Transcript Document

CIS 350 – I
Game Programming
Instructor: Rolf Lakaemper
Introduction
To
Collision Detection
Parts of these slides are based on
www2.informatik.uni-wuerzburg.de/ mitarbeiter/holger/lehre/osss02/schmidt/vortrag.pdf
by Jakob Schmidt
What ?
The problem:
The search for intersecting planes of
different 3D models in a scene.
Collision Detection is an important
problem in fields like computer
animation, virtual reality and game
programming.
Intro
The problem can be defined as
if, where and when
two objects intersect.
Intro
This introduction will deal with
the basic problem:
IF
two (stationary) objects
intersect.
Intro
The simple solution:
Pairwise collision check of all
polygons the objects are made
of.
Intro
Problem:
• complexity O(n²)
• not acceptable for reasonable
number n of polygons
• not applicable for realtime
application
Bounding Volumes
Part 1: Bounding Volumes
Reduce complexity of collision
computation by substitution of
the (complex) original object
with a simpler object
containing the original one.
Bounding Volumes
The original objects can only
intersect if the simpler ones
do. Or better: if the simpler
objects do NOT intersect, the
original objects won’t either.
Bounding Volumes
How to choose BVs ?
• Object approximation behavior (‘Fill
efficiency’)
• Computational simplicity
• Behavior on (non linear !)
transformation (incl. deformation)
• Memory efficiency
Bounding Volumes
Different BVs used in game
programming:
• Axes Aligned Bounding Boxes (AABB)
• Oriented Bounding Boxes (OBB)
•Spheres
• k-Discrete Oriented Polytopes (k DOP)
AABB
Sphere
OBB
k-DOP
Bounding Volumes
Axes Aligned Bounding Box (AABB)
• Align axes to the
coordinate system
• Simple to create
• Computationally efficient
• Unsatisfying fill efficiency
• Not invariant to basic
transformations, e.g.
rotation
Bounding Volumes
Axes Aligned Bounding Box (AABB)
Collision test: project BBs
onto coordinate axes. If
they overlap on each axis,
the objects collide.
Bounding Volumes
Oriented Bounding Box (OBB)
Align box to object such that it fits optimally
in terms of fill efficiency
Computationally expensive
Invariant to rotation
Complex intersection check
Bounding Volumes
The overlap test is based on the
Separating Axes Theorem
(S. Gottschalk. Separating axis theorem. Technical Report TR96-024,Department
of Computer Science, UNC Chapel Hill, 1996)
Two convex polytopes are disjoint iff there
exists a separating axis orthogonal to a face
of either polytope or orthogonal to an edge
from each polytope.
Bounding Volumes
Each box has 3 unique face orientations, and
3 unique edge directions. This leads to 15
potential separating axes to test (3 faces from
one box, 3 faces from the other box, and 9
pairwise combinations of edges).
Bounding Volumes
Sphere
• Relatively complex to compute
• Bad fill efficiency
• Simple overlap test
• invariant to rotation
Bounding Volumes
K-DOP
•Easy to compute
•Good fill efficiency
•Simple overlap test
•Not invariant to rotation
Bounding Volumes
k-DOP is considered to be a trade off
between AABBs and OBBs.
Its collision check is a general version of
the AABB collision check, having k/2
directions
Bounding Volumes
k-DOPs are used
e.g. in the game
‘Cell Damage’
(XBOX, Pseudo
Interactive, 2002)
Bounding Volumes
How to Compute and Store k-DOPs:
k-directions
k-directions Bi define halfplanes (they are
the normals to the halfplanes) , the
intersection of these halfplanes defines
the k-DOP bounding volume.
Halfplane Hi = {x | Bi x – di <= 0}
Bounding Volumes
3D Example: UNREAL-Engine
Bounding Volumes
2D Example for halfplanes defining a
k-DOP
Normal vector
Bi
Bounding Volumes
Again: halfplane Hi = {x | Bi x – di <= 0}
•If the directions Bi are predefined, only
the distances di must be stored to specify
the halfplane Hi . This is one scalar value
per direction.
•If the directions are NOT predefined, Bi
and di must be stored (3D case: 4 values)
Bounding Volumes
How to compute di :
Hessian Normal Form
( Bx – d = 0 with d = Bp) with
unit vector
of a plane automatically gives
distance d if a single point p on
the plane is known.
Bounding Volumes
Compute distances of all vertices to plane
Bx = 0, i.e. multiply (dot product) each
vertex with the unit normal vector B
Unit vector B
Bx = 0
Bounding Volumes
di is the minimum distance of the
object to the plane Bx = 0
Bx = 0
di
Bounding Volumes
Collision
Given: k non kollinear directions Bi and
V = set of vertices of object.
Compute di = min{Bi v| v in V } and
Di = max{Bi v| v in V}. di and Di define an
interval on the axis given by Bi .
This is the interval needed for the
collision detection !
Bounding Volumes
Collision
D1
Plane defined by B1
Interval [d1, D1] defined by B1
B1
d1
Bounding Volumes
Part 2: Collision on different scales:
Hierarchies
Hierarchies
Idea:
To achieve higher exactness in
collision detection, build a
multiscale BV representation
of the object
Hierarchies
Hierarchies
Use the hierarchy from coarse to fine
resolution to exclude non intersecting
objects
Hierarchies
The hierarchy is stored in a tree, named
by the underlying BV scheme:
AABB – tree
OBB – tree
Sphere – tree
kDOP – tree
Hierarchies
Sphere Trees are used for
example in
“Gran Tourismo”
Hierarchies
Simple example:
• Binary tree
• Each node contains all
primitives of its subtree
• Leaves contain single
primitive
Hierarchies
Hierarchies
Hierarchies
Recursive
Collision
Detection
Returns TRUE if BBs
overlap.
How could this be improved
to give a precise overlap test ?
)
Hierarchies
Hierarchies
How to create a hierarchy tree
Top down:
• Use single BV covering whole object
• Split BV
• Continue recursively until each BV
contains a single primitive
Hierarchies
Bottom up:
• Start with BV for each primitive
• Merge
Hierarchies
Example for top down using OBBs :
Hierarchies
Comparison AABB / OBB
Multiple Objects
Part 3: Collision between
Multiple Objects
Multiple Objects
Virtual environment usually consists of more
than 2 objects. Pairwise detailed collision
between all objects is too slow. Solution again:
1. Exclude non colliding objects
2. Check collision between remaining
objects
Multiple Objects
Methods to exclude non colliding objects:
1.Grid Method
Or
2. Sort and Sweep (AABB)
Multiple Objects
Grid Method:
Create 3d grid volume overlay
Only check collision between objects
sharing at least one cell
Multiple Objects
2D example
Multiple Objects
Sort and Sweep
• Create single AABB for each object
• Project BVs onto coordinate axes
• Create a sorted list of start and endpoints for
each coordinate axis, hence store the
intervals created by each object
(Cont’d)
Multiple Objects
• Traverse each list
• If startpoint of object i is hit, insert i into
‘active list’
• If endpoint of object i is hit, remove i from
‘active list’
• If 2 objects i1,i2 are active at the same time
they overlap in the dimension processed
• Objects overlapping in all single dimensions
overlap in world
Multiple Objects
X
S3
S1
E3
S2
E1
E2
s3
s1
3
e3
1
s2
e1
2
Y
S1
S2
E1
S3
E2
E3
e2
OVERLAP 1,2
s1
s2 e1 s3
e2 e3
Multiple Objects
Note: sort and sweep for a single step is
relatively expensive.
Since not all objects are transformed for the
next frame, the list is not created newly for
each frame, but updated.