[Paraview] questions concerning vtkFloatArray

[Burlen Loring]

Natalie Happenhofer wrote:
> Hi Burlen,
> thanks for your answer!

No problem.
> I still have a few questions:
>
> -) I don´t understand the structure of the vtkFloatArray (or in 
> general, of the vtkDataArray): I have not points, but tuples? How is 
> the data organised? 
You should obtain a current/recent copy of the vtk users guide. 
Basically the documentation of VTK is split in a number of places, as 
someone who is developing vtk codes, you need access to them all, since 
no there is no single source that has it all. In addition you will want 
the vtk text, and the paraview guide, and you will constantly be reading 
the online docs as well: 
http://www.vtk.org/doc/nightly/html/classes.html The online docs have 
examples linked in. Take a look at 
http://www.vtk.org/doc/nightly/html/classvtkFloatArray.html for example. 
This is the nightly but you may want to look at the specific release you 
are using instead.

> Let´s assume, I have as an input a structured grid with 1000 points 
> and scalar attribute data attached. In Vtk the points section would be 
> described as a vtkDataArray with 1000 tuples and each tuple has 3 
> components. The scalar attribute data is another vtkDataArray with 
> 1000 Tuples and each tuple has just 1 component. If it was vector 
> attribute data, there would be again 3 components/ tuple. Did I get 
> that right?
 Yep, pretty much. vtkPoints internally uses a vtkDataArray as you 
described.
> So, in my filter, I want to get the scalar attribute data and actually 
> perform arithmetic operations with it, so I want to "get the values 
> out" of the vtkDataArray, do some calculations, and assign the new 
> array as scalar attribute data to the output. The grid itself, i.e the 
> points would stay untouched, it is just that I have as input 
> structured grid, rectilinear grid and structured points. In order to 
> assign the geometry to the output, which has the format structured 
> grid, is it ok to cast my rectilinear grid or my structured points to 
> vtkStructuredGrid and use then the "CopyStructure"-Method? Or will 
> this not work and I have to compute the points from the rectilinear 
> grid and the structured points manually and insert it to the output 
> structured grid?
check this out:
http://www.vtk.org/doc/nightly/html/classvtkDataSet.html#a257bc67d26310ad0d6bef97c066c099
>
> -)to use the RequestData() method I have to specify the input in the 
> server manager xml file, right?
Nope. This has to do with the way the VTK pipelines are structured. The 
vtk users guide has an entire chapter devoted to writing filters. 
Alternatively, you can take a look at some simple filters. I beleive the 
vtkShrinkFilter is a very simple one that you can use as a model, until 
you get the books ;)

