department of computer science and engineering computing handles and tunnels in 3d models tamal k....
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Department of Computer Science and Engineering
Computing handles and tunnels in 3D models
Tamal K. Dey
Joint work: Kuiyu Li, Jian Sun, D. Cohen-Steiner
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Projects in Jyamiti group
• Surface reconstrucion (Cocone)• Delaunay meshing (DelPSC)• Shape feature processing (Segmatch,
Cskel)
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Handle loops/Tunnel loops
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Handle loops/Tunnel loops
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Topological Simplification
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Use of Handles and Tunnels
• Topological simplification / feature recognition / parameterization• El-Sana and Varshney[1997] use α-hulls to search for
tunnels.• Guskov and Wood[2001] propose a surface growing
strategy to remove small handles.• Nooruddin and Turk[2003] use morphological operations to
remove small handles.• Wood, Hoppe, Desbrun, and Schroder[2004] use Reeb
graph to remove handles.• Shattuck and Leahy[2001] use Reeb graph to compute
handles.• From WUSTL Model/surface editing (Zhou, Ju, and Hu[2007]
use medial axis to detect loops) and work by Grimm et al.
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• Nontrivial loops can neither be handle nor tunnel.
• Handle and tunnel loops have to be non-separating.
Nontrivial loops as first attempt?
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• Loops on a surface• Compute polygon schemas [Vegter-Yap90][Dey-Schipper95]
• Linear time algorithm• Compute optimal systems of loops [VL05][EW06]
• Verdiere and Lazarus gave an algorithm for computing a system of loops which is shortest among the homotopy class of a given one.
• Erickson and Whittlesey gave a greedy algorithm to compute the shortest system of loops among all systems of loops.
• Compute optimal cut graph [EH04]
• Computing a shortest cut graph is NP-hard.
Related work for loops on surface
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A better idea
• Intuitively, a loop on a connected, closed, surface M in R3 is:• handle if it spans a disk
(surface) in the bounded space bordered by M.
• tunnel if it spans a disk (surface) in the unbounded space bordered by M.
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Cycle, Boundary and Homology
• p-chain: c=aii, i is p-simplex.
(e2+e3+e4) is a 1-chain.
• If c'=ai'i, then c+c'=(ai+ai')i.
• c + c = 0 for Z2 coefficients.
• Boundary operator p = [v0,v1,…,vp] = i=0
p[v0,…,~vi,…,vp]
• e.g.: t = (e2+e3+e4)
• e.g.: e0+ e1+ e4 = (v0+v2)+(v0+v1)+(v1+v2) = 0
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Cycle, Boundary and Homology
• p-cycle: if pc = 0.
• p-boundary: if there’s (p+1)-chain d, s.t. c=pd .• p-cycle group Zp = kernel (p).• p-boundary group Bp = image (p+1).• Hp= Zp / Bp
• Each element of Hp represents an equivalent class.
• c1 and c2 are in the same class if c1 + c2 is in Bp.• Trivial elements in Hp bounds (p+1)-chain
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Handle and tunnel loops – definitions [Dey-Li-Sun 07]
• Let M be a connected, closed, surface in R3. M separates R3 into inside I and outside O. Both I and O have M as boundary.
• Handle: A loop on M whose homology class is trivial in H1(I) and non-trivial in H1(O).
• Tunnel: A loop on M whose homology class is trivial in H1(O) and non-trivial in H1(I).
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• Consider a thickened trefoil• handle loop (green)• tunnel loop (red)
Nasty knots
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Handle and tunnel loops – existence
• Theorem (Dey, Li, Sun[2007])• For any connected closed surface M of
genus g in R3, there exist g handle loops {hi}i=1…g forming a basis for H1(O) and g tunnel loops {ti}i=1...g forming a basis for H1(I).
• Furthermore, {hi}i=1...g and {ti}i=1...g form a basis for H1(M).
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First algorithm for GR models
[Dey-Li-Sun 07]
• Graph retractability assumption : A surface is graph retractable if both I and O deformation retract to a graph, denoted I and O, respectively.
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• I and O are disjoint core graphs, each with g loops : {Kj
I}j=1g and {Kj
O}j=1g and {Kj}j=1
2g all together.
• How to compute I and O ?
Inner and outer cores
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Linking number
• J and K are two disjoint loops in R3. • In a regular projection, there are two ways
in which J crosses under K. • The linking number, lk(K, J), is the sum of
these signed crossings.
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Linking number Theorem
• A loop on M is a handle iff lk(, KiI) 0 for at
least one inner core and lk(, KiO)=0 for all outer
cores.
• A loop on M is a tunnel iff lk(, KiO) 0 for at
least one outer core and lk(, KiI)=0 for all inner
cores.
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Minimally linked basis
• A loop is minimally linked if it links once only with one core loop.
• Theorem : There exist 2g minimally linked loops, denoted {Jj}j=1
2g, such that lk(Ki, Jj)=ij.
• Half of them linked with KiI’s, denoted {JjI}j=1
g, are handle loops.
• Other half linked with KiO’s, denoted {JjO}j=1
g, are tunnel loops.
• {[JjI]} j=1g form a basis for H1(I) and {[JjO]}j=1
g form a basis for H1(O). Hence {[Jj]}j=1
2g form a basis for H1(M).
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Topological algorithm• Assume I and O are given.
• Step1: Compute {Ki}i=12g using the spanning tree of I and
O.• Step2: Compute a system of 2g loops on M, denoted
{j}j=12g. (say by Dey-Schipper or Vegter-Yap linear time
algorithms)
• Step3: Compute lk(Ki, j) for all i and j. Let A be the 2gx2g matrix {lk(Ki, j)}.• A is the transform matrix from a minimally linked basis
{[Jj]}j=12g to basis {[j]} j=1
2g.
