[vtkusers] Streamlines from scalars of structured points.Needing help!

[stefano mininel]

Thank you very much for your help.
This solution is definitely something I wouldn't find easily or in a short time in a lonely
documentation/examples search.
I cannot say I've understood completely how it works, but it works perfectly!
As soon as I'll have some spare time I'll try to have a look at the source of this
vtkAssignAttribute and understand it better.

Meanwhile, a question, just to have a general idea: am I correct if I think of the
vtkAssignAttribute filter as a sort of necessary interface between vtk4.0 data structures and the
classes/filters that expect in their input data in the way of older vtk versions (to call upon
such data the now deprecated xx->GetPointData()->GetVectors() or GetScalars()  ).

By the way, I'm amazed by the quickness of your help. I think that about 15 minutes between
sending my question and receiving the answer is a sort of record :-)

    Stefano Mininel


Berk Geveci wrote:

> This is common problem. vtkImageGradient does not produce
> any vectors. Instead it produces a scalar array with 3 components.
> If you are using vtk, you can use the vtkAssignAttribute
> filter to label the scalars as also vectors. Something like:
> vtkAssignAttribute* aa = ...;
> aa->SetInput(...);
> aa->Assign(vtkDataSetAttributes::SCALARS, vtkDataSetAttributes::VECTORS,
>            vtkAssignAttribute::POINT_DATA);
>
> bla->SetInput(aa->GetOutput());
>
> If you are using an older version of VTK, you have to use
> vtkFieldDataToAttributeDataFilter and vtkAttributeDataToFieldDataFilter
> to shuffle things around.
>
> -Berk
>
> On Thu, 2002-05-30 at 10:10, stefano mininel wrote:
> > I know there should be an easy answer but right now I'm blocked.
> > I've got a
> > "vtkStructuredPoints *pPotGrid;"
> > with its scalars containing electric potential values around a current
> > dipole source (calculated from a previous PDE numerical solution and
> > copied in this vtk object for visualization of the results).
> > I want to show some current lines, so I tried with
> >
> > "
> > // using sphere source to have a bunch of points around the dipole
> > position
> > vtkSphereSource *pNearPoints = vtkSphereSource::New();
> >     pNearPoints->SetCenter(xDipPos,yDipPos,zDipPos);
> >     pNearPoints->... // other sphere settings
> >
> > // setting the integrator for the streamlines
> >  vtkRungeKutta4 *pIntegrator = vtkRungeKutta4::New();
> >
> > vtkStreamLine *pStreamer = vtkStreamLine::New();
> >   pStreamer->SetInput(pPotGrid);
> >   pStreamer->SetSource(pNearPoints->GetOutput());
> >   pStreamer->... // other streamline settings, quite standard and copied
> > from vtk examples
> >
> > ... // all the other things in visualization pipeline
> > "
> >
> > But upon rendering I receive the message "ERROR: in... No vector data
> > defined!"
> > True, I say! The structured points object contains only scalars. I need
> > vectors.
> > So I go with
> >
> > "
> > vtkImageGradient* grad = vtkImageGradient::New();
> >     grad->SetDimensionality( 3 );
> >     grad->SetInput(pPotGrid);
> > "
> > and put
> > "pStreamer->SetInput(grad->GetOutput());"
> >
> > But I receive the same error message.
> > Probably I doing something very stupid but I'm lost in a maze, and I
> > hope someone can enlighten me.
> > Thanks in advance for any tip/help/suggestion/insult (erh ... not the
> > last I hope!).
> >
> >     Stefano Mininel
> >
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