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Eurographics Symposium on Geometry Processing (2003)L. Kobbelt,
P. Schröder, H. Hoppe (Editors)
CLODs: Dual Hierarchies for Multiresolution Collision
Detection
Miguel A. Otaduy and Ming C. Lin
Department of Computer Science, University of North Carolina,
Chapel Hill, U.S.A
AbstractWe present “contact levels of detail” (CLOD), a novel
concept for multiresolution collision detection. Given apolyhedral
model, our algorithm automatically builds a “dual hierarchy”, both
a multiresolution representation ofthe original model and its
bounding volume hierarchy for accelerating collision queries. We
have proposed variouserror metrics, including object-space errors,
velocity dependent gap, screen-space errors and their
combinations.At runtime, our algorithm uses these error metrics to
select the appropriate levels of detail independently at
eachpotential contact location. Compared to the existing exact
collision detection algorithms, we observe significantperformance
improvement using CLODs on some benchmarks, with little degradation
in the visual rendering ofsimulations.
Categories and Subject Descriptors (according to ACM CCS): I.3.3
[Computer Graphics]: Hierarchy and GeometricTransformations
1. Introduction
Collision detection is an important problem in physically-based
modeling, computer animation, virtual environments,electronic
prototyping, robotics and many other engineeringapplications.
Collision detection is often one of the compu-tational bottlenecks
in achieving interactive simulation forcomplex environments. The
running time of any collisionquery algorithm depends on both the
input size (the combi-natorial complexity of the models) and the
output size (thenumber of contact points). Despite the huge body of
litera-ture, the existing techniques cannot offer the desired
perfor-mance for many real-time applications, such as haptic
ren-dering of large structures, real-time interaction with
massiveCAD models, and interactive dynamic simulations of com-plex
objects.
In this paper, we investigate the use of
multiresolutiontechniques for collision detection. Model
simplification hasbeen an active research area for the past decade.
Many effi-cient and effective simplification algorithms have been
pro-posed to accelerate the visual rendering of highly
complexmodels in real time. Applications of mesh simplification
al-gorithms to the problem of collision detection can poten-tially
accelerate collision queries. However, to date only rel-atively
simple algorithms have been proposed for convexpolytopes. A simple
approach would be to generate a seriesof simplified
representations, also known as levels-of-detail(LOD), and use them
directly for collision detection. But,collision queries require
auxiliary data structures, such asbounding volume hierarchies (BVH)
or spatial partitioning,to achieve good runtime performance. These
data structures
often take a substantial amount of memory, while the
LODsthemselves can require a large amount of storage as well.
Torepresent the auxiliary data structures for each static LODcan
quickly increase the memory requirement to the pointwhere the
memory access time may dominate the overallruntime performance. On
the other hand, constructing BVHsfor on-the-fly simplification can
be time consuming.
Main Contribution: We present the notion of “contact lev-els of
detail” (CLODs) that is based on a novel data struc-ture and
algorithms for multiresolution collision detection.The data
structure is a “dual hierarchy” that serves both asa BVH for
accelerating collision queries and a multiresolu-tion
representation of the original model for computing con-tact
information. We have also proposed several error met-rics,
including object-space errors, velocity dependent gap,screen-space
errors and their combinations. At runtime, thealgorithm uses these
error metrics to select adaptive CLODat each potential contact
location independently. In addition,CLODs offer the capability to
perform time-critical colli-sion queries for real-time
applications. We have tested ourapproach on a diverse set of
benchmarks with challengingcontact scenarios. We observed
noticeable performance im-provement with little degradation in the
visual rendering ofsimulation results on some of the
benchmarks.
Organization: Section 2 briefly surveys previous work. Wepresent
the general data structure and associated proximityquery algorithm
for contact levels-of-detail in section 3. Sec-tions 4 and 5
describe a specific implementation of hierarchyconstruction using
convex hulls and its use for runtime con-tact queries respectively.
We discuss the implementation is-
c© The Eurographics Association 2003.
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Otaduy and Lin / CLODs for Multiresolution Collision
Detection
sues and demonstrate the effectiveness of our approach insection
6.
