1 Street Generation for City Modeling Xavier Décoret, François Sillion iMAGIS GRAVIR/IMAG - INRIA.
Maintenance of the Visibility of a Moving Viewpoint, and ... · Soutenance de thèse 22 mai 2006...
Transcript of Maintenance of the Visibility of a Moving Viewpoint, and ... · Soutenance de thèse 22 mai 2006...
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Maintenance of the Visibility of a Moving Viewpoint, andApplications
Samuel Hornus, Université Joseph FourierSoutenance de thèse
22 mai 2006
Préparée dans l'équipe ARTIS du laboratoire GRAVIR - IMAGUMR C5527 between CNRS, INPG, INRIA and UJF
and a project of INRIA.
Samuel Hornus () Soutenance de thèse lundi 22 mai 1 / 37
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Outline
1 Introduction
2 Connexity in the 3D Visibility Complex
3 Visibility Maintenance among Convex Polytopes in Space
4 Conclusions
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Visibility in Image Synthesis and Computational Geometry
Computer Graphics oldest goal: create images of virtual worlds.
[Wonka et al. 2006]Let's look at some examples of visibility problems. . .
Example in the plane: given polygonal scene description and viewpoint V
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Visibility in Image Synthesis and Computational Geometry
Example in the plane: given polygonal scene description and viewpoint V
V
Ordered around the viewpoint: table, bottle, lamp
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Visibility in Image Synthesis and Computational Geometry
Compute objects visible from V
V
Ordered around the viewpoint: table, bottle, lamp
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Visibility in Image Synthesis and Computational Geometry
Compute segments visible from V
V
Possibly ordering the segments circularly
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Visibility in Image Synthesis and Computational Geometry
Compute parts visible from V . E.g. lit/shadowed parts if V is a light source
Adding discontinuity positions (◦), to clip the invisible parts
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Visibility in Image Synthesis and Computational Geometry
Further, the observer might vary (from-cell visibility)
V
Precomputation of visible sets, or area-lights[Durand et al. 00, Haumont et al. 05]
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Visibility in Image Synthesis and Computational Geometry
V (or the objects) might move: update visibility during motion
E.g., if no preprocessing is available
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Kinetic Data Structures (KDS)
Framework for design & analysis of algo. for maintaining an attribute ofcontinuously moving items [Basch et al. 97].
Event queue
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Kinetic Data Structures (KDS)
Framework for design & analysis of algo. for maintaining an attribute ofcontinuously moving items [Basch et al. 97].
Event queue
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Kinetic Data Structures (KDS)
Framework for design & analysis of algo. for maintaining an attribute ofcontinuously moving items [Basch et al. 97].
time
Event queueSamuel Hornus () Soutenance de thèse lundi 22 mai 4 / 37
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Kinetic Data Structures (KDS)
Framework for design & analysis of algo. for maintaining an attribute ofcontinuously moving items [Basch et al. 97].
time
Event queueSamuel Hornus () Soutenance de thèse lundi 22 mai 4 / 37
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Kinetic Data Structures (KDS)
Framework for design & analysis of algo. for maintaining an attribute ofcontinuously moving items [Basch et al. 97].
time
Event queueSamuel Hornus () Soutenance de thèse lundi 22 mai 4 / 37
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Kinetic Data Structures (KDS)
Framework for design & analysis of algo. for maintaining an attribute ofcontinuously moving items [Basch et al. 97].
time
Event queueSamuel Hornus () Soutenance de thèse lundi 22 mai 4 / 37
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Kinetic Data Structures (KDS)
Framework for design & analysis of algo. for maintaining an attribute ofcontinuously moving items [Basch et al. 97].
time
Event queueSamuel Hornus () Soutenance de thèse lundi 22 mai 4 / 37
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Contributions on Visibility Maintenance
In the �rst part, we consider three problems on:
Exact and continuous object-visibility maintenance.
With moving point observer (viewpoint).
And possibly continuously moving objects.
... and a property of the visibility complex.
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Contrib #1: Vis. Maintenance among Points in the Plane
Goal: maintain the ordering of n pointsaround V , as V moves along linesegment trajectories given on-line.
Optimal algorithm.
V
Canadian Conference on Computational Geometry 2005; O. Devillers, V.Dujmovi¢, H. Everett, S. Hornus, S. Whitesides, and S. Wismath.
