Planar L-Drawings of Directed Graphs · 2017. 11. 8. · There is a variety of drawing styles for...
Transcript of Planar L-Drawings of Directed Graphs · 2017. 11. 8. · There is a variety of drawing styles for...
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Planar -Drawings of Directed Graphs
Graph Drawing and Network Visualization 2017 · Boston
Steven Chaplick, Markus Chimani, Sabine Cornelsen,Giordano Da Lozzo, Martin Nollenburg, Maurizio Patrignani,Ioannis G. Tollis, Alexander Wolff
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Drawing Directed Graphs
There is a variety of drawing styles for directed graphs, e.g.
Layered layout Kandinsky style layout Overloadedorthogonal layout
[Foßmeier, Kaufmann 1996][Kornaropoulos, Tollis 2011]
[Sugiyama, Tagawa, Toda 1981]
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Drawing Directed Graphs
There is a variety of drawing styles for directed graphs, e.g.
Layered layout Kandinsky style layout Overloadedorthogonal layout
[Foßmeier, Kaufmann 1996][Kornaropoulos, Tollis 2011]
[Sugiyama, Tagawa, Toda 1981]
In 2016 Angelini et al. introduced L-drawings:
exclusive x- and y-coordinates per vertexoutgoing edges attach verticallyincoming edges attach horizontallysmall arcs indicate L-bendscrossings and “confluent” overlaps allowedexist for any graphink minimization is NP-hard
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Drawing Directed Graphs
There is a variety of drawing styles for directed graphs, e.g.
Layered layout Kandinsky style layout Overloadedorthogonal layout
[Foßmeier, Kaufmann 1996][Kornaropoulos, Tollis 2011]
[Sugiyama, Tagawa, Toda 1981]
In 2016 Angelini et al. introduced L-drawings:
exclusive x- and y-coordinates per vertexoutgoing edges attach verticallyincoming edges attach horizontallysmall arcs indicate L-bendscrossings and “confluent” overlaps allowedexist for any graphink minimization is NP-hard
1
![Page 5: Planar L-Drawings of Directed Graphs · 2017. 11. 8. · There is a variety of drawing styles for directed graphs, e.g. Layered layout Kandinsky style layout Overloaded [F o meier,](https://reader036.fdocuments.net/reader036/viewer/2022062510/611c60a44a234e6e516df7bd/html5/thumbnails/5.jpg)
Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Planar L-Drawings
Definitions:Planar L-drawing if crossing-freeUpward planar L-drawing if all edges y-increasingUpward-rightward planar L-drawing if all edges x- and y-increasing
planar upward planar upward-rightward planar
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Planar L-Drawings
Definitions:Planar L-drawing if crossing-freeUpward planar L-drawing if all edges y-increasingUpward-rightward planar L-drawing if all edges x- and y-increasing
planar upward planar upward-rightward planar
Observation:Planar L-drawings correspond to planar1-bend Kandinsky drawings with extraconstraints on cyclic edge orders of vertices.
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Overview of Results
planar upward (-rightward) planar
planar st-graphs
NP-complete
linear time
characterization
directed plane graphs+ port assignment
directedplanar graphs
constructive linear time algorithm
Overview of Results
Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs3
![Page 8: Planar L-Drawings of Directed Graphs · 2017. 11. 8. · There is a variety of drawing styles for directed graphs, e.g. Layered layout Kandinsky style layout Overloaded [F o meier,](https://reader036.fdocuments.net/reader036/viewer/2022062510/611c60a44a234e6e516df7bd/html5/thumbnails/8.jpg)
Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Overview of Results
planar upward (-rightward) planar
planar st-graphs
NP-complete
linear time
characterization
directed plane graphs+ port assignment
directedplanar graphs
constructive linear time algorithm
Overview of Results
Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs3
![Page 9: Planar L-Drawings of Directed Graphs · 2017. 11. 8. · There is a variety of drawing styles for directed graphs, e.g. Layered layout Kandinsky style layout Overloaded [F o meier,](https://reader036.fdocuments.net/reader036/viewer/2022062510/611c60a44a234e6e516df7bd/html5/thumbnails/9.jpg)
Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Planar L-Drawings of Directed Graphs
Any planar L-drawing implies 4-modal embedding.
There are planar directed graphs that do not admit planar L-drawings.
6-modal in any embedding
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Planar L-Drawings of Directed Graphs
Any planar L-drawing implies 4-modal embedding.
There are planar directed graphs that do not admit planar L-drawings.
6-modal in any embedding
There are graphs with 4-modal embedding but no planar L-drawing.
every vertex is 4-modal . . .
. . . but rightmost vertex in L-drawingcan be at most bimodal
octahedron4
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
NP-Completeness
Theorem: Deciding whether a directed graph admits a planarL-drawing is NP-complete.
