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**Order-k Voronoi Diagram in the Plane**

Computational Geometry, WS 2006/07 Lecture 16 Dr. Dominique Schmitt Laboratoire MAGE - FST Université de Haute-Alsace Algorithmen & Datenstrukturen, Institut für Informatik Fakultät für Angewandte Wissenschaften Albert-Ludwigs-Universität Freiburg

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**(Very) Basic properties on circles**

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**Voronoi diagram (reminder)**

S : a set of n sites in the plane V(s) : the set of points of the plane closer to s than to any other site = the Voronoi region of s V(3) Vor(S) : the partition of the plane formed by the Voronoi regions, their edges, and vertices

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**Voronoi regions, edges, and vertices**

c(s,t) : the set of centers of all the empty circles passing through s and t u(Q) : the center of the empty circle passing through a set Q of at least 3 co-circular sites V(s) : the set of centers of all the empty circles passing through s

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**Generalizations of the Voronoi diagram**

d-dimensional space, non-Euclidean space, non-Euclidean distance, weighted sites, sites are not points, k nearest neighbors - ...

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**Farthest point Voronoi diagram**

V-1(s) : the set of points of the plane farther from s than from any other site Vor-1(S) : the partition of the plane formed by the farthest point Voronoi regions, their edges, and vertices

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**Farthest point Voronoi regions**

Construction of V-1(7) Property The farthest point Voronoi regions are convex

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**Farthest point Voronoi regions**

Property If the farthest point Voronoi region of s is non empty then s is a vertex of conv(S)

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**Farthest point Voronoi regions**

Property If s is a vertex of conv(S) then the farthest point Voronoi region of s is non empty Property The farthest point Voronoi regions are unbounded

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**Farthest point Voronoi regions**

Property If s is a vertex of conv(S) then the farthest point Voronoi region of s is non empty Property The farthest point Voronoi regions are unbounded Corollary The farthest point Voronoi edges and vertices form a tree

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**Farthest point Voronoi edges and vertices**

edge : set of points equidistant from 2 sites and closer to all the others V-1(4) x V-1(1) => c-1(s,t)

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**Farthest point Voronoi edges and vertices**

edge : set of points equidistant from 2 sites and closer to all the others V-1(4) => c-1(s,t) V-1(7) V-1(2) vertex : point equidistant from at least 3 sites and closer to all the others => u-1(Q)

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**Duality in planar partitions (reminder)**

Delaunay diagram : a partition dual to the Voronoi diagram => denoted by Del(S)

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**Farthest point Delaunay diagram**

Property The farthest point Delaunay edges do not intersect c-1(2,4)

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**Farthest point Delaunay regions**

Property The farthest point Delaunay diagram of S is a partition of conv(S) in polygons inscribable in “full” circles u-1(2,4,7) => Construction in O(n log n)

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**Order-2 Voronoi diagram**

V(s,t) : the set of points of the plane closer to each of s and t than to any other site Property The order-2 Voronoi regions are convex

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**Order-2 Voronoi regions**

V(s,t) : the set of points of the plane closer to each of s and t than to any other site Property The order-2 Voronoi regions are convex Construction of V(3,5)

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**Order-2 Voronoi edges edge : set of centers of circles passing through**

2 sites s and t and containing 1 site p V(2,3) c3(1,2) => cp(s,t) V(1,3) Question Which are the regions on both sides of cp(s,t) ? => V(p,s) and V(p,t)

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**Order-2 Voronoi vertices**

vertex : center of a circle passing through at least 3 sites and containing either 1 or 0 site vertex : center of a circle passing through at least 3 sites and containing 1 site u5(2,3,7) => up(Q) or u(Q) u(3,6,7,5)

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**Order-2 Voronoi vertices**

vertex : center of a circle passing through at least 3 sites and containing either 1 or 0 site V(5,2) u5(2,3,7) V(5,7) => up(Q) or u(Q) V(5,3) Question Which are the regions incident to up(Q) ? => V(p,q) with q Q

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**Order-2 Voronoi vertices**

vertex : center of a circle passing through at least 3 sites and containing either 1 or 0 site V(5,7) V(3,5) => up(Q) or u(Q) u(3,6,7,5) Question Which are the regions incident to up(Q) ? V(3,6) V(6,7) => V(p,q) with q Q Question Which are the regions incident to u(Q) ? => V(q,q’) with q and q’ consecutive on the circle circumscribed to Q

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**Construction of Vor2(S) from Vor(S)**

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**Construction of Vor2(S) from Vor(S)**

V(3) in Vor(S) The set of points of the plane closer to 3 than to any other site of S V(3,5) V(5) in Vor(S\{3}) The set of points of the plane closer to 5 than to any other site of S\{3} => The intersection of V(3) and Vor(S\{3}) is a subset of Vor2(S)