Good luck
Burlen
> thx a lot!
> Natalie
>
> > Date: Thu, 11 Sep 2008 09:46:16 -0400
> > From: burlen.loring at kitware.com
> > To: nataliehapp at hotmail.com
> > CC: paraview at paraview.org
> > Subject: Re: [Paraview] questions concerning vtkFloatArray
> >
> > Hi Natalie,
> > Some comments follow:
> >
> > > int dims[3],i,j,k;
> > > // pointer for casting
> > > vtkRectilinearGrid *rect_input = vtkRectilinearGrid::New();
> > > vtkStructuredGrid *structgrid_input = vtkStructuredGrid::New();
> > >
> > >
> > > vtkDataArray *scaldat = this ->
> > > vtkAlgorithm::GetInputArrayToProcess(0,input);
> > > vtkDataArray *new_scaldat;
> > >
> > > vtkFloatArray *scalars = vtkFloatArray::New();
> > > vtkFloatArray *new_scalars = vtkFloatArray::New();
> > >
> > > scalars = vtkFloatArray::SafeDownCast(scaldat); //this is now the
> > > input as vtkFloatArray
> >
> > This is going to create a memory leak. You New()'d a vtkFloatArray and
> > never called Delete() before reassigning.
> >
> > > if((DataSetType == VTK_RECTILINEAR_GRID) && (rect_input =
> > > vtkRectilinearGrid::SafeDownCast(input)))
> > > {
> > > rect_input -> GetDimensions(dims);
> > > }
> > >
> > > else if((DataSetType == VTK_STRUCTURED_GRID )&& (structgrid_input =
> > > vtkStructuredGrid::SafeDownCast(input)))
> > > {
> > > structgrid_input->GetDimensions(dims);
> > > }
> > >
> >
> > Also these are going to create leaks. You never call Delete() on the
> > data sets you New()'d before reassigning.
> >
> > > My first problem occurs in the part, where I already got the input
> > > Scalar array and casted it to vtkFloatArray. In order to perform
> > > arithmetic operations with the data in it, I want to assign each 
> point
> > > of the vtkFloatArray to a "real" float array, but to do that, I need
> > > to get the vtkIDList of the vtkFloatArray , so I can use the
> > > GetPoint(vtkId x)-method.
> > >
> > > How do I get a vtkIDList of my vtkFloatArray?
> > There is no GetPoint() method for a vtkFloatArray. Do you mean 
> GetTuple?
> > The Tuples are ordered such the tuple 0 is associated with point/cell 0
> > for point/cell data arrays. Are you looking to get the points 
> associated
> > with the data set or the scalar values associated with the points?
> > >
> > > secondly,
> > > i don´t really now how to set the output correctly.
> > > I want to use a method like CopyStructure() so that the grid is 
> copied
> > > from input to output, but as the input is a DataSet and the output
> > > StructuredGrid, does this method compute points and so for the
> > > structuredGrid Format or do I have to assign them manually?
> > CopyStructure, does copy the geometry. Both the source and destination
> > data set have to be the same type.
> >
> > >
> > > Thirdly, I don´t know how to assign my new vtkFloatArray as Attribute
> > > Data to the output. SetScalars() does not work here, the compiler 
> says
> > > that this is not a member function of vtkStructuredGrid. But surely
> > > there must be a method to assign attribute data to a structured grid?
> > if you have point data then you can:
> >
> > output->GetPointData()->AddArray(someArray);
> >
> > There are similar calls for CellData and FieldData.
> >
> > I would advise taking a close look at the examples in the vtk user
> > guide. In the filters I have seen you will implement RequestData and 
> get
> > the input and output from an instance of vtkInformation. here is an
> > example using unstructured grids:
> >
> > int MyPv3Filter::RequestData(
> > vtkInformation *req,
> > vtkInformationVector **input,
> > vtkInformationVector *output)
> > {
> > vtkDataObject *tmpDataObj;
> > // Get the inputs
> > tmpDataObj
> > = input[0]->GetInformationObject(0)->Get(vtkDataObject::DATA_OBJECT());
> > vtkUnstructuredGrid *usgIn
> > = dynamic_cast<vtkUnstructuredGrid *>(tmpDataObj);
> > // Get the outputs
> > tmpDataObj
> > = output->GetInformationObject(0)->Get(vtkDataObject::DATA_OBJECT());
> > vtkUnstructuredGrid *usgOut
> > = dynamic_cast<vtkUnstructuredGrid *>(tmpDataObj);
> >
> > // do stuff here
> >
> > return 1;
> > }
> >
> > In regard to the input arrays, they can be easily obtained from
> > PointData, CellData, or FieldData via a call such as
> >
> > usgIn->GetPointData()->GetArray(x).
> > >
> > > thx,
> > > Natalie
> > >
> > > 
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> >
> >
> > --
> > Burlen Loring
> > Kitware, Inc.
> > R&D Engineer
> > 28 Corporate Drive
> > Clifton Park, NY 12065-8662
> > Phone: 518-371-3971 x137
> >
>
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-- 
Burlen Loring
Kitware, Inc.
R&D Engineer
28 Corporate Drive
Clifton Park, NY 12065-8662
Phone: 518-371-3971 x137