Proof: (sketch)
reduction from NP-complete HV-rectilinear planarity testing[Didimo, Liotta, Patrignani 2014]
Given biconnected degree-4 planar graph G with edges labeled H and V ,decide if G admits drawing with horizonal H-edges and vertical V -edges.
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
NP-Completeness
Theorem: Deciding whether a directed graph admits a planarL-drawing is NP-complete.
Proof: (sketch)
reduction from NP-complete HV-rectilinear planarity testing[Didimo, Liotta, Patrignani 2014]
Given biconnected degree-4 planar graph G with edges labeled H and V ,decide if G admits drawing with horizonal H-edges and vertical V -edges.
core gadget: 4-wheel graph has basically two planar L-embeddings
w
c
v
zu
w
c
v
z
uwc
v
z
u
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
NP-Completeness
Theorem: Deciding whether a directed graph admits a planarL-drawing is NP-complete.
Proof: (sketch)
reduction from NP-complete HV-rectilinear planarity testing[Didimo, Liotta, Patrignani 2014]
Given biconnected degree-4 planar graph G with edges labeled H and V ,decide if G admits drawing with horizonal H-edges and vertical V -edges.
core gadget: 4-wheel graph has basically two planar L-embeddings
in HV -graph G replace vertices by 4-wheel and edges by H-/V -gadgets
4-wheel
H-gadget
V -gadget
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
NP-Completeness
Theorem: Deciding whether a directed graph admits a planarL-drawing is NP-complete.
Proof: (sketch)
reduction from NP-complete HV-rectilinear planarity testing[Didimo, Liotta, Patrignani 2014]
Given biconnected degree-4 planar graph G with edges labeled H and V ,decide if G admits drawing with horizonal H-edges and vertical V -edges.
core gadget: 4-wheel graph has basically two planar L-embeddings
in HV -graph G replace vertices by 4-wheel and edges by H-/V -gadgets
4-wheel
H-gadget
V -gadget
resulting graph G′ hasplanar L-drawing
⇔G has HV -drawing
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Overview of Results
planar upward (-rightward) planar
planar st-graphs
NP-complete
linear time
characterization
directed plane graphs+ port assignment
directedplanar graphs
constructive linear time algorithm
Overview of Results
Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs6
![Page 16: Planar L-Drawings of Directed Graphs · 2017. 11. 8. · There is a variety of drawing styles for directed graphs, e.g. Layered layout Kandinsky style layout Overloaded [F o meier,](https://reader036.fdocuments.net/reader036/viewer/2022062510/611c60a44a234e6e516df7bd/html5/thumbnails/16.jpg)
Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Planar st-Graphs and Bitonic st-Orderings
A planar st-graph G is a directed acyclic graph with exactlyone source s and one sink t, both embeddable on same face.
s
t
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Planar st-Graphs and Bitonic st-Orderings
A planar st-graph G is a directed acyclic graph with exactlyone source s and one sink t, both embeddable on same face.
s
t planar st-graphs always admit straight-lineupward planar drawingshave st-ordering π respecting edge directions
[Di Battista, Tamassia 1988]
1
2
3
45
6
7
7
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Planar st-Graphs and Bitonic st-Orderings
A planar st-graph G is a directed acyclic graph with exactlyone source s and one sink t, both embeddable on same face.
s
t planar st-graphs always admit straight-lineupward planar drawingshave st-ordering π respecting edge directions
[Di Battista, Tamassia 1988]
1
2
3
45
6
7
st-ordering π of plane st-graph (planar st-graph+ embedding) is bitonic if successors of eachvertex form bitonic sequence [Gronemann 2014, 2016]
increasing decreasing347
8
6
5
7
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Planar st-Graphs and Bitonic st-Orderings
A planar st-graph G is a directed acyclic graph with exactlyone source s and one sink t, both embeddable on same face.
s
t planar st-graphs always admit straight-lineupward planar drawingshave st-ordering π respecting edge directions
[Di Battista, Tamassia 1988]
1
2
3
45
6
7
st-ordering π of plane st-graph (planar st-graph+ embedding) is bitonic if successors of eachvertex form bitonic sequence [Gronemann 2014, 2016]
increasing decreasing347
8
6
5
For a planar st-graph G define a bitonic pair (E , π) as anupward planar embedding E of G with a bitonic st-ordering π.
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Characterization
Theorem: A planar st-graph admits an upward-planar L-drawingif and only if it admits a bitonic pair.
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Characterization
Theorem: A planar st-graph admits an upward-planar L-drawingif and only if it admits a bitonic pair.
Proof: (sketch)
⇒
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Characterization
Theorem: A planar st-graph admits an upward-planar L-drawingif and only if it admits a bitonic pair.