2. Related Work
This research is built upon a large collection of knowledge
inmesh simplification and collision detection. We briefly sur-vey
related work in this section.
2.1. Model Simplification
Many techniques for mesh decimation and model simplifi-cation
have been proposed for the last few years. We referreaders to an
excellent book on this subject18. There has beensome growing
interest in perception-based simplification forinteractive
rendering17. However, these techniques are basedon different
criteria than those for collision detection. Al-though we
precompute the level of detail (LOD) hierarchyoffline, the way we
select the appropriate LOD on the fly is“contact dependent” at each
potential contact location acrossthe object surfaces. It shares a
more common theme withview-dependent simplification18, which uses
higher resolu-tion representations on the silhouette of the object
and muchcoarser approximations on the rest of the object that is
not asnoticeable to the viewpoint.
El-Sana and Varshney4 proposed to construct a continu-ous,
multiresolution hierarchy of the model during prepro-cessing. At
run time, a high-detail representation is used inthe region of
contact, and a coarser representation fartheraway. The proposed
approach only applies to haptic render-ing using a point probe to
explore a 3D model. It does notextend naturally to collision
queries between two interacting3D objects, since multiple disjoint
contacts can occur simul-taneously at widely varying locations
without much spatialcoherence. The latter problem is the focus of
our paper.
2.2. Collision Detection
Data structures based on hierarchical representations havebeen
extensively used in the design of efficient algo-rithms for
collision detection between geometric models (seesurvey12, 14).
Examples of typical bounding volumes includespheres and
axis-aligned bounding boxes, due to their sim-plicity in performing
overlap tests between two such vol-umes. Other hierarchies include
octrees, k-d trees, k-DOPs,OBBTrees, trees based on S-bounds,
R-trees and their vari-ants, etc.12, 14. Other spatial
representations are BSP’s andtheir extensions to multi-space
partitions, space-time boundsor four-dimensional tests (see a brief
survey22), and manymore.
O’Sullivan and Dingliana20 investigated LOD techniquesfor
collision simulations and studied various factors affect-ing
collision perception, including separation, eccentricity,causality,
distractors, and accuracy of simulation results.Their work is based
on the model of human visual percep-tion and validated by
psychophysical experiments. The fea-sibility of using these factors
for scheduling interruptible
collision detection among large numbers of visually homo-geneous
objects is also demonstrated. Instead of addressingthe scheduling
of multiple collision events among many ob-jects, we focus
primarily on the problem of contact queriesbetween two highly
complex objects.
Recently techniques based on GPU acceleration have alsobeen
proposed for collision queries9, 16. However, these ap-proaches are
not necessarily faster for rigid bodies whereprecomputation can be
effectively carried out off-line, sincethe readback from
framebuffer and depth buffer cannot bedone fast enough.
The concept of time-critical collision detection wasfirst
introduced by Hubbard11 using sphere-trees. Collisionqueries are
performed as far down the sphere-trees as timeallows, without
traversing the entire hierarchy. The con-tact information was
derived from two colliding boundingspheres and could deviate
arbitrarily from the actual con-tact normals and contact locations.
This idea can be appliedto any type of bounding volume hierarchies
(BVH). An er-ror metric is often desirable for interactive
applications toformally and rigorously quantify the amount of error
intro-duced. However, no tight error bounds have been providedusing
such approaches. This can be problematic, especiallywhen contact
normals and contact points are required tocompute a plausible
collision response.
Some techniques limited to convex polytopes have beenproposed to
exploit hierarchical representations and motioncoherence for fast
distance computation 2, 7. The progres-sive hull representation 23
also presents some favorable char-acteristics for collision
detection purposes, but to the bestof our knowledge no data
structure for collision detectionhas been defined using this
representation. In addition, thesetechniques suffer from a surface
enlargement problem dueto meeting containment conditions, that can
result in no-table surface deviations when being used for
multiresolutioncollision detection. Our implementation of CLODs
createsa BVH of convex hulls, taking advantage of the work byEhmann
and Lin3. Our work differs from theirs in the sensethat we create a
new multiresolution representation of theobjects to be used in
multiresolution collision detection.