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Contrib #2: Vis. Maintenance Among Convex Sets in 2DExposition of Hall-Holt's Visible Zone algorithm
Tangents through V describe thevisibility polygon (in green).
Goal: maintain the visibility polygonas V moves along algebraictrajectory given on-line.
V
O0
O2
O3
O1
∞
∞Under mild assumptions, the Visible Zone algorithm is optimal.
We explain the algorithm and give a new and simpler proof of a crucialproperty used for the algorithm.
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Contrib #3: Vis. Maintenance Among Polytopes in Space
The visibility polyhedra encodes the set of objects visible from V
Goal: maintain the visibility polyhedra as V moves along arbitrary
pseudo-algebraic trajectory.
Give a non-optimal algorithm to do so, together with hints at how toimprove it. More on that in a few minutes.Early results presented at DIMACS Workshop, 2002.Samuel Hornus () Soutenance de thèse lundi 22 mai 8 / 37
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Contrib #4: Connectedness in the 3D Visibility Complex
3D Visibility complex = 4D set encoding visibility relationships betweenobjects in space. Current algorithms for constructing it seem di�cult[Durand] or are not (yet) implementable [Goaoc 04].
1 We prove a topological property of the 3D visibility complex.
2 And apply this property to a simple algorithm to construct thevisibility complex. More on that in a few minutes.
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Graphics Contributions
Computer graphics applications, related to visibility and motion:
real-time rendering of large indoor scenes
real-time rendering of shadows
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Contrib #5: Automatic Cells-and-Portals Decomposition
Given an input polygonal scene, we give an algorithm that builds acells-and-portals graph suitable for portal rendering (well sized cells).
()
INRIA Research Report 4898 (2003), S. Lefebvre and S. Hornus.Samuel Hornus () Soutenance de thèse lundi 22 mai 11 / 37
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Contrib #5: Automatic Cells-and-Portals Decomposition
Given an input polygonal scene, we give an algorithm that builds acells-and-portals graph suitable for portal rendering (well sized cells).
Invisible polygons (portals) separate cells (�rooms�)
INRIA Research Report 4898 (2003), S. Lefebvre and S. Hornus.Samuel Hornus () Soutenance de thèse lundi 22 mai 11 / 37
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Contrib #5: Automatic Cells-and-Portals Decomposition
Given an input polygonal scene, we give an algorithm that builds acells-and-portals graph suitable for portal rendering (well sized cells).
()Cells and portals de�ne a graph
INRIA Research Report 4898 (2003), S. Lefebvre and S. Hornus.Samuel Hornus () Soutenance de thèse lundi 22 mai 11 / 37
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Contrib #5: Automatic Cells-and-Portals Decomposition
Given an input polygonal scene, we give an algorithm that builds acells-and-portals graph suitable for portal rendering (well sized cells).
�Portal rendering� is a graph traversal
V
INRIA Research Report 4898 (2003), S. Lefebvre and S. Hornus.Samuel Hornus () Soutenance de thèse lundi 22 mai 11 / 37
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Contrib #7: ZP+, Correct Z-pass Stencil Shadows
Corrects a �aw in well-known algorithm. Generally faster than previouswork. [Laine 05] combines best of ZP+ and previous work.
Doom 3[HHLH05] ACM Symposium on Interactive 3D Graphics 2005; S. Hornus,J. Hoberock, S. Lefebvre and J. C. Hart.
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Outline
1 Introduction
2 Connexity in the 3D Visibility Complex
3 Visibility Maintenance among Convex Polytopes in Space
4 Conclusions
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Connexity in the 3D Visibility ComplexThe Visibility Complex
3D visibility complex:
All visibility relationships.
Structures the 4D set of �light rays� between objects.
My contributions:
1 Theorem: boundaries of its 4-dimensional �cells� are path-connected.
2 Applied to a simple algorithm to construct the visibility complex.
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Connexity in the 3D Visibility ComplexThe Visibility Complex
Consider a set O of pairwise disjointconvex sets in space in 3D (�gures are2D).
The free space F is outside the objects.
Free segments (in green).
Maximal free segments (in blue).
Let S be the set of maximal freesegments. Each maximal free segmenthas 2 blockers in (O ∪ {∞})2
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Connexity in the 3D Visibility ComplexThe Visibility Complex
Consider a set O of pairwise disjointconvex sets in space in 3D (�gures are2D).
