rvtk-example-5

RVTK Example: Sources and Filters.

As you can guess building data sets manually from scratch is not much of a fun. So the special classes were introduced into VTK that make this task more easy. The sources are used in vtk for data set generation from a small number of input parameters, and filters generate output datasets from input datasets. It is hard to classify all filters and sources available in VTK. Various sources and filters constitute the major part of VTK code.
To get the way filters and sources can be used we try to elaborate a little our example and add spheres at vertices of tetrahedra and run tubes along its edges.

vtkSphereSource()->Sphere
  Sphere$SetRadius( 0.05)
  Sphere$SetPhiResolution(20)
  Sphere$SetThetaResolution(20)

Here is an example of the sphere source - setting the radius of the sphere and number of meridians and parallels we get a data set with 400 vertices. This shere we will use as a stamp to create four spheres at the vertices of tetrahedra

vtkGlyph3D() ->Vertices
  Vertices$SetInput(model)
  Vertices$SetSource(Sphere$GetOutput())

vtkGlyph3D is an example of a filter. SetInput method is uniformly used by filters to set an input dataset for filter. But note that the rea;ization of this method for particular filter class may require different type of input parameter. Also there is no rule of thumb to determine which part of dataset will be used for generation of output dataset. I.e. Point- and CellData as well as the geometry of input dataset may influence the properties of output dataset. To make things more complicated different classes of filters may have different number of inputs (and outputs as well). vtkGlyph3D has two inputs - SetInput determines the position of glyphs and SetSource - theirs shape. For vtkGlyph3D's method SetScaleModeToScaleByScalar() enables the scaling of glyphs according to the PointData of input dataset (in that case Raduis of sphere glyph is an unit length). This is the default behaviour, the SetScaleModeToDataScalingOff() method is used to turn this feature off. GetOutput method (as you may guess) produces an output dataset (here we use it to get an output from Sphere source). As well as in case with GetInput method the type of return value is not defined - various filters/sources returns different descendants of vtkDataSet class.
To build the "tube" part of the data set we will follow the different routine. To specify the geometry scheme of tubes it is not hard to build a data set consisting of 6 lines but we use another filter which extracts tetrahedra edges. One more filter will be applied to convert the extracted edge lines into tubes:

vtkExtractEdges()->ee
  ee$SetInput(model)
vtkTubeFilter()-> Tubes
  Tubes$SetInput(ee$GetOutput())
  Tubes$SetRadius(.03)
  Tubes$SetNumberOfSides(6)

The purpose of the last filter is to merge two parts together (otherwise we could create the polydata mappers and the actors to render the individual parts of the model). All filters used before expect polydata as inputs and return polydata. vtkAppendPolydata filter merges any number of polygonal data sets together:

vtkAppendPolyData()->mf
  mf$AddInput(Vertices$GetOutput())
  mf$AddInput(Tubes$GetOutput())

The rendering of resulting dataset is just the same as before:

vtkPolyDataMapper()-> TubeMapper
  TubeMapper$SetInput(mf$GetOutput())
vtkActor()->TubeActor
  TubeActor$SetMapper(TubeMapper)
  TubeActor$GetProperty()$SetOpacity(0.6)
ren$AddActor(TubeActor)

As a last remark (once more) - the way the filters handle input data is not always very intuitive and some consulting with VTK documentation is particularly useful (although this is a good thing in general). It is not obvious which part of input data set will influence the result. For example (see Volcano demo) vtkWarp filter converts vtkImageData into Polydata by extracting the point data of vtkImageData and warps the geometry of the image (rectangular equally spaced grid) by adding this extaracted data to the Z coordinates of points. As you see in this case the geometry and the associated data both influence the filter's result.

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