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**Construction of Vor2(S) from Vor(S)**

V(5) in Vor(S) The set of points of the plane closer to 5 than to any other site of S V(3,5) V(3) in Vor(S\{5}) The set of points of the plane closer to 3 than to any other site of S\{5} => The intersection of V(5) and Vor(S\{5}) is a second subset of Vor2(S)

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**Construction of Vor2(S) from Vor(S)**

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**Construction of Vor2(S) from Vor(S)**

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**Construction of Vor2(S) from Vor(S)**

The algorithm For every region V(s) of Vor(S) - construct Vor(S\{s}) - compute the intersection of Vor(S\{s}) with V(s) Stick the different pieces together The time complexity Construction of one Voronoi diagram of n sites => O(n log n) Construction of n such diagrams => O(n2 log n)

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**Construction of Vor2(S) from Vor(S)**

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Vor(S) and Vor2(S) V(3,5) c(3,5) V(6,7) c(1,3) V(1,3) c(6,7)

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**Vor(S) and Vor2(S) Property**

Every order-1 Voronoi edge c(s,t) is included in one and only one order-2 Voronoi region V(s,t). Conversely, every order-2 Voronoi region contains one and only one order-1 Voronoi edge. => c(s,t) V(s,t)

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**Vor(S) and Vor2(S) Property**

Every order-1 Voronoi edge c(s,t) is included in one and only one order-2 Voronoi region V(s,t). Conversely, every order-2 Voronoi region contains one and only one order-1 Voronoi edge. x y y c(s,t) x c(s,t)

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**Vor(S) and Vor2(S) Corollary The order-1 and order-2**

Voronoi edges do not intersect Corollary V(s,t) is an order-2 region iff c(s,t) is an order-1 edge <=> V(s,t) is a region in Vor2(S) iff V(s) and V(t) share an edge in Vor(S)

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**Construction of Vor2(S) from Vor(S)**

A better algorithm For every region V(s) of Vor(S) - let S’ be the set of sites whose regions in Vor(S) share an edge with V(s) - construct Vor(S’) - compute the intersection of Vor(S’) with V(s) Stick the different pieces together Space complexity Number of regions of Vor2(S) = number of edges of Vor(S) => O(n)

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**Construction of Vor2(S) from Vor(S)**

Time complexity ns = number of edges of V(s) construction of 1 Vor(S’) => O(ns log ns) construction of all Vor(S’) => O( (ns log ns)) < O(( ns ) log n) < O(n log n)

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**Construction of Vor2(S) from Vor(S)**

Theorem The ordre-2 Voronoi diagram of n sites can be computed from the order-1 Voronoi diagram in O(n) time.

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**Order-2 Delaunay diagram**

Dual vertices The dual of an order-2 Voronoi region V(s,t) is the midpoint of st. => g(s,t)

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**Order-2 Delaunay vertices**

Property The order-2 Delaunay vertices are distinct points Proof g(s,t) a vertex of Del2(S) <=> V(s,t) a region of Vor2(S) <=> c(s,t) an edge of Vor(S) <=> st an edge of Del(S)

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**Order-2 Delaunay edges Dual edges The dual of an order-2**

Voronoi edge cp(s,t) is the segment connecting g(p,s) and g(p,t). => ep(s,t) g(4,1) g(4,6) e4(1,6) V(4,1) V(4,6) c4(1,6) ep(s,t) : image of st by an homothety of center p and ratio 1/2

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**Order-2 Delaunay edges Property**

The ordre-2 Delaunay edges do not intersect. Proof Let ep(s,t) and ep’(s’,t’) be 2 distinct order-2 Delaunay edges.

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**Order-2 Delaunay edges Property**

The ordre-2 Delaunay edges do not intersect. Exercise Deal with the other cases.

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**Order-2 Delaunay regions**

Region dual of up(Q) The polygon with vertices g(p,q) with q Q. Region dual of u(Q) The polygon with vertices g(q,q’) with {q,q’} consecutive on the circle circumscribed to Q.

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**Constructing Del2(S) from Del(S)**

For every region of Del(S) compute its inscribed polygon For every site s of S - determine the polygon of the neighbors of s in Del(S) - construct the order-1 Delaunay diagram of this polygon - compute its image by an homothety with center s and ratio 1/2 Stick the different pieces together

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**Order-k Voronoi and Delaunay diagrams**

V(T) : the set of points of the plane closer to each site of T than to any other site (with |T| = k) g(T) : the center of gravity of T

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**Order-k Voronoi and Delaunay diagrams**

Theorem The size of the order-k diagrams is O(k(n-k)) Theorem The order-k diagrams can be constructed from the order-(k-1) diagrams in O(k(n-k)) time Corollary The order-k diagrams can be iteratively constructed in O(n log n + k2(n-k)) time

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