The basic idea of CLODs has been successfully applied tohaptic
rendering21. Here we generalize CLODs by
definingimplementation-independent data structures and
algorithms,present more diverse error metrics, and apply CLODs
torigid body dynamic simulation.
3. Contact Levels of Detail
In this section, we present our approach for constructing
hi-erarchies of CLODs. First we analyze the advantage of usingCLODs
in hierarchical collision detection, then we describethe associated
data structures, as well as the process for cre-ating them, and
finally we explain how CLODs are used inruntime collision
queries.
c© The Eurographics Association 2003.
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Otaduy and Lin / CLODs for Multiresolution Collision
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3.1. Hierarchical Collision Detection
Bounding volume hierarchies (BVHs) are commonly usedfor
collision detection between general geometric objects.To perform
intersection tests, two models are queried by re-cursively
traversing their BVHs in tandem. Each recursivestep tests whether a
pair of bounding volumes a and b, onefrom each hierarchy, overlap.
If a and b do not overlap, therecursion branch is terminated.
Otherwise, if they do over-lap, the algorithm is applied
recursively to their children. Ifa and b are both leaf nodes, the
primitives within them aretested directly.
The test between the two BVHs can be described by thebounding
volume test tree (BVTT)13, a tree structure thatholds in each node
the result of the query between twoBVs. When temporal coherence is
present, collision tests canbe accelerated by generalized front
tracking (GFT)3. GFTcaches the front of the BVTT, where the result
of the queriesswitches from "true" to "false", for collision query
in the nexttime step. The overall cost of a collision test is
proportionalto the number of nodes in the front of the BVTT.
When large areas of the two objects are in a close proxim-ity, a
larger portion of the BVTT front is close to the leaves,and it
consists of a larger number of nodes. The size of thefront also
depends on the resolution at which the objects aremodeled; higher
resolution implies a BVTT with a greaterdepth. We can draw the
conclusion that the cost of a colli-sion query depends on two key
factors:• The size of the contact area• The resolution of the
models
Figure 1: a) Large contact area in high resolution; b)
Largecontact area in low resolution; c) Small contact area in
highresolution
The contact between two real objects typically occursalong a
certain contact area. With polygonalized models, thismay result in
multiple contact points. The finer the resolu-tion of the objects,
the larger the number of contact points,as seen in Fig. 1. However,
employing a larger resolutionmay have little effect on the forces
computed between theobjects, because these forces are computed as a
sum of con-tact forces arising from a net of contact points. We can
arguethat intuitively a larger contact area allows the objects to
bedescribed at a coarser resolution. This is also supported
bystudies on tactual perception21. In this paper we exploit
thishypothesis to create multiresolution representations of
theobjects, and use them at each contact location independentlyfor
selecting the appropriate resolution of each approxima-tion. Using
CLODs we can achieve nearly constant cost for
collision queries by exploiting, among other factors, the
rela-tion between contact area and the resolution of local
contactfeatures.
3.2. Data Structure
In order to perform efficient multiresolution collision
detec-tion, we need to achieve two main objectives:1. Create
accurate multiresolution representations.2. Embed the
multiresolution representations in effective
bounding volume hierarchies.Multiresolution representations are
often created by per-
forming mesh decimation on the given polyhedral models.The
difficulty arises when trying to embed these represen-tations in
BVHs. If we consider each LOD of the object asone whole model, each
LOD would require a distinct BVHfor collision detection. This would
result in a very inefficientcollision query, because the front of
the BVTT would have tobe updated for the BVH of each LOD. Instead,
we introducea procedure to create one unique dual hierarchy that
servesas both a multiresolution representation and a BVH.
Assuming that the input model is described as a trianglemesh M0,
the data structure for CLODs is composed of:• A sequence of LODs
{M0,M1, ...,Mn−1}, where Mi+1
is obtained by applying simplification operations and re-moving
high resolution geometric detail from Mi.
• For each mesh Mi, a partition of the set of triangles of
Miinto disjoint clusters {ci,0,ci,1, ...,ci,m}.
• For each cluster ci, j, a bounding volume Ci, j.• A tree T
formed by all the BVs of clusters, where BVs of
clusters in Mi are children of BVs of clusters in Mi+1, andall
the BVs except the ones corresponding to M0 have atleast one
child.