The free space F is outside the objects.
Free segments (in green).
Maximal free segments (in blue).
Let S be the set of maximal freesegments. Each maximal free segmenthas 2 blockers in (O ∪ {∞})2
F
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Connexity in the 3D Visibility ComplexThe Visibility Complex
Consider a set O of pairwise disjointconvex sets in space in 3D (�gures are2D).
The free space F is outside the objects.
Free segments (in green).
Maximal free segments (in blue).
Let S be the set of maximal freesegments. Each maximal free segmenthas 2 blockers in (O ∪ {∞})2
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Connexity in the 3D Visibility ComplexThe Visibility Complex
Consider a set O of pairwise disjointconvex sets in space in 3D (�gures are2D).
The free space F is outside the objects.
Free segments (in green).
Maximal free segments (in blue).
Let S be the set of maximal freesegments. Each maximal free segmenthas 2 blockers in (O ∪ {∞})2
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Connexity in the 3D Visibility ComplexThe Visibility Complex
Consider a set O of pairwise disjointconvex sets in space in 3D (�gures are2D).
The free space F is outside the objects.
Free segments (in green).
Maximal free segments (in blue).
Let S be the set of maximal freesegments. Each maximal free segmenthas 2 blockers in (O ∪ {∞})2
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The visibility Complex (2/4)
The visibility complex partitions S inmaximal sets of segments having thesame set of blockers.
And each set is separated in connectedcomponents.
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The visibility Complex (2/4)
The visibility complex partitions S inmaximal sets of segments having thesame set of blockers.
And each set is separated in connectedcomponents.
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The visibility Complex (3/4)
In 2D, the visibility complex VC is a 2-dimensional cellular complex over S.Each k-cell, of dimension k ≤ 2, is homeomorphic to a k-disc. [Pocchiolaand Vegter 96].
a part of a 1-cell
one 2-cell, cell or facetwo 0-cells
bitangents
tangents non-tangents
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The visibility Complex (4/4)
In 3D, the visibility complex VC is not a cellular complex over S.Intuitively, tiny objects `create' tunnels through 4-cells [Durand et al. 02].
A
BThe green loop of segmentsis not contractiblein the 4-cell AB
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Our result
Theorem
Let C be a 4-cell of the visibility complex. Let ∂C be its boundary. Then,
∂C is path-connected.
A
B
Remarks:
The visibility complex is not a cell-complex, but S is (e.g., arangementin Plücker space [Goa04]).
S is a Haussdorf space: in which tools from algebraic topology workwell.
The proof of the theorem uses 3 sub-lemmas. . .
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Lemmas for the proof of theorem
In order to prove the theorem, we manipulate homology groups.
Let X be a topological space.
H0(X ) is the zeroth homology group. H0(X ) = Zk ; k is the number ofconnected components.
H1(X ) is the �rst homology group. We have H1(X ) = 0 if X is1-connected.
Each lemma translates in an homological identity:
1 Segment space S is path-connected ⇒ H0(S) = Z2 Segment space S is one-connected ⇒ H1(S) = 0
3 The complement CC of 4-cell C is path-connected⇒ H0(CC ) = Z
We have H0(∂C ) = Zk . We want to prove that k = 1.
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Proof of theorem
We enlarge ∂C a little to obtain an open neighborhood B of ∂C with
H0(∂C ) = H0(B) = Zk , k ≥ 1
De�ne U = B ∪ C , V = B ∪ CC (C , U and V are connected).
C B U V
∂C
Using Mayer-Vietoris sequence on U and V , we obtain the followingshort exact sequence of morphisms of groups:
0φ3−→ Zk φ2−→ Z2 φ1−→ Z φ0−→ 0
Generally, such a sequence 0→ A→ B → C → 0 is said to split,which means B ≈ A⊕ C .In our case: Z2 ≈ Zk ⊕ Z ≈ Zk+1, therefore k = 1, that is, ∂C ispath-connected.
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Proof of theorem
We enlarge ∂C a little to obtain an open neighborhood B of ∂C with
H0(∂C ) = H0(B) = Zk , k ≥ 1
De�ne U = B ∪ C , V = B ∪ CC (C , U and V are connected).
