vtkGraph

Section: Visualization Toolkit Filtering Classes

Usage

vtkGraph is the abstract base class that provides all read-only API for graph data types. A graph consists of a collection of vertices and a collection of edges connecting pairs of vertices. The vtkDirectedGraph subclass represents a graph whose edges have inherent order from source vertex to target vertex, while vtkUndirectedGraph is a graph whose edges have no inherent ordering.

Graph vertices may be traversed in two ways. In the current implementation, all vertices are assigned consecutive ids starting at zero, so they may be traversed in a simple for loop from 0 to graph->GetNumberOfVertices() - 1. You may alternately create a vtkVertexListIterator and call graph->GetVertices(it). it->Next() will return the id of the next vertex, while it->HasNext() indicates whether there are more vertices in the graph. This is the preferred method, since in the future graphs may support filtering or subsetting where the vertex ids may not be contiguous.

Graph edges must be traversed through iterators. To traverse all edges in a graph, create an instance of vtkEdgeListIterator and call graph->GetEdges(it). it->Next() returns lightweight vtkEdgeType structures, which contain the public fields Id, Source and Target. Id is the identifier for the edge, which may be used to look up values in assiciated edge data arrays. Source and Target store the ids of the source and target vertices of the edge. Note that the edge list iterator DOES NOT necessarily iterate over edges in order of ascending id. To traverse edges from wrapper code (Python, Tcl, Java), use it->NextGraphEdge() instead of it->Next(). This will return a heavyweight, wrappable vtkGraphEdge object, which has the same fields as vtkEdgeType accessible through getter methods.

To traverse all edges outgoing from a vertex, create a vtkOutEdgeIterator and call graph->GetOutEdges(v, it). it->Next() returns a lightweight vtkOutEdgeType containing the fields Id and Target. The source of the edge is always the vertex that was passed as an argument to GetOutEdges(). Incoming edges may be similarly traversed with vtkInEdgeIterator, which returns vtkInEdgeType structures with Id and Source fields. Both vtkOutEdgeIterator and vtkInEdgeIterator also provide the wrapper functions NextGraphEdge() which return vtkGraphEdge objects.

An additional iterator, vtkAdjacentVertexIterator can traverse outgoing vertices directly, instead needing to parse through edges. Initialize the iterator by calling graph->GetAdjacentVertices(v, it).

vtkGraph has two instances of vtkDataSetAttributes for associated vertex and edge data. It also has a vtkPoints instance which may store x,y,z locations for each vertex. This is populated by filters such as vtkGraphLayout and vtkAssignCoordinates.

All graph types share the same implementation, so the structure of one may be shared among multiple graphs, even graphs of different types. Structures from vtkUndirectedGraph and vtkMutableUndirectedGraph may be shared directly. Structures from vtkDirectedGraph, vtkMutableDirectedGraph, and vtkTree may be shared directly with the exception that setting a structure to a tree requires that a "is a tree" test passes.

For graph types that are known to be compatible, calling ShallowCopy() or DeepCopy() will work as expected. When the outcome of a conversion is unknown (i.e. setting a graph to a tree), CheckedShallowCopy() and CheckedDeepCopy() exist which are identical to ShallowCopy() and DeepCopy(), except that instead of emitting an error for an incompatible structure, the function returns false. This allows you to programmatically check structure compatibility without causing error messages.

To construct a graph, use vtkMutableDirectedGraph or vtkMutableUndirectedGraph. You may then use CheckedShallowCopy to set the contents of a mutable graph type into one of the non-mutable types vtkDirectedGraph, vtkUndirectedGraph. To construct a tree, use vtkMutableDirectedGraph, with directed edges which point from the parent to the child, then use CheckedShallowCopy to set the structure to a vtkTree.

To create an instance of class vtkGraph, simply invoke its constructor as follows

  obj = vtkGraph

Methods

The class vtkGraph has several methods that can be used. They are listed below. Note that the documentation is translated automatically from the VTK sources, and may not be completely intelligible. When in doubt, consult the VTK website. In the methods listed below, obj is an instance of the vtkGraph class.