• A-1 = {aji} exists and has integer entries.
• [Jj] = i=12g aji[i].
• Step4: Obtain Jj by concatenating i’s according to the above expression.
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An implementation to compute system of handle and tunnel loops
with small size
• Compute I and O• Basic idea: Collapse the inside
(outside) Voronoi diagram to obtain I (O).
• The curve-skeleton [Dey-Sun06] captures the geometry better.
• Each skeleton edge (e) gets associated with an additional value called geodesic size (g(e)) indicating the local size of M.
• Establish graph structure on the curve skeleton.
• The geodesic size for a graph edge, g(E) = min{g(e): e is a skeleton edge in E}.
• Issue: not always work for any graph retractable surface, e.g., a thickening of a house with two room.
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Core graphs by curve-skeletons[DS05]
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An implementation to compute system of handle and tunnel loops
with small size
• Compute core loops {Kj}j=12g
• Compute the maximal spanning tree for I and O using geodesic sizes as weight.
• Add the remaining edges, Ei’s, to form Ki’s.
• Compute minimally linked {Jj}j=1
2g
• Basic idea: compute Ji’s at different location indicated by Ei’s.
• Let e be the skeleton edge with the smallest geodesic size in Ei. Let p be one of the vertices of the dual Delaunay triangle of e.
• Compute an optimal system of loops by geodesic exploration and apply topological algorithm.
• In our experiments, one of the loop in the system of loops itself satisfies the condition to be Ji’s.
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Results
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A Persistence based algorithm [Dey-Li-Sun-Cohen-Steiner 08]
• A fast algorithm based on persistent homology introduced by Edelsbrunner, Letscher, Zomorodian[2002].
• The algorithm does not require any extra structure such as medial axis, curve skeleton, or Reeb graphs.
• It works on a larger class of models. Both surface mesh and isosurface with volume grids can be input.
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Filtration in persistent homology
• Filtration of a simplicial complex K is a nested sequence of complexes:
• 0 = K-1 < K0 < K1 < …< Kn = K
• Assume Ki - Ki-1 = i
• Persistence homology studies how the homology groups change over the filtration.
• In our algorithm, we use the pairing concept in the persistent homology.
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Positive and Negative simplices
• Once a p-simplex is added to Ki-1, it is• Positive if it creates a non-boundary p-
cycle.• Negative if it kills an existing (p-1)-cycle.
• A negative p-simplex is always paired with a unique positive (p-1)-simplex ' where kills a (p-1)-cycle created by '.
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Positive and negative simplices
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Pairing algorithm – pair ()
1) c=p2) d is youngest positive (p-1)-simplex in c3) while (d is paired and c Φ) do
• Let c' be the cycle killed by the simplex paired with d
• c=c'+c• d is the youngest positive (p-1)-simplex in c
end while4) if c is not empty, then
is negative p-simplex and paired with d else
is positive p-simplex.
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Computing handle and tunnel loops
• Topological algorithm• Two refinements for improving loop quality.• Assume M, I, and O denote the simplicial
complexes of surface, inside, and outside respectively; g is the genus of M.
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Topological algorithm
• 3 Steps: • 1) Simplices of M are added to the filtration,
generating 2g unpaired positive edges;• 2) Simplices of I are added, g previously
unpaired edges get paired, each pair corresponds to a handle loop;
• 3) Simplices of O are added, the rest of g unpaired edges get paired, each pair corresponds to a tunnel loop.
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Topological algorithm cont.
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Refinement 1
• For a negative triangle , a series of homologous loops are obtained.
• Let L1, L2,…,Lk be those that lie on M, sorted by the time when they are found. Set the handle (or tunnel) loop L() for as L1.
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Refinement 2
• A handle (or tunnel) loop is found when a cross section of M get filled.
• Cross sections of small size will get filled first if we add triangles in I and O to the filtration in increasing order of their geodesic sizes.
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Computing geodesics – for refinement 2
• Geodesic size of an edge E =(A1, A2): geodesic distance between points A1' and A2' on M, where Ai'=Ai if Ai is on M; otherwise, Ai' is the closet point on M for Ai
• Geodesic size size of a triangle t: max {g(e)|e is in t}
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Entire algorithm
• 1) Compute geodesic size for edges and triangles in I and O;
• 2) For each simplex on M, pair();• 3) E = unpaired positive edges on M;• 4) In the order of increasing geodesic size,
pair() for each triangle in I;• 5) if ( is negative and its paired positive
edge is in E), then output a handle: hi = L();
• 6) Do similarly as 4) and 5) to get tunnels.
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Entire algorithm
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Gearbox (genus 78)
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Isosurface Fuel (genus 8)
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Isosurface (Engine, genus 20)
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Aneurysm
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Knotty cup and noise
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Experiments
• We implemented our algorithm in C++. All experiments were done on a Dell PC with 2.8GHz Intel Pentium D CPU and 1GB RAM.
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Feature Detection
• Compute handle (tunnel) features based on handle (tunnel) loops. • Sweep the handle (tunnel)
loop until the length become too long.
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Topological simplification
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Simplification (Buddha)
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• A simple, topologically correct algorithm for handles and tunnels.
• Mathematical proof of geometric qualities of the loops?
• What about surfaces with boundaries?• Handletunnel software available from http://www.cse.ohio-state.edu/~tamaldey/handle.html
Conclusions and future work
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Pegasus (genus 5)
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2-torus (genus 2)