The Hierarchical Layout uses generic parameters, common to other graph layouts, and specific parameters applicable in hierarchical layouts only. Refer to the following sections for general information on parameters among the graph layouts:

The Hierarchical Layout parameters are described in detail in this topic under:

The IlvHierarchicalLayout class supports the following generic parameters defined in the IlvGraphLayout class:

The following paragraphs describe the particular way in which these parameters are used by this subclass.

The layout algorithm stops if the allowed time setting has elapsed. (For a description of this layout parameter in the IlvGraphLayout class, see Allowed Time.) If the layout stops early because the allowed time elapsed, the nodes and links are not moved at all and remain in the same position they had before the call of layout.

The layout algorithm does not reshape the links that are specified as fixed. (For more information on link parameters in the IlvGraphLayout class, see Preserve Fixed Links and Link Style.)

The layout algorithm does not move the nodes that are specified as fixed. (For more information on node parameters in the IlvGraphLayout class, see Preserve Fixed Nodes.) Moreover, the layout algorithm ignores fixed nodes completely and also does not route the links that are incident to the fixed nodes. This can result in undesired overlapping nodes and link crossings. However, this feature is useful for individual, disconnected components that can be laid out independently.

The following parameters are specific to the IlvHierarchicalLayout class:

The flow direction parameter specifies the direction in which the majority of the links should point. If the flow direction is to the top or to the bottom, the node levels are oriented horizontally and the links mostly vertically. If the flow direction is to the left or to the right, the node levels are oriented vertically and the links mostly horizontally.

If the flow direction is to the bottom, the nodes of the level with index 0 are placed at the top border of the drawing. The nodes with level index 0 are usually the root nodes of the drawing (that is, the nodes without incoming links). If the flow direction is to the top, the nodes with level index 0 are placed at the bottom border of the drawing. If the flow direction is to the right, the nodes are placed at the left border of the drawing.

To specify the flow direction, use the following method:

The valid values for direction are:

To obtain the current choice, use the following method:

If the layout uses horizontal levels, the nodes of the same level are placed approximately at the same y-coordinate. The nodes can be justified, depending on whether the top border, or the bottom border, or the center of all nodes of the same level should have the same y-coordinate.

If the layout uses vertical levels, the nodes of the same level are placed approximately at the same x-coordinate. In this case, the nodes can be justified to be aligned at the left border, at the right border, or at the center of the nodes that belong to the same level.

To specify the level justification, use the following method:

If the flow direction is to the top or to the bottom, the valid values for justification are:

If the flow direction is to the left or to the right, the valid values for justification are:

To obtain the current value, use the following method:

The layout algorithm positions the nodes and routes the links. To avoid overlapping nodes and links, it creates bend points for the shapes of links. The link style parameter controls the position and number of bend points. The link style can be set globally, in which case all links have the same kind of shape, or locally on each link such that different link shapes occur in the same drawing.

To set the global link style, use the following method:

The valid values for style are:

To obtain the current choice, use the following method:

All links have the same style of shape unless the global link style is MIXED_STYLE. Only when the global link style is MIXED_STYLE can each link have an individual link style.

To set and retrieve the style of an individual line, use the following methods:

The valid values for the local link style are the same as for the global link style:

The layout algorithm positions the end points of links (the connector pins) at the nodes automatically. The connector style parameter specifies how these end points are calculated.

To set the connector style, use the following method:

The following options are available for style:

To obtain the current choice, use the following method:

The layout algorithm tries to place the nodes such that all links are short, point in the flow direction, and do not cross each other. However, this is not always possible. Often, links cannot have the same length. If the graph has cycles, some links must be reversed against the flow direction. If the graph is nonplanar, some links have to cross each other.

The link priority parameter controls which links should be selected if long, reversed, or crossing links are necessary. Links with a low priority are more likely to be selected than links with a high priority. This does not mean that low-priority links are always longer, reversed, or crossed, because the graph may have a structure such that no long, reversed or crossing links are necessary.

To set or retrieve the link priority, use the following methods.

The default value of the link priority is 1.0. Negative link priorities are not allowed.

For an example of using the link priority, consider a cycle A->B->C->D->E->A. It is impossible to lay out this graph without reversing any link. Therefore, the layout algorithm selects one link to be reversed. To control which link is selected, you can give one link a lower priority than the others. This link will be reversed. In Figure 4.32, the bottom layout shows the use of the link priority. The link C->D was given the priority 0.5, while all the other links have the priority 1.0. Therefore C-D is reversed. The top layout in Figure 4.32 shows what happens when all links have the same priority. Link E->A is reversed.

In Step 1 of the layout algorithm (the leveling phase), the nodes are partitioned into levels. These levels are indexed starting from 0. For instance, when the flow direction is to the bottom, the nodes of the level with index 0 are placed at the topmost horizontal level line, and the nodes with larger level index are placed at a lower position than the nodes with smaller level index (see Figure 4.26). The layout algorithm calculates these level indices automatically.