Proof: (sketch)
⇒
12
345
67
89
y-coordinates induce st-ordering ππ is bitonic due to upward L-properties
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Characterization
Theorem: A planar st-graph admits an upward-planar L-drawingif and only if it admits a bitonic pair.
Proof: (sketch)
⇒
12
345
67
89
y-coordinates induce st-ordering ππ is bitonic due to upward L-properties
⇐ use π for y-coordinatesincrementally construct partial order ≺ as basis for x-coordinates
Gi−1
vi
u1u2 u4u3≺ ≺ ≺
vi
u1u4u2 u3≺ ≺
Gi≺≺
Gi−1
invariant: outer face of Gisimple cycle ordered by ≺insert each vi btw. lasttwo predecessors
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Characterization
Theorem: A planar st-graph admits an upward-planar L-drawingif and only if it admits a bitonic pair.
Proof: (sketch)
⇒
12
345
67
89
y-coordinates induce st-ordering ππ is bitonic due to upward L-properties
⇐ use π for y-coordinatesincrementally construct partial order ≺ as basis for x-coordinates
invariant: outer face of Gisimple cycle ordered by ≺insert each vi btw. lasttwo predecessorsGi−1
vi
u1
vi
Gi≺ ≺
Gi−1
special case: just one predecessor → augment graph similar to [Gronemann 2016]
`
smax
u1`
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Finding Bitonic Pairs
Assumption: st-graph G is biconnected and has edge (s, t)
We say G is v-monotonic or (strictly) v-bitonic if for every vertex vsubgraph induced by successors of v (− transitive edges) is a path pp is monotonic or (strictly) bitonic
v v v
monotonic ⊂ bitonic strictly bitonic ⊂ bitonic
apex of v and p
9
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Finding Bitonic Pairs
Assumption: st-graph G is biconnected and has edge (s, t)
We say G is v-monotonic or (strictly) v-bitonic if for every vertex vsubgraph induced by successors of v (− transitive edges) is a path pp is monotonic or (strictly) bitonic
v v v
monotonic ⊂ bitonic strictly bitonic ⊂ bitonic
apex of v and p
Goal: augment G into G∗ by adding edges s.t. G∗ is v-bitonic
Theorem:∼ [Gronemann 2016]
Plane st-graph G admits bitonic st-ordering iff G∗ is v-bitonic.Any st-ordering of G∗ is bitonic st-ordering of G.
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Finding Bitonic Pairs
Assumption: st-graph G is biconnected and has edge (s, t)
We say G is v-monotonic or (strictly) v-bitonic if for every vertex vsubgraph induced by successors of v (− transitive edges) is a path pp is monotonic or (strictly) bitonic
v v v
monotonic ⊂ bitonic strictly bitonic ⊂ bitonic
apex of v and p
Goal: augment G into G∗ by adding edges s.t. G∗ is v-bitonic
Theorem:∼ [Gronemann 2016]
Plane st-graph G admits bitonic st-ordering iff G∗ is v-bitonic.Any st-ordering of G∗ is bitonic st-ordering of G.
→ The task of finding a bitonic pair of G reduces to findingan augmentation G∗ of G that is v-bitonic.
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Finding a v-Bitonic Augmentation
Visiting the SPQR-tree of biconnected planar st-graph G rooted at edge (s, t)in bottom-up fashion find augmentation G∗ and embedding (if one exists).
s
t
1
2
3 4
56
7
8
10
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Finding a v-Bitonic Augmentation
Visiting the SPQR-tree of biconnected planar st-graph G rooted at edge (s, t)in bottom-up fashion find augmentation G∗ and embedding (if one exists).
s
t
1
2
3 4
56
7
8
An SPQR-node µ with source sµ is oftype M if the augmented pertinent graph is sµ-monotonictype B if the augmented pertinent graph is strictly sµ-bitonic
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Finding a v-Bitonic Augmentation
Visiting the SPQR-tree of biconnected planar st-graph G rooted at edge (s, t)in bottom-up fashion find augmentation G∗ and embedding (if one exists).
s
t
1
2
3 4
56
7
8
An SPQR-node µ with source sµ is oftype M if the augmented pertinent graph is sµ-monotonictype B if the augmented pertinent graph is strictly sµ-bitonic
When processing an SPQR-node µ our primary goal is to make it type Mand otherwise type B. If both fails, no v-bitonic augmentation of G exists.
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Processing SPQR-Nodes
Q-node: trivially type M
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Processing SPQR-Nodes
Q-node: trivially type M
S-node: replace each virtual edge by augmented pertinent graph ofchild node with arbitrarily flipped embedding.Node type is inherited from bottom child.
sµ
B/M
B/M. . .