• For every BV, Ci, j, the maximum directed Hausdorff dis-tance
h(Ci, j) from its descendent BVs.The tree T of BVs, together with
the Hausdorff distances,
serves as the BVH for culling purposes in collision
detection.Directed Hausdorff distances are necessary because in
ourdefinition of CLODs the set of BVs associated with one
par-ticular LOD may not bound the surface of previous LODs.The
Hausdorff distances are used to perform conservativecollision
tests, as later explained in Sec. 5.1.
An additional constraint is added to the data structure,such
that the coarsest LOD, Mi−1 is partitioned into one sin-gle cluster
ci−1,0. Therefore, the root of the BVH will be theBV of the
coarsest LOD. Descending to the next level of thehierarchy will
yield the children BVs, whose union enclosesthe next LOD. At the
end of the hierarchy, the leaf BVs willenclose the original surface
M0.
The process of creating the CLODs, depicted in Fig. 2,starts by
grouping the triangles of the original surface intoclusters. The
size and properties of these clusters depend onthe type of BV that
is used for the BVH, and will be suchthat the collision query
performance between two BVs is op-timized. The next step is to
compute the BV of each cluster.
c© The Eurographics Association 2003.
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Otaduy and Lin / CLODs for Multiresolution Collision
Detection
Figure 2: Constructing Dual Hierarchies: (a) Initial surface;
(b) Clusters of triangles; (c) BVs for each cluster (in this
case,AABBs); (d) Mesh simplification; (e) BV of the union of
clusters after some conditions are met.
After this initialization, we start a mesh decimation
processwith a bottom-up construction of the BVH. This is achievedby
merging clusters and computing the BV of their union.
The atomic simplification operations need to satisfy
thefollowing:
• Constraints imposed by the BVH: The containment ofsurface
geometry inside the BVs has to be preserved aftereach
simplification operation. This may impose topologi-cal and/or
geometric constraints.
• Design requirements to achieve better efficiency: Theunion of
clusters is possible when certain conditions aremet. The BVH will
be more effective in collision pruning,if these conditions are
taken into account when designingthe atomic simplification
operations.
In Sec. 4, we present a specific implementation of CLODsthat
uses convex hulls as the BVs.
3.3. Runtime Queries with CLOD
Using CLODs, multiresolution collision detection can
beimplemented by slightly modifying the typical collision
de-tection procedures using BVHs. For each node x of theBVTT, we
perform a collision query. If the query returns"false", we do not
need to descend to the children. If thequery result is "true", then
we perform a test for selective re-finement. This test can embed
various perceptual error met-rics, and it determines if the
resolution of x is fine enoughto describe the contact information
at each query location. Ifthe refinement test returns "true", then
we can directly com-pute contact information for x, otherwise we
descend to itschildren in the BVTT. Descending to the children
involvesdescending to the children BVs, as well as refining the
sur-face representation. This approach handles the selective
re-finement at each query location independently in a very
effi-cient way.
CLODs can also be used to perform time-critical
collisiondetection11. We need to store the nodes of the BVTT
front,and assign priorities to each node based on the
refinementtest.
In Sec. 5 we describe how we implement the collisionqueries
using CLODs based on convex hulls, as well as var-ious error
metrics that we have designed.
4. Dual Hierarchies of Convex Hulls
In this section we describe a particular implementation ofCLODs
using BVHs of convex hulls. We first address thereasons for
choosing convex hulls as the BVs, and then de-scribe the details of
constructing dual hierarchies.
4.1. Selection of the BVs
Overlap tests between convex hulls can be executed rapidlyin
expected constant time with motion coherence7. Further-more, convex
hulls provide at least equally good, if not su-perior, fitting to
the underlying geometry as OBBs6 or k-DOPs12.
