radius of the sphere - COLLISION
Collision with Volume
In front of 1 plane, but intersecting 0. No collision
In front = 2Intersecting = 0No Collision
In front = 2Intersecting = 1No Collision
Rule: Must be intersecting all the planes we’re in front of...?
Collision with Volume
In front = 1Intersecting = 1COLLISION
In front = 2Intersecting = 2COLLISION
Rule: Must be intersecting all the planes we’re in front of...?
In front = 0COLLISION
Simple Collision System Demo
http://www.youtube.com/watch?v=5Xe2twpDe_8
XNACollisions_1
Duration: 1:14s
Using a Full Physics Engine
• Examples– Jitter (jitter-physics.com)
• Full & free use of dll• Source code access US$400
– BEPUPhysics (www.bepu-games.com) • Free for non-commercial use• Open Source as of last week!!
– JigLibX (jiglibx.codeplex.com)• Free• Open-source
JigLibX Phyics Demo
http://www.youtube.com/watch?v=Sc7XOJgCVdc
XNACollisions_2
Duration: 1:00s
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