You can affect how the levels are partitioned by specifying the level indices for some nodes. The nodes are placed in the specified level.

To specify the level index of a node, use the following method:

The default value is -1. If the default value is used or if a node is set to a negative level index, the layout algorithm automatically calculates an appropriate level index during the leveling phase of the algorithm.

To obtain the current level for a node, use the following method:

Figure 4.33 illustrates a hierarchical layout with the same level specified for each node of the graph.

In Step 2 of the layout algorithm (the crossing reduction phase), the nodes are ordered within the levels. All nodes that belong to the same level get a position index starting from 0. For instance, when the flow direction is to the bottom, the node with the position index 0 is placed in the left-most position within its level. The nodes with a larger position index are placed more to the right than the nodes with a smaller position index in the same level. The nodes of different levels are independent. The node of the first level with the position index 0 is left of the node of the first level with the position index 1, but not necessarily left of a node of another level with position index 0. Note that long links crossing a level also obtain a position index (see Figure 4.26). The layout algorithm calculates these position indices automatically.

You can affect how the nodes are positioned within each level by specifying the position index of some nodes. The nodes are placed at the specified position within their level.

To specify the position index of a node, use the following method:

The default value is -1. If the default value is used, if a node is set to a negative position index, or if a node is set to a position index that is larger than the number of nodes of its level, the layout calculates automatically an appropriate position index during the crossing reduction step.

To obtain the current position index of a node, use the following method:

The spacing of the layout is controlled by three kinds of spacing parameters: the minimal offset between nodes, the minimal offset between parallel segments of links and the minimal offset between a node border and a bend point of a link or a link segment that is parallel to this border. The offset between parallel segments of links is at the same time the offset between bend points of links. All three kind of parameters occur in both directions: horizontally and vertically.

The spacing parameters can be set for the horizontal direction by the following methods:

The spacing parameters can be set for the vertical direction by the following methods:

The spacing parameters can be obtained by the corresponding methods:

For a layout with horizontal levels (the flow direction is to the top or to the bottom), the horizontal node offset is the minimal distance between nodes of the same level. The vertical node offset is the minimal distance between nodes of different levels, that is, the minimal distance between the levels. For non-orthogonal link styles, the horizontal link offset is basically the minimal distance between bend points of links. The horizontal node-link offset is the minimal distance between the node border and the bend point of a link. For horizontal levels, the vertical link offset and the vertical node-link offset play a role only if the link shapes are orthogonal.

Similarly, for a layout with vertical levels (the flow direction is to the left or to the right), the vertical node offset controls node distances within the levels. The horizontal node offset is the minimal distance between the levels. In this case, the vertical link offset and the vertical node-link offset always play a role, while the horizontal link offset and the horizontal node-link offset affect the layout only with orthogonal links.

For orthogonal links, the horizontal link offset is the minimal distance between parallel, vertical link segments. The vertical link offset is the minimal distance between parallel, horizontal link segments. However, the layout algorithm cannot always satisfy these offset requirements. If a node is very small but has many incident links, it may be impossible to place the links orthogonally with the specified minimal link distance on the node border. In this case, the algorithm places some link segments closer than the specified link offset.

In some circumstances, you may need to use a sequence of layouts on the same graph. For example:

The Hierarchical Layout supports sequences of layout that “do not change very much.” It allows you to preserve the level index of a node and the position index of the node within the level. By doing this, the relative position of a node compared to a previous layout does not change. The algorithm calculates new coordinates for the nodes and new routings for the links to adjust the absolute positions of the objects in order to clean up the drawing. The relative order such as “node A is in a level above node B” or “node A is left of node B” can be preserved.

The parameters for hierarchical sequences are described in:

The layout algorithm allows you to access the level index that was calculated for a node by a previous layout. To do this, use the following method:

If the node was never laid out, this method returns -1. Otherwise, it returns the previous level index of the node.

The method can be used to specify the level index for the next layout in the following way:

When this is done, it ensures that the node is placed at the same level as in the previous layout.

If the graph is detached from the layout algorithm, the calculated level index of a node is set back to -1.

The layout algorithm allows you to access the position index within a level that was calculated for a node by a previous layout. To do this, use the following method:

If the node was never laid out, this method returns -1. Otherwise, it returns the previous position index of the node within its level.

To ensure that the node is placed at the same level at the same relative position as in the previous layout, use the following:

If the graph is detached from the layout algorithm, the calculated position index of a node is set back to -1.

The following example shows how to use the calculated and specified level and position indices to refine a layout. Assume that you want to place a leaf node in the level with the highest index (see Figure 4.36). Since you do not know before laying out the graph how many levels will be created, you cannot specify the highest level index at the beginning. You must perform a first layout, iterate over the nodes, and retrieve the highest calculated level. If the leaf node is not already placed at the highest level index, you must specify this level index for the leaf node while preserving the level and position indices of all other nodes. Then perform the layout a second time. The following code fragment shows how to do this.