C B U V
∂C
Using Mayer-Vietoris sequence on U and V , we obtain the followingshort exact sequence of morphisms of groups:
0φ3−→ Zk φ2−→ Z2 φ1−→ Z φ0−→ 0
Generally, such a sequence 0→ A→ B → C → 0 is said to split,which means B ≈ A⊕ C .In our case: Z2 ≈ Zk ⊕ Z ≈ Zk+1, therefore k = 1, that is, ∂C ispath-connected.Samuel Hornus () Soutenance de thèse lundi 22 mai 21 / 37
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Application to the construction of the visibility complex
Assume we have constructed the 3-skeleton VC(3) of the visibility complexVC (its cells of dimension 3 and lower, see manuscript).
3-cell <3-cells
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Application to the construction of the visibility complex
We see VC(3) as a graph whose edges are the 3-cells, and nodes are cells ofdimension < 3.
3-cell <3-cells
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Application to the construction of the visibility complex
We label each 3-cell with its three adjacent 4-cells.
3-cell <3-cells
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Application to the construction of the visibility complex
Let C be a 4-cell. Its boundary is connected. So we can retrieve it as aconnected component in the graph VC(3).
3-cell <3-cells
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Application to the construction of the visibility complex
()
3-cell <3-cells
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Application to the construction of the visibility complex
()
3-cell <3-cells
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Application to the construction of the visibility complex
()
3-cell <3-cells
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Outline
1 Introduction
2 Connexity in the 3D Visibility Complex
3 Visibility Maintenance among Convex Polytopes in Space
4 Conclusions
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Visibility Maintenance among Convex Polytopes in SpaceProblem statement
k disjoint convex polytopes inspace.
Viewpoint V .
Viewmap: partition of thesphere of directions around V .
Goal: maintain the viewmap asV moves continuously.
V
The motion of V is given on-line as pseudo-algebraic trajectories.Polytopes can move too.
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Visibility Maintenance among Convex Polytopes in SpaceProblem statement
k disjoint convex polytopes inspace.
Viewpoint V .
Viewmap: partition of thesphere of directions around V .
Goal: maintain the viewmap asV moves continuously.
The motion of V is given on-line as pseudo-algebraic trajectories.Polytopes can move too.
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Visibility Maintenance among Convex Polytopes in SpaceProblem statement
k disjoint convex polytopes inspace.
Viewpoint V .
Viewmap: partition of thesphere of directions around V .
Goal: maintain the viewmap asV moves continuously.
The motion of V is given on-line as pseudo-algebraic trajectories.Polytopes can move too.
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Visibility Polyhedra
The viewmap, alone, does not containenough information for its maintenance.We need additional information.
The viewmap is the same as the visibilitypolyhedron: the set of all visible points
V
We extend the visibility polyhedron into a radial decomposition RV of thefreespace, centered on V .Let us �rst describe the radial decomposition in 2D. . .
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Visibility Polyhedra
The viewmap, alone, does not containenough information for its maintenance.We need additional information.
The viewmap is the same as the visibilitypolyhedron: the set of all visible points
VO0
O2
O3
O1
∞
∞
We extend the visibility polyhedron into a radial decomposition RV of thefreespace, centered on V .Let us �rst describe the radial decomposition in 2D. . .
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Radial Decomposition in 2D
V
(a) (c)(b)
(a) Draw tangents at each silhouette points.(b) Tangents (or walls) partition freespace in faces. Blue faces form thevisibility polygon.(c) Each cell can be seen as a one-dimensional set of segments.The 2D radial decomposition can be maintained without further datastucture.
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Radial Decomposition in 3D
We add walls in free space, supported by silhouette edges.
2 radial walls
V
t-vertex
silhouette edge supporting 2 radial walls
These walls partition the free space into 3D faces. . .
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Radial walls and 3D Faces of the Radial Decomposition
B
A
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Faces of RV
A face f of RV is
a 3D set of points (blue)
a 2D set of segments
V
Each face has a front blocker (A), and a back blocker (B):
The front blocker is a polytope or the viewpoint V
The back blocker is a polytope or the sky, ∞
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Faces of RV
A face f of RV is
a 3D set of points (blue)
a 2D set of segments
V
A
B
Each face has a front blocker (A), and a back blocker (B):
The front blocker is a polytope or the viewpoint V
The back blocker is a polytope or the sky, ∞
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Maintenance of RV
Together, the faces of RV are self-maintenable.
Therefore RV is maintenable.
