VTK formats are developed for the Visualization Tool Kit library (VTK) and can be used for 3D visualization with software such as VisIt or ParaView. Networks are stored as VTK unstructured network data and can be output in fours different VTK formats:

  • -vtk: the legacy format
  • -vtk_ascii: ASCII version of the vtk format
  • -vtu: a more recently developed XML version of the vtk format,
  • -vtu_ascii: ASCII version of the vtu format

The specifications for these formats can be found in this PDF file. See also here and there for additional information.