[vtk-developers] Re: String Array Support in VTK -- update

[Utkarsh Ayachit]

Update on String Array support.

I've checked in the code to intergrate vtkStringArrays with the
vtkDataArrays. The only change from my earlier email to this regard is
that the methods from vtkFieldData, that were claimed to be removed, are
now flagged as legacy and will be removed following the next release.

Utkarsh.

Utkarsh Ayachit wrote:
> Hey folks,
> 
> Currently, Sandia and Kitware are working on intergrating the support
> for string arrays with the main stream data arrays, so that string
> arrays can be passed around are field/point/cell data and used by
> filters etc. This email summarizes the changes that we are making to the
> present data array/field data/readers/writes. The changes will be
> committed to the main CVS trunk by this weekend. Your
> suggestions/objections are welcome, as always.
> 
> * vtkDataArray now has a new superclass: vtkAbstractArray.
> vtkAbstractArray provides type-agnostic API to access array elements.
> vtkDataArray provides a double API to access values -- such an API is
> deliberately missing from vtkAbstractArray, since arrays like string
> arrays are not castable to doubles.
> 
> * vtkAbstractArray has the notion of components/tuples, but does not
> make any assumptions about the arragment of these. It's up to the
> concrete subclasses how the data is represented in memory.
> 
> * vtkFieldData used to provide tuple API to get/set values from data
> arrays contained in it. However, now that FieldData can have arrays that
> are not vtkDataArrays, this API is not longer valid. The following are
> the methods that are no longer available:
>   double* GetTuple(const vtkIdType);
>   void SetTuple(const vtkIdType i, const double* tuple);
>   void GetTuple(const vtkIdType i, double* tuple);
>   void InsertTuple(const vtkIdType i, const double* tuple);
>   void InsertNextTuple(cons double*);
>   double GetComponent(const vtkIdType i, const int j);
>   void SetComponent(const vtkIdType i, const int j, const double c);
>   void InsertComponent(const vtkIdType i, const int j, const double c);
> However, new API is provided to copy tuples arround from one field data
> to another using:
>   void SetTuple(const vtkIdType i, const vtkIdType j, vtkFieldData* source);
>   void InsertTuple(const vtkIdType i, const vtkIdType j, vtkFieldData*
> source);
>   vtkIdType InsertNextTuple(const vtkIdType j, vtkFieldData* source);
> 
> * vtkFieldData provide a vtkDataArray* GetArray(int index) to retrieve
> arrays in the field data. This function can will now return a NULL if
> the array at the given index is not a data array. It provides
>   vtkAbstractArray* GetAbstractArray(int index);
> to retrieve an array even if it isn't a vtkDataArray.
> 
> * Since vtkFieldData can contain arrays that aren't subclasses of
> vtkDataArray, so can vtkDataSetAttributes. However, an attribute can
> only be a vtkDataArray subclass i.e. one cannot set a vtkStringArray as
> the SCALARS array for a Point Data, for example.
> 
> * String Array IO is only supported in XML file formats. If deemed
> important, support may be added for lecacy file formats.
> 
> *** Array Iterators ***
> 
> Another addition to the VTK Arrays, is the notion of Array Iterator. A
> vtkArrayIterator subclass is be provided for every vtkAbstractArray
> subclass. The iterator is a random access iterator. The iterators are
> not available in any of the wrapped languages, however, in C++ they can
> be used to write templatized code to processes the array values without
> any implicit type conversion and consequent loss of precision.
> vtkArrayIteratorTemplateMacro is provided to simply switching between
> different types of the arrays. Examples of use of this macro and the
> iterators can be found in vtkXMLWriter.cxx / vtkXMLDataReader.cxx /
> vtkXMLStructuredDataReader.cxx.
> 
> Utkarsh.
>