The CL algorithm

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Ring and star topologies are similar in several ways. Look at Ring topology and Star topology to get an idea of their similarities.

Ring topology

Star topology

Both topologies are composed of nodes drawn in a circle. For the Circular Layout algorithm, the only difference between the ring and star topologies is that the star has a special node, called the star center, that is drawn at the center of the circle.

Depending on the clustering mode (see Clustering mode (CL)), the layout allows you to specify the order of the nodes on the circle (see Cluster membership and order of the nodes in a cluster (CL)) or it computes the order automatically such that the number of link crossings is small.

The network topology can be composed of more than one ring or star. These rings and stars can be partially interconnected; that is, two or more clusters can have a common node as shown in figure Rings interconnected by common nodes. They can also be interconnected by links between nodes of two different clusters as shown in figure Rings interconnected by links.

Rings interconnected by common nodes

Rings interconnected by links

The Circular Layout algorithm lays out ring and star topologies in a way that preserves the visual identity of each cluster and avoids overlapping nodes and clusters. See the sample drawings in CL samples.

To understand how layout is performed in the clustering modes AUTOMATIC and BY_CLUSTER_IDS , consider a graph in which each node represents a ring or star cluster of a network topology. Add a link between two nodes each time there is an interconnection between the corresponding clusters. The Circular Layout algorithm is designed for the case where the graph obtained in this manner is a tree (that is, a graph with no cycles). If cycles exist, the layout is performed using a spanning tree of the graph.

Starting from a root cluster (either a ring or a star), the clusters that are connected to the root cluster are drawn in a circle that is concentric to the root cluster. The radius of the circle is computed to avoid overlapping clusters. Next, the algorithm lays out the clusters connected to these last clusters on a larger circle, and so on. Each circle is called a level.

For networks that are not connected (that is, disconnected groups of clusters exist in the graph), more than one spanning tree exists. Each spanning tree is laid out separately and placed near the others. You can see it in the sample drawings in CL samples.

In the clustering mode BY_SUBGRAPHS , each subgraph (cluster) keeps its initial position. The subgraphs can be placed either by a different layout algorithm or interactively.

Example of CL

In CSS

Below is a sample CSS specification using the Circular Layout algorithm. Since the Circular Layout places nodes and reshapes the links, it is usually not necessary to specify an additional link layout in CSS. The CSS specification can be loaded as a style file into an application that uses the IlvDiagrammer class (see Graph layout in Rogue Wave JViews Diagrammer).

SDM {
    GraphLayout : "true";
    LinkLayout  : "false";
}

GraphLayout {
    graphLayout    : "Circular";
    clusteringMode : "BY_CLUSTER_IDS";
    offset         : "20";
    levelOffset    : "30";
}

When using the clustering mode BY_CLUSTER_IDS (see Clustering mode (CL)), you need to specify the clustering information (see see Cluster membership and order of the nodes in a cluster (CL)). When using the clustering mode AUTOMATIC , you can optionally specify clustering information, but the algorithm tries to obtain missing cluster information automatically by analyzing the topology. When using the clustering mode BY_SUBGRAPHS , each cluster must be a subgraph.

In Java

The following code sample uses the IlvCircularLayout class. This code sample shows how to perform a Circular Layout on a grapher directly, without using a diagram component or any style sheet:

 ...
import ilog.views.*;
import ilog.views.graphlayout.*;
import ilog.views.graphlayout.circular.*;
 ...
IlvGrapher grapher = new IlvGrapher();
IlvManagerView view = new IlvManagerView(grapher);
IlvCircularLayout layout = new IlvCircularLayout();
layout.attach(grapher);

layout.setClusteringMode(IlvCircularLayout.BY_CLUSTER_IDS);
 
 ... /* Fill in the grapher with nodes and links here */
 
// create identifier for cluster 0
IlvClusterNumber clusterId = new IlvClusterNumber(0);
 
// specify the cluster identifier for cluster 0
// Assume there are three nodes: node1, node2, node3
// the ordering of the nodes: node1 -> node2 -> node3
layout.setClusterId(node1, clusterId, 0); // index 0
layout.setClusterId(node2, clusterId, 1); // index 1
layout.setClusterId(node3, clusterId, 2); // index 2
 
// create identifier for cluster 1
clusterId = new IlvClusterNumber(1);
 
// specify the cluster identifier for cluster 1
// Assume there are three nodes: node4, node5, node6
// the ordering of the nodes: node4 -> node5 -> node6
layout.setClusterId(node4, clusterId, 1); // index 1
layout.setClusterId(node5, clusterId, 2); // index 2
layout.setClusterId(node6, clusterId, 0); // index 0
 
try {
        IlvGraphLayoutReport layoutReport = layout.performLayout();

        int code = layoutReport.getCode();

        System.out.println("Layout completed (" +
          layoutReport.codeToString(code) + ")");
}
catch (IlvGraphLayoutException e) {
        System.err.println(e.getMessage());
}

Current section

Circular layout (CL)