Table 5.45, “Layout Stages” lists the layout stages that can be used with all tree layout algorithms to enhance the layout process. Layout module TreeLayoutModule.java presents the setup of the tree layout algorithms in an application context. It also demonstrates how to set up and use the tree reduction layout stage.

Table 5.45. Layout Stages

Classname	Description
TreeReductionStage	Adds support for tree-like graphs.

Class TreeReductionStage is a layout stage that transforms graphs into proper trees. It automatically removes all non-tree edges prior to an algorithm's run and re-inserts them thereafter. When the edges are re-inserted, different routing styles can be applied.

Class TreeLayouter is a layouter mainly used for directed trees that have a unique root element. Starting with the root node the nodes are arranged either from top to bottom, left to right, right to left, or bottom to top. The edges of a graph can either be routed as straight lines or in an orthogonal bus-like fashion.

TreeLayouter knows a number of data provider keys which are used to retrieve supplemental layout data for a graph's elements. The data is bound to the graph by means of a data provider, which is registered using a given look-up key. Table 5.46, “Data provider look-up keys” lists all look-up keys for TreeLayouter.

Binding supplemental layout data to a graph is described in the section called “Providing Supplemental Layout Data”.

Table 5.46. Data provider look-up keys

Key	Element Type	Value Type	Description
EDGE_LABEL_LAYOUT_KEY	Edge	LabelLayoutData[]	For each edge an array of LabelLayoutData objects that encode size and preferred placement for all labels of the edge.
NODE_LABEL_LAYOUT_KEY	Node	LabelLayoutData[]	For each node an array of LabelLayoutData objects that encode size and preferred placement for all labels of the node.

These options configure class TreeLayouter in detail.

void setMinimalLayerDistance(double minimalDistance)

void setMinimalNodeDistance(double minimalDistance)

void setLayoutOrientation(byte orientation)

TreeLayouter tl = new TreeLayouter();
// Use left-to-right main layout direction.
tl.setLayoutOrientation(LayoutOrientation.LEFT_TO_RIGHT);

void setPortStyle(int portStyle)

void setLayoutStyle(int style)

void setBusAlignment(double busAlignment)

void setVerticalAlignment(double verticalAlignment)

void setChildPlacementPolicy(byte childPlacementPolicy)

setEnforceGlobalLayering(boolean enabled)

Class BalloonLayouter is a tree layouter that positions the subtrees rooted at a node in a radial fashion around that node. It is ideally suited for huge trees (say, 10,000 nodes) since it computes fast layouts that are quite compact.

Figure 5.65. Sample layout of a tree that contains 10,000 nodes

These options configure class BalloonLayouter in detail.

void setRootNodePolicy(byte policy)

void setPreferredRootWedge(int wedgeAngle)

void setPreferredChildWedge(int wedgeAngle)

void setMinimalEdgeLength(int minimalLength)

void setCompactnessFactor(double factor)

void setAllowOverlaps(boolean allowOverlaps)

void setConsiderNodeLabelsEnabled(boolean enabled)

Class HVTreeLayouter allows to layout a tree such that each subtree rooted at a node can either have a horizontal or vertical layout. For each node the layout orientation of its child nodes can be specified using the data provider look-up key SUBTREE_ORIENTATION.

Class HVTreeLayouter knows a number of data provider keys which are used to retrieve supplemental layout data for a graph's elements. The data is bound to the graph by means of a data provider, which is registered using a given look-up key. Table 5.47, “Data provider look-up keys” lists all look-up keys for HVTreeLayouter.

Binding supplemental layout data to a graph is described in the section called “Providing Supplemental Layout Data”.

Table 5.47. Data provider look-up keys

Key	Element Type	Value Type	Description
SUBTREE_ORIENTATION	Node	Object	For each root node either HORIZONTAL_SUBTREE or VERTICAL_SUBTREE to indicate horizontal or vertical subtree placement.

These options configure class HVTreeLayouter in detail.

void setHorizontalSpace(double space)

void setVerticalSpace(double space)

Class HVTreeLayouter allows to arrange subtrees either horizontally or vertically. To specify the desired placement, a data provider holding such supplemental layout data can be bound to the graph. The data provider is expected to be registered with the graph using key SUBTREE_ORIENTATION. Note that in the absence of the data provider horizontal placement is chosen.

Class ARTreeLayouter generates compact orthogonal tree drawings. As a layout constraint a preferred aspect ratio (relation of width to height) can be given. This is especially useful when the graph should fit perfectly on a page of given size.

Figure 5.66. Sample layouts of the same tree using different preferred aspect ratio settings

Using aspect ratio 2.0	Using aspect ratio 1.0	Using aspect ratio 0.5

Class ARTreeLayouter knows a number of data provider keys which are used to retrieve supplemental layout data for a graph's elements. The data is bound to the graph by means of a data provider, which is registered using a given look-up key. Table 5.48, “Data provider look-up keys” lists all look-up keys for ARTreeLayouter.

Binding supplemental layout data to a graph is described in the section called “Providing Supplemental Layout Data”.

Table 5.48. Data provider look-up keys

Key	Element Type	Value Type	Description
RATIO	Node	Number	For each root node a Number object encapsulating a double value that indicates the subtree's aspect ratio.
ROOT_PLACEMENT	Node	Object	For each root node one of PLACEMENT_TOP, PLACEMENT_CORNER, PLACEMENT_CORNER_SIDE, or PLACEMENT_CORNER_TOP to indicate the root node's placement.
ROUTING_POLICY	Node	Object	For each root node either ROUTING_HORIZONTAL or ROUTING_VERTICAL to indicate horizontal or vertical edge routing.

These options configure class ARTreeLayouter in detail.

void setHorizontalSpace(double space)

void setVerticalSpace(double space)

void setBendDistance(double distance)

void setAspectRatio(double aspectRatio)