Most importantly the fitting property is related to the
per-formance of the query to obtain contact information of a
cer-tain CLOD. As explained in Sec. 3.3, when the refinementtest
determines that a CLOD does not need to be refined, wemust get
contact information at that level. That implies get-ting contact
information from the triangles of that specificCLOD. If the BVs at
that level are one of AABB, OBB ork-DOPs, the efficiency of getting
contact information usingtriangles is related to the number of
triangles in each cluster.However, with convex hulls, if we ensure
that the clusters arethemselves convex surface patches, the contact
informationat the triangle level is obtained for free when
performing thequery between BVs3.
The initial clusters will be defined as the surface patchesof a
convex surface decomposition1, 3. We follow the defini-tion of
convex surface patches by Ehmann and Lin 3, whichimposes two types
of convexity constraints on the process ofcreating dual
hierarchies:
• Local constraints: the interior edges of convex patcheshave to
remain convex after simplification operations areapplied.
• Global constraints: the enclosing convex hulls cannotprotrude
the surface of the object.
However, it is important to note that convex hulls posesome
limitations on the types of models that can be handled.In our
implementation, the convex hulls are formed by con-vex surface
decomposition, which requires the input modelsto be described as
2-manifold oriented surfaces.
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4.2. Construction of the Dual Hierarchy
As already mentioned, the process is initialized by perform-ing
a convex surface decomposition of the object and com-puting the
convex hulls of the patches. This is followed by asimplification
loop represented schematically in Fig. 3.
We apply atomic simplification operations taking into ac-count
convexity constraints. After each operation, we test ifthe union of
every pair of neighboring convex patches is avalid convex patch. If
so, we merge the patches, constructthe convex hull of the union,
and set parent children rela-tionships between the BVs. We have
chosen to generate anew LOD every time that the number of convex
patches ishalved.
Due to the convexity constraints, we might reach a pointwhere no
more simplification operations are possible. Wecomplete the
construction of the hierarchy by unconstrainedpairwise merging of
patches3. The levels of the hierarchycreated in this manner cannot
be used as CLODs in collisionqueries, but are necessary to complete
the BVH.
Figure 3: Generation of CLODsThere are various aspects of the
simplification process
central to our construction of CLODs that need to be de-fined:•
The type of simplification operation• The assignment of priorities
for simplification• The local retriangulation after the
simplification
We select edge collapse as the atomic simplification op-eration.
There are many options for prioritizing edges andselecting the
position of the resulting vertices: minimizationof energy
functions10, optimization approaches15, quadricerror metrics for
measuring surface deviation5, and more.However, none of these
approaches meets the convexityconstraints or takes into account the
factors that maximizethe efficiency of CLODs. Another possibility
is to employ
the progressive hulls representation23, which presents
theinteresting property of containment for collision
detectionpurposes. However, in this representation successive
LODsare not simply multiresolution representations of the
initialmesh, because they also undergo an enlargement processwhich
can result in noticeable visual artifacts. Our local
sim-plification operations are inspired by multiresolution
analy-sis and signal processing of meshes.
4.2.1. Multiresolution Analysis of MeshesAn LOD M j of a mesh M0
at resolution r j is defined as anapproximation of M0 that stores
all the surface detail at res-olutions lower than r j. Following
this definition, we havedecided to prioritize the edges to be
collapsed based on theirresolution. We use the same definition of
edge resolution asin 21, which is inspired by the definition of
resolution forfunctions in the 1D setting. By the same definition
of LODs,the collapse of an edge at resolution r j involves
removingthe geometric detail at resolutions higher than r j. We
imple-ment this by incorporating a filtering step in the
placementof vertices resulting from edge collapse operations.
We have compared this approach to other techniques forplacing
the vertices resulting from edge collapses, and it hasproved to
accelerate the process of merging convex patches.This is probably
because the filtering step tends to removeconcavities at high
resolution, while other techniques fail toremove these concavities
in favor of preserving other prop-erties. Note that a more
efficient merging of convex patcheshas beneficial consequences in
the performance of CLODsat runtime.
4.2.2. Filtered Edge CollapseWe employ a local simplification
operation filtered edgecollapse 21 subject to local and global
convexity constraints.This operation is composed of the following
steps:1. A topological edge collapse.2. An initialization process
that sets the position of the re-
sulting vertex using quadric error metrics 5.3. Unconstrained
filtering of the position of the vertex
using Guskov’s minimization of second order
divideddifferences8.
