Lecture 12: Delaunay Triangulations - Utrecht University · Introduction Triangulations Delaunay...

IntroductionTriangulations

Delaunay Triangulations


Computational Geometry

Lecture 12: Delaunay Triangulations

Computational Geometry Lecture 12: Delaunay Triangulations1



Motivation: Terrains

a terrain is the graph of afunction f : A⊂ R2→ Rwe know only height values for aset of measurement points

how can we interpolate theheight at other points?

using a triangulation

– but which?








– but which?








– but which?

?







using a triangulation– but which?

0

10

6

20

36

28

1000

980

990

1008

8904 23interpolated height = 985

q

0

10

6

20

36

28

1000

980

990

1008

8904 23

interpolated height = 23

q




Triangulation

Let P = {p1, . . . ,pn} be a point set. A triangulation of P is amaximal planar subdivision with vertex set P.

Complexity:

2n−2− k triangles

3n−3− k edges

where k is the number of points in Pon the convex hull of P.




Angle Vector of a Triangulation

Let T be a triangulation of P with m triangles and 3mvertices. Its angle vector is A(T) = (α1, . . . ,α3m) whereα1, . . . ,α3m are the angles of T sorted by increasing value.

Let T′ be another triangulation of P.We define A(T) > A(T′) if A(T) islexicographically larger than A(T′).

T is angle optimal if A(T)≥ A(T′)for all triangulations T′ of P.

α1

α2 α3

α4α5

α6

A(T) = (α1, . . . ,α6)








α1

α2 α3

α4α5

α6

A(T) = (α1, . . . ,α6)




Edge Flipping

α ′1

α ′4

α ′3

α ′5

α ′2

α ′6

pi

p j

pk

pl

piα4

α1

α3

α5

α2

α6

p j

pk

pl

edge flip

Change in angle vector:α1, . . . ,α6 are replaced by α ′1, . . . ,α

′6.

The edge e = pipj is illegal if min1≤i≤6 αi < min1≤i≤6 α ′i .

Flipping an illegal edge increases the angle vector.




Edge Flipping

α ′1

α ′4

α ′3

α ′5

α ′2

α ′6

pi

p j

pk

pl

piα4

α1

α3

α5

α2

α6

p j

pk

pl

edge flip


′6.






Characterisation of Illegal Edges

How do we determine if an edge is illegal?

Lemma: The edge pipj is illegalif and only if pl lies in the interiorof the circle C.




Characterisation of Illegal Edges

How do we determine if an edge is illegal?

Lemma: The edge pipj is illegalif and only if pl lies in the interiorof the circle C. pi

p j

pk

pl

illegal




Thales Theorem

Theorem: Let C be a circle, ` a lineintersecting C in points a and b, andp,q,r,s points lying on the same sideof `. Suppose that p,q lie on C, r liesinside C, and s lies outside C. Then

]arb > ]apb = ]aqb > ]asb,

where ]abc denotes the smallerangle defined by three points a,b,c.

` C

p

q

r

s

a

b

pi

p j

pk

pl

illegal




Thales Theorem




` C

p

q

r

s

a

b

pi

p j

pk

pl

illegal




Thales Theorem




` C

p

q

r

s

a

b

pi

p j

pk

pl

illegal

α

α ′

β

β ′

α ′ < α

β ′ < β




Legal Triangulations

A legal triangulation is a triangulation that does not containany illegal edge.

Algorithm LegalTriangulation(T)Input. A triangulation T of a point set P.Output. A legal triangulation of P.1. while T contains an illegal edge pipj

2. do (∗ Flip pipj ∗)3. Let pipjpk and pipjpl be the two triangles adjacent

to pipj.4. Remove pipj from T, and add pkpl instead.5. return T

Question: Why does this algorithm terminate?




PropertiesRandomized Incremental ConstructionAnalysis

Voronoi Diagram and Delaunay Graph

Let P be a set of n points in theplane.

The Voronoi diagram Vor(P) isthe subdivision of the plane intoVoronoi cells V(p) for all p ∈ P.

Let G be the dual graph ofVor(P).The Delaunay graph DG(P) isthe straight line embedding of G.

Question: How can wecompute the Delaunay graph?










Vor(P)










Vor(P)

G










Vor(P)

DG(P)





Planarity of the Delaunay Graph

Theorem: The Delaunay graph of a planar point set is a planegraph.