The visibility polyhedron (or viewmap) of V is a subset of RV .
Therefore the viewmap can be maintained by maintaining RV .
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Maintaining a Face
Each face of RV is also a 2D set of segments, each with a uniquedirection: A face of RV can be described as a spherical polygon on thesphere of directions.
t-vertex
AB
Perspective view from V The face AB
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Maintaining a Face
Each face of RV is also a 2D set of segments, each with a uniquedirection: A face of RV can be described as a spherical polygon on thesphere of directions.
AB
Perspective view from V The face AB
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Faces of RV as Spherical Polygons
V is in the middle of the lot. Sphere of directions around V .
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Maintaining Each Face
EEE events (3 edges visually meet at a same point) are easy to detect.
VE events (1 vertex crosses an edge) are di�cult to detect andcorrespond to topological change in a face.
In order to detect VE events, we triangulate each face:VE event ⇔ collapse of a triangle.
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Maintaining Each Face by triangulation
t-vertex
f
f
High triangle count in the triangulation of RV .
Yields a large event queue: approximatelyO(s(silhouette edges) + m(t-vertices)).
We present �rst steps to reduce the number of events.Samuel Hornus () Soutenance de thèse lundi 22 mai 34 / 37
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Towards A Scene Sensitive Pseudo-Triangulation
t-vertex
f
f
Number of pseudo triangles: O(m + k)+ separation sensitive term.
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Conclusions
Exact visibility maintenance:1 2D with points: optimal algorithm.2 2D with convex objects: new simpler proof.3 3D with convex objects: arbirary motion.
3D visibility complex:1 New connexity result.2 Applied to visibility complex construction.
First (to my knowledge / together with [Haumont 03]) automaticdecomposition of indoor scene suitable for real-time rendering.
Stencil shadows: new technique (ZP+), �symmetrical� to previouswork (Z-fail) � take advantage of triangle-strips for large meshes �generally faster � instrumental to new techniques [Laine 05].
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Future Work
3D visibility maintenance: More work to do on maintainingpseudo-triangulation (e.g., canonical pseudo-triangulation)
3D visibility complex: More to do on the topology of the 3D visibilitycomplex ? maybe helpful for optimal visibility maintenance in 3D.
The end
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Future Work
3D visibility maintenance: More work to do on maintainingpseudo-triangulation (e.g., canonical pseudo-triangulation)
3D visibility complex: More to do on the topology of the 3D visibilitycomplex ? maybe helpful for optimal visibility maintenance in 3D.
The end
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Julien Basch, Leo Guibas, and J. Hershberger.Data structures for mobile data.In Proc. 8th Symposium on Discrete Algorithms (SODA'97), pages747�756, 1997.
Olivier Devillers, Vida Dujmovi¢, Hazel Everett, Samuel Hornus, SueWhitesides, and Stephen Wismath.Maintaining visibility information of planar point sets with a movingviewpoint.In Proc. 17th Canadian Conference on Computational Geometry, 2005.
Frédo Durand, George Drettakis, and Claude Puech.The 3D visibility complex.ACM Transactions on Graphics, 2002.
Frédo Durand, George Drettakis, Joëlle Thollot, and Claude Puech.Conservative visibility preprocessing using extended projections.Proceedings of SIGGRAPH 2000, July 2000.Held in New Orleans, Louisiana.
Xavier Goaoc.
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Structures de visibilité globales: tailles, calculs et dégénérescences.PhD thesis, Université de Nancy, 2004.
Samuel Hornus, Jared Hoberock, Sylvain Lefebvre, and John C. Hart.ZP+: correct z-pass stencil shadows.In ACM International Symp. on Interactive 3D Graphics and Games.ACM Press, April 2005.
Denis Haumont, Otso Mäkinen, and Shaun Nirenstein.A low dimensional framework for exact polygon-to-polygon occlusionqueries.In Proc. 16th Eurographics Symposium on Rendering, pages 211�222,June 2005.
Samuli Laine.Split-plane shadow volumes.In Proceedings of Graphics Hardware 2005, pages 23�32. EurographicsAssociation, 2005.
Michel Pocchiola and Gert Vegter.The visibility complex.
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International Journal on Computational Geometry and Applications,6(3):279�308, 1996.Special issue devoted to the proceedings of the 9th Annual ACMSymposium on Computational Geometry (SoCG'93).
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