4. Solving an optimization problem to minimize the dis-tance of
the vertex to its unconstrained position whiletaking into account
the local convexity constraints.
5. A bisection search between the initial position of the
ver-tex and the one that meets the local constraints, to find
alocation where the global convexity constraints are alsomet.The
filtered edge collapse operation is described in further
detail in 21.
In Fig. 5 (see color plate) we show an example of a
dualhierarchy of CLODs. The process is applied to the model of
awrinkled torus. Fig. 5-b shows a detailed view of the
convexdecomposition of the original surface. In the next CLODs,the
filtering smoothens the concave areas, and the convex
c© The Eurographics Association 2003.
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patches grow forming triangle strips on the hyperbolic re-gions.
Likewise, the simplification operations coarsen thetriangulation.
Also notice that initially the simplification op-erations are
concentrated in the region where the wrinklesmerge and is also the
area with the finest detail.
5. Runtime Queries
In this section we discuss our implementation of
collisionqueries using CLODs of convex hulls, present various
errormetrics for selecting the appropriate CLODs, and demon-strate
the application to rigid body dynamic simulation.
5.1. Collision Queries between CLODs of Convex Hulls
A typical collision query between two BVs a and b deter-mines if
a and b are closer than a distance δ. The collisiondetection
algorithms based on BVHs must ensure that if aleaf node x of the
BVTT returns "true" to the collision query,then all its ancestors
must return "true" too. This is usuallyachieved by containing the
surface of each object inside theunion of the BVs at every level of
its BVH. In our implemen-tation of CLODs this containment property
does not hold,but we can ensure the correctness of the collision
detectionby modifying the collision distance δab between two
convexhulls a and b:
δab = δ+h(a)+h(b) (1)
where h(a) and h(b) are maximum directed Hausdorff dis-tances
from the descendant BVs of a and b to a and b re-spectively.
5.2. Error Metrics
We have designed a functional φ to evaluate the resultingerror,
when we stop a collision query at a node ab of theBVTT. φa =
sar2a
(2)
where sa is the surface deviation of the convex hull a
withrespect to the original surface and ra is its resolution.
Bothvalues are precomputed. The functional φ can be regardedas a
measure of the volume of the fictitious features that arefiltered
out when using a as the CLOD.
As introduced in Sec. 3.1, larger contact areas allow themodels
to be described at coarser resolution. We take intoaccount this
observation by averaging the functional φ overan estimated contact
area D. Thus, we compute a weightedsurface deviation s∗ as:
s∗ab =max(φa,φb)
D(3)
s∗ can been considered as the surface deviation errorsweighted
by a constant that depends on both the contact areaand the
resolutions of local surface features. The contact areaD is
estimated based on precomputed per-vertex support ar-eas (see 21
for more details).
The node of the BVTT ab is refined if s∗ab is larger than
athreshold value, s0. This threshold can be determined basedupon
different perceptual metrics.
Size Dependant Metric
s0 will be determined as a fixed percentage of the radii ofthe
objects involved in the contact queries.
Velocity Dependant Metric
Set s0 as a value proportional to the relative velocity ofthe
colliding objects at the contact location. This is basedon the
observation that the gap between the objects is lessnoticeable as
the objects move faster.
View Dependant Metric
Determine s0 based on screen-space errors. Given N pix-els of
error, a distance l from the camera to the contact lo-cation, a
distance n to the near plane of the view frustum, asize f of the
frustum in world coordinates, and a size i of theimage plane in
pixels:
s0 =N · l · f
n · i(4)
Selective refinement using CLODs can be implementedcombining any
of these error metrics. It can also supportother types of
perceptual error metrics20.
Constant Frame-rate: Another important feature is the factthat
CLODs can be used for time-critical collision detection.The factor
s
∗
s0 , computed at every potential contact location,can be used to
prioritize the refinement. To achieve a guar-anteed framerate for
real-time applications, the algorithmwill perform as many collision
queries as possible, within afixed time interval. The query event
queue will be prioritizedbased on s
∗
s0 .
