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1 On Balloon Drawings of Rooted Trees Chun-Cheng Lin and Hsu-Chun Yen Dept. of Electrical Engineering, National Taiwan University

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2 Balloon Drawing of Rooted Trees Definition. The balloon drawing of a rooted tree is a drawing having the following constraints: all the children of the same parent are placed on the circumference of the cycle centered at their parent, there exist no edge crossings in the drawing, and with respect to the root, the deeper an edge is, the shorter its drawing length becomes.

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3 Two models of balloon drawing (I) The fractal model – top-down method ( Koike & Yoshihara, 1993 ) Displaying self-similarly Evenly angle degree Edge length formula r m = r m-1 120 o r1r1 r2r2 r3r3

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4 Two models of balloon drawing (II) The subtree with nonuniform sizes (SNS) model – Bottom-up method ( Carriere & Kazman, 1995 ) Allow the subtrees with the same parent to reside in circles of different sizes inner circle outer circle r1r1 r2r2 r4r4 r3r3 11 r

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5 Comparison Q. For an unordered rooted tree (i.e., changing the order of subtrees is allowed), what is a good balloon drawing under the SNS model depend on? The balloon drawings of the same ordered tree under different models are given: The drawing based on the SNS model results in a clearer display on large subtrees than that under the fractal model. the SNS model the fractal model A. Angular resolution and aspect ratio of angle.Goal: optimize them.

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6 Preliminaries Unordered tree The order of subtrees is not significant. Angular resolution ( denoted by AngResl ) the minimum degree of two neighboring edges incident to the common vertex. Range: ( 0 o, 360 o ) Larger AngResl is better. Aspect ratio ( denoted by AspRatio ) the ratio of the maximum degree to the minimum degree of the angles incident to a vertex Range: ( 1, ) Smaller AspRatio approaching to one gives a more balanced display α min α max AngResl =α min AspRatio = α max / α min

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7 Reduction to star graphs Definition. The balloon drawing of a star graph with children of nonuniform size is a drawing in which circles associated with different children of the root do not overlap, and all the children of the root are placed on the circumference of a circle centered at the root. M1M1 M2M2 m1m1 m2m2 The balloon drawing under the SNS model (for the level zero)(for the nozero level)

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8 Balloon Drawing of Unordered Trees Changing the order of subtrees affects the angular resolution as well as the aspect ratio of the drawing. Example. Larger AngResl and smaller AspRatio. swap M 2 and m 2 M1M1 M2M2 m1m1 m2m2 min max M1M1 M2M2 m1m1 m2m2 min max

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9 Independence Optimizing AngResl and AspRatio on each level is Independent. Swapping any two subtrees inside the outer circle doesn’t affect the optimization on other levels.

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10 slice 1 11 c2c2 coco 1 1 1’1’ 2 2 3 3 4 4 c1c1 c4c4 c3c3 slice 4 slice 3 slice 2

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11 Procedure 1 OptBalloonDrawing -Order { 1,…, n } in ascending order as either m 1, m 2, …, m k-1, m k, M k, M k-1, …, M 2, M 1 if n is even, or m 1, m 2, …, m k-1, m k, mid, M k, M k-1, …, M 2, M 1 if n is odd, where m i (resp. M i ) is the i-th minimum (resp. maximum) among all, and mid is the median if n is odd. -Output a drawing witnessed by the following circular permutation: = (M 1, m 2, M 3, m 4, … (, mid) …, M 4, m 3, M 2, m 1 ) e.g. (n = 10) (the drawing has 10 slices) { 1,…, 10 } m 1 < m 2 < m 3 < m 4 < m 5 < M 5 < M 4 < M 3 < M 2 < M 1 M1M1 M3M3 m2m2 M2M2 m3m3 m1m1 M5M5 m4m4 m5m5 M4M4 Subindex difference = 1

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12 Theorem. Procedure 1 achieves optimality in angular resolution as well as in aspect ratio for star graphs. Basic idea to prove the correctness of the algorithm: Claim. optAngResl must be (M i + m i-1 )/2 for some i {2,…,k}, which can be generated by Procedure 1. Similarly, the minimum of the largest angle must be (M j-1 + m j )/2 for j {2,…,k}, which can be generated by Procedure 1. Since the permutation , generated by Procedure 1, simultaneously has both the maximum degree of the smallest angle and the minimum degree of the largest angle of any drawing, also witnesses the optAspRatio. Q.E.D

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13 An experimental result

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14 Balloon drawing with uneven angles Area of balloon drawing The size of the cycle enclosing the drawing The drawing under the SNS model may not be minimal (of even angle type) (of uneven angle type)

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15 Given the initial drawing of a star graph (with uneven angles) and a real number r, determine how to flip the drawing of subtrees so that AspRatio r (resp. AngResl r). The Aspect Ratio (resp. Angular resolution ) problem slice 2 slice 5 slice 4 slice 3 slice 1

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16 Matching A set of edges such that any two edges shares no common node. Maximum matching A matching of the maximum cardinality Perfect matching For a graph with n nodes, the largest possible matching consists of n/2 edges The maximum matching problem for bipartite graphs with n vertices and m edges can be found in time. Perfect!!

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17 Theorem. Both the Aspect Ratio Problem and the Angular Resolution Problem can be solved in O(n 2.5 ) time. pf. Consider the Aspect Ratio Problem ; the other problem can be proved by a slight modification. A1A1 A2A2 A3A3 A4A4 A1A1 A2A2 A3A3 A4A4

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18 Algorithm. Iteratively selects an pair (x,y) where x {b i, b’ i } and y {b i 1, b’ i 1 } such that x+y is assumed to be the ‘smallest’ angle. A bipartite graph G (x,y) is constructed in such a way that a drawing respecting the aspect ratio r exists iff G (x,y) has a perfect matching. Take the following example for illustration: Assume is the smallest angle. The nodes with odd index are placed on the upper level. A bipartite graph G (x,y) Perfect matching a balloon drawing Delete the edges (s,t) where s + t > r b4b4 b’ 4 b1b1 b’ 1

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19 Local magnetic spring model Magnetic spring model (Sugiyama and Misue, 1995) The graph is placed on a global magnetic field Edge magnetized spring Our local magnetic spring model

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20 Experimental results and applications Experimental results Applications

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21 Thank you for your attention.

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