B/M. . .
tµ
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Processing SPQR-Nodes
Q-node: trivially type M
S-node: replace each virtual edge by augmented pertinent graph ofchild node with arbitrarily flipped embedding.Node type is inherited from bottom child.
P-node:if two or more children are of type B→ successors of sµ have two apices, so regardless of embedding nosµ-bitonic augmentation exists
sµ
tµ
sµ
tµ
B B
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Processing SPQR-Nodes
Q-node: trivially type M
S-node: replace each virtual edge by augmented pertinent graph ofchild node with arbitrarily flipped embedding.Node type is inherited from bottom child.
P-node:if two or more children are of type B→ successors of sµ have two apices, so regardless of embedding nosµ-bitonic augmentation exists
if one child is of type B and one child is Q-node for (sµ, tµ)→ apex of type-B node 6= tµ, but tµ must be apex of sµ;again sµ has two apices and no sµ-bitonic augmentation exists
sµ
tµ
sµ
tµ
B
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Processing SPQR-Nodes
Q-node: trivially type M
S-node: replace each virtual edge by augmented pertinent graph ofchild node with arbitrarily flipped embedding.Node type is inherited from bottom child.
P-node:if two or more children are of type B→ successors of sµ have two apices, so regardless of embedding nosµ-bitonic augmentation exists
if one child is of type B and one child is Q-node for (sµ, tµ)→ apex of type-B node 6= tµ, but tµ must be apex of sµ;again sµ has two apices and no sµ-bitonic augmentation exists
else embed child of type B or Q-node for (sµ, tµ) rightmost(if any) and connect successors of sµ in order of embedding;node type is M ⇔ rightmost child is of type M
sµ
tµ
MM M M/B
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Processing SPQR-Nodes
Q-node: trivially type M
S-node: replace each virtual edge by augmented pertinent graph ofchild node with arbitrarily flipped embedding.Node type is inherited from bottom child.
P-node:if two or more children are of type B→ successors of sµ have two apices, so regardless of embedding nosµ-bitonic augmentation exists
if one child is of type B and one child is Q-node for (sµ, tµ)→ apex of type-B node 6= tµ, but tµ must be apex of sµ;again sµ has two apices and no sµ-bitonic augmentation exists
else embed child of type B or Q-node for (sµ, tµ) rightmost(if any) and connect successors of sµ in order of embedding;node type is M ⇔ rightmost child is of type M
R-node: more complicated, see paper
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Upward Planar L-Drawings
Theorem: It can be tested in linear time whether a planar st-graphG admits an upward-planar L-drawing.If it does, it can also be constructed in linear time.
Proof: (sketch)process SPQR-tree to find v-bitonic augmentation G∗ and embedding E∗in root node (if any)any st-ordering π of G∗ yields bitonic pair (E , π) of GG has bitonic pair ⇔ G admits upward-planar L-drawingall steps can be implemented in linear time
vi
u1u4u2 u3≺ ≺
Gi≺≺
Gi−1
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Upward Planar L-Drawings
Theorem: It can be tested in linear time whether a planar st-graphG admits an upward-planar L-drawing.If it does, it can also be constructed in linear time.
Proof: (sketch)process SPQR-tree to find v-bitonic augmentation G∗ and embedding E∗in root node (if any)any st-ordering π of G∗ yields bitonic pair (E , π) of GG has bitonic pair ⇔ G admits upward-planar L-drawingall steps can be implemented in linear time
Remark: Same approach can be used to decide existence and constructupward-rightward-planar L-drawings.
vi
u1u4u2 u3≺ ≺
Gi≺≺
Gi−1
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Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Summary
linear time→ see paper
directed plane graphs+ port assignment
planar st-graphscharacterizationconstructive linear time algorithm
planar upward (-rightward) planar
NP-completedirectedplanar graphs
Summary
Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
directed plane graphs
upward planar graphs
bimodal graphs
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![Page 40: Planar L-Drawings of Directed Graphs · 2017. 11. 8. · There is a variety of drawing styles for directed graphs, e.g. Layered layout Kandinsky style layout Overloaded [F o meier,](https://reader036.fdocuments.net/reader036/viewer/2022062510/611c60a44a234e6e516df7bd/html5/thumbnails/40.jpg)
Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
Summary
linear time→ see paper
directed plane graphs+ port assignment
planar st-graphscharacterizationconstructive linear time algorithm
planar upward (-rightward) planar
NP-completedirectedplanar graphs
Summary
Chaplick, Chimani, Cornelsen, Da Lozzo, Nollenburg, Patrignani, Tollis, Wolff · Planar L-Drawings of Directed Graphs
directed plane graphs
?upward planar graphs
bimodal graphs
13