5.3. Application to Rigid Body Dynamic SimulationIn rigid body
dynamic simulations, many of the factors in-volved in the selection
of CLODs, such as the velocity ofthe objects, the contact area and
the distance to the camera,will vary dynamically. This results in
transitions in the re-finement tests and thus switching between
selected CLODs.Rigid body dynamic simulations often require the
executionof time-stepping algorithms to search for the time
instantsprior to collision events. With CLODs the result of the
colli-sion queries may vary depending on the selected
resolution,therefore special treatment is necessary so that
switchingCLODs do not generate inconsistencies or deadlock
situa-tions in the time-stepping algorithms. (The analogy in
visualrendering is the “popping” effects often seen during
LODswitching.)
Given a node xi of the BVTT, with collision query “false”at
times ti and ti+1 of the simulation, and a node x j, childof xi,
with collision query “true” at both time instants, ifthe refinement
test of xi switches from “false” to “true” att ∈ [ti, ti+1], the
time stepping method will encounter an in-consistency. It will try
to search for a non-existent collisionevent in the interval [ti,
ti+1]. We solve this problem by es-timating a collision time tc
interpolating the separation dis-tance of the node xi at ti and the
penetration depth of the nodex j at ti+1. We apply the contact
response and restart the nu-merical integration from tc with xi as
the active CLOD.
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6. Implementation and Performance
In this section we describe the implementation issues and
thebenchmarks used to analyze the performance and effective-ness of
CLODs.
6.1. Benchmark ModelsFig. 6 in color plate shows the set of
benchmark models.They are:
• A wrinkled torus falling along a spiral peg• A spoon sliding
inside a cup• A soup of numbers settling in a bowl
In Table 1 we present the statistics of the dual
hierarchiescreated to represent the benchmark models. Note that in
allcases the models are simplified to a coarsest CLOD with
acomplexity in the order of 1000 triangles and 100 convexpieces.
This limitation is imposed by the topology of the ob-jects and the
localized influence of the simplification opera-tions. The
statistics of the numbers represent an average ofall the models
from number 0 to number 9.
Models Cup Spoon Spiral Torus Bowl Numbers
Trisorig 64000 86016 32160 32000 12288 1080
BVsorig 15490 16125 6448 5919 4336 161
Trissimp 1532 1074 716 644 634 306
BVssimp 241 61 100 92 63 37
CLODs 7 9 7 7 8 3
LODs 14 14 13 13 13 9
Table 1: Models and Associated Hierarchies. The numberof
triangles (Trisorig) and convex patches (BVsorig) of theinitial
mesh of the models; the number of triangles (Trissimp)and convex
patches (BVssimp) of the coarsest valid CLOD;and CLODs and total
number of LODs in the hierarchies.
6.2. Runtime Performance
The rigid body simulations captured in the snapshots inFig. 6 on
the color plate were computed using impulsivemethods19. These
simulations were performed on a Pentium-4 2.4Ghz processor PC with
2.0GB of memory, a NVidiaGeForce-FX graphics card and Windows2000
OS.
The timing profile of Fig. 4 shows query times and thenumber of
nodes in the BVTT front for a simulation of thespoon sliding inside
the cup. We have compared results ob-tained with an exact collision
detection algorithm3 againstthe results of our CLOD technique using
the size dependanterror metric. In particular, we have used values
of s0 of 3.5%,0.35%, and 0.035% of the radius of the cup. As the
timingsshow, CLODs with s0 = 3.5% perform at least as good as
theexact algorithm for most of the simulation duration.
Duringseveral time intervals, we observe a performance gain of
al-most one order of magnitude. In the other two benchmarks,
we have not observed as significant performance gains
usingCLODs. Thus, their timing profiles are omitted here, giventhe
page limit.
We have also evaluated CLODs with velocity and viewdependant
error metrics. In Fig. 7 (see color plate) coarseCLODs are selected
when the spoon hits the bottom of thecup, and fine resolution CLODs
are selected when the spoonslides along the side of the cup. In the
first case the polygoncount of the representations is roughly 10
times larger thanin the second case.