Ci j

pi

p j

contained in V(pi)

contained in V(p j)





Delaunay Triangulation

If the point set P is in general position then the Delaunay graphis a triangulation.

vf





Empty Circle Property

Theorem: Let P be a set of points in the plane, and let T bea triangulation of P. Then T is a Delaunay triangulation of Pif and only if the circumcircle of any triangle of T does notcontain a point of P in its interior.





Delaunay Triangulations and Legal Triangulations

Theorem: Let P be a set of points in the plane. A triangulationT of P is legal if and only if T is a Delaunay triangulation.

pi

p j

pk

pl

C(pi p j pk)

pm C(pi p j pm)

e





Angle Optimality and Delaunay Triangulations

Theorem: Let P be a set of points in the plane. Anyangle-optimal triangulation of P is a Delaunay triangulation ofP. Furthermore, any Delaunay triangulation of P maximizesthe minimum angle over all triangulations of P.





Randomized Incremental Construction

Algorithm DelaunayTriangulation(P)Input. A set P of n+1 points in the plane.Output. A Delaunay triangulation of P.1. Initialize T as the triangulation consisting of an outer

triangle p0p−1p−2 containing points of P, where p0 is thelexicographically highest point of P.

2. Compute a random permutation p1,p2, . . . ,pn of P\{p0}.3. for r← 1 to n4. do5. Locate(pr,T)6. Insert(pr,T)7. Discard p−1 and p−2 with all their incident edges from T.8. return T






pr

pi

pk

p j

pr

p j

pi

pkpl

pr lies in the interior of a triangle pr falls on an edge






Insert(pr,T)1. if pr lies in the interior of the triangle pipjpk2. then Add edges from pr to the three vertices of pipjpk, thereby

splitting pipjpk into three triangles.3. LegalizeEdge(pr,pipj,T)4. LegalizeEdge(pr,pjpk,T)5. LegalizeEdge(pr,pkpi,T)6. else (∗ pr lies on an edge of pipjpk, say the edge pipj ∗)7. Add edges from pr to pk and to the third vertex pl of the

other triangle that is incident to pipj, thereby splitting thetwo triangles incident to pipj into four triangles.

8. LegalizeEdge(pr,pipl,T)9. LegalizeEdge(pr,plpj,T)10. LegalizeEdge(pr,pjpk,T)11. LegalizeEdge(pr,pkpi,T)






LegalizeEdge(pr,pipj,T)1. (∗ The point being inserted is pr, and

pipj is the edge of T that may need tobe flipped. ∗)

2. if pipj is illegal3. then Let pipjpk be the triangle

adjacent to prpipj along pipj.4. (∗ Flip pipj: ∗) Replace pipj

with prpk.5. LegalizeEdge(pr,pipk,T)6. LegalizeEdge(pr,pkpj,T)

pr pi

p j pk






=⇒

pr

All edges created are incident to pr.

Correctness: Are new edges legal?






pi

p jpl

pr

CC′

Correctness:For any new edge there is an empty circle through endpoints.New edges are legal.






Initializing triangulation: treat p−1 and p−2 symbolically.No actual coordinates.Modify tests for point location and illegal edges to work as iffar away.

Point location: search data structure.Point visits triangles of previous triangulations that contain it.






split ∆1

flip pi p j

flip pi pk

pi

pk

∆1∆1

∆2

∆2

∆3

∆3

pr ∆1 ∆2 ∆3

∆2

∆3pi

p j ∆1 ∆2 ∆3

∆4

∆4 ∆5

∆6

∆7

∆7∆6

∆1 ∆2 ∆3

∆4

∆3

∆5

∆4 ∆5





Analysis

1 Expected total number of triangles created in O(n)2 Expected total number of triangles visited while search

point location data structure: O(n logn)We will only consider the first (see book for second)





Analysis

Lemma: Total number of triangles created is at most 9n+1.

How many triangles are created when inserting pr?

Backwards analysis: Any point of p1, . . . ,pr has the sameprobability 1/r to be pr.

Expected degree of pr ≤ 6.

Number of triangles created ≤ 2degree(pr)−3(Why? Count flips.)

2 ·6−3 = 9

+ outer triangle





Analysis






2 ·6−3 = 9

+ outer triangle





Analysis

Theorem: The Delaunay triangulation of n points can becomputed in O(n logn) expected time.


Lecture 12: Delaunay Triangulations - Utrecht University · Introduction Triangulations Delaunay...

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