0 5 10 15 20 25 30 35 400
10
20
30
40
50exact0.035%0.35%3.5%
0 5 10 15 20 25 30 35 400
1000
2000
3000
4000
5000
QUERY TIME (ms)
BVTT FRONT (num. nodes)
Figure 4: Comparison of query time and size of the BVTTfront for
exact full res. collision detection and CLODs
6.3. Limitations and AnalysisWe have chosen a diverse set of
benchmarks with varyingmodel complexity and observed the
performance of CLODson different simulation scenarios. As a result
of the modifi-cation to the collision tolerance introduced in Sec.
5.1, themultiresolution collision detection algorithm using
CLODscannot prune as efficiently as the exact algorithm3, whenthe
objects are separated by a distance notably larger thanδ. However,
when they come close to contact, the tempo-ral coherence in the
front of the BVTT is much higher withCLODs than with the exact
algorithm. This can be verified inFig. 4, where the number of nodes
does not vary much withCLODs. This also explains the spikes present
in the timingsfor the exact algorithm. These spikes take place at
the in-stants when the objects are about to interpenetrate. Even
ifthe number of nodes in the front of the BVTT is smallerfor the
exact algorithm than for CLODs with s0 = 0.035%,the coherence is
lower, because suddenly many nodes in theBVTT come closer than δ,
thus yielding longer query times.
We believe that the insignificant performance gain in
thesimulation of the torus falling along the spiral is due to
sim-ilar reasons. Because of the high traveling speed and the
im-pacts of the falling torus, the simulated objects are rarely
inclose proximity or have low spatial coherence. As for
thesimulation of the numbers settling in a bowl, the
polygonalcomplexity of the models of the numbers is not high
enoughto benefit from CLODs.
c© The Eurographics Association 2003.
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Otaduy and Lin / CLODs for Multiresolution Collision
Detection
To summarize, we can conclude that CLODs perform re-markably
better than exact collision detection algorithms (upto one order of
magnitude) for situations of sliding or restingcontacts between
complex models. These are in fact some ofthe most challenging
contact scenarios, because large areasof the objects are in
parallel close proximity6.7. Summary and Future Work
We have introduced the concept of “contact levels of de-tail”
(CLOD) for multiresolution collision queries. CLODsare based on a
dual hierarchy that serves as both a multires-olution
representation of the input model and its boundingvolume hierarchy
for collision queries. The runtime algo-rithm dynamically selects
adaptive CLODs at each potentialcontact independently using various
error metrics. This ap-proach considerably speeds up the overall
performance ofcollision queries in complex environments with
challengingcontact scenarios.
This research may be extended in several directions:• A further
investigation of the problem of switching
CLODs and its implications on the contact response mod-els of
various simulation methods;
• Optimization of the simplification process to obtain
moreeffective hierarchies, perhaps incorporating an operationwith a
more global support than the current ones;
• The implementation of CLODs with other types of BVHs.
Acknowledgments
This research is supported in part by a fellowship of
theGovernment of the Basque Country, National Science Foun-dation,
Office of Naval Research, U.S. Army Research Of-fice, and Intel
Corporation. We would like to thank StephenEhmann and Young Kim for
their help on integratingSWIFT++ and DEEP with our collision
detection. We arealso grateful to Dinesh Manocha, Jack Snoeyink,
Mark Fos-key, and the anonymous reviewers for their feedback on
theearlier drafts of this paper.
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Otaduy and Lin / CLODs for Multiresolution Collision
Detection
Figure 5: From left to right, original mesh and detail view of
CLOD0, CLOD1, CLOD2, CLOD4 and CLOD6. Mesh simplifica-tion is
combined with the construction of the BVH. Different colors
correspond to different convex patches.
Figure 6: Snapshots of our benchmark simulations. From left to
right: torus falling along a spiral peg, a spoon in a cup, and
asoup of numbers in a bowl.
Figure 7: Velocity dependent adaptive selection of CLODs used in
detecting contact between a cup and a spoon. In blue andgreen, the
vicinity of the contact locations shown at the resolution of the
adaptively selected CLODs. In the right, coarseresolution is
selected when the spoon falls inside the cup. In the left, fine
resolution is selected when the spoon slides slowlyalong the side
of the cup.
c© The Eurographics Association 2003.