[vtkusers] Writing VTK filters in Python and C

[David Gobbi]

Hi Robb,

A couple years ago I wrote classes to move vtkImageData (also works
for vtkStucturedPoints) into and out of a NumPy array.  These
classes were in VTK 3.2, but are not in VTK 4.x because they weren't
sufficiently well tested:

http://public.kitware.com/cgi-bin/cvsweb.cgi/~checkout~/vtk/python/vtkImageExportToArray.py?rev=1.1&content-type=text/plain&cvsroot=vtk
http://public.kitware.com/cgi-bin/cvsweb.cgi/~checkout~/vtk/python/vtkImageImportFromArray.py?rev=1.1&content-type=text/plain&cvsroot=vtk

They might work right off-the-bat (or might not) but in any case
you can take a peek inside to see how they work.  They convert
the NumPy array into a python string, and then have VTK access
the raw data in the string via a 'void *'.

In fact any vtkDataArray can be created from a python string using
>>> array.SetVoidArray(s,len(s)/typesize,1)
where 'typesize' is 1 for UnsignedCharArray, 2 for ShortArray,
8 for DoubleArray, etc. and 'save' must be set to 1 because VTK
cannot be allowed to free memory that was allocated by python.

Vice-versa doesn't work, i.e. void *GetVoidPointer() isn't available
in python.  Maybe sometime in the future it will be.

 - David

--
  David Gobbi, MSc                       dgobbi at irus.rri.ca
  Advanced Imaging Research Group
  Robarts Research Institute, University of Western Ontario

On Wed, 20 Feb 2002, Robert Brown wrote:

> I've got a vtkProgrammableFilter set up but the way I was getting the vtkStructuredPoints input was not working.  I got a void pointer which Python didn't know  how to deal with.  I've got a vtkStructuredPoints object -- do I have to use the point by point get and set functions to work with it?  I'd like to copy everything into a Numpy array, then put it back into the structuredpoints (ImageData) for output.
>
> Thanks,
>
> Robb
>
> Yes, Ken and Berk are right (of course).  The vtkProgrammableFilter
> will do the trick.  Note that the 'vtkStructuredPoints' data type
> is equivalent to vtkImageData for most intents and purposes here.
>
> Look at the man pages for the particular data type you want to use
> as well as the pages for vtkDataArray.  Building data sets has
> changed slightly (for the better) in VTK 4.x compared to 3.x.
>
>  - David
>
>
> On Fri, 15 Feb 2002, Ken Martin wrote:
>
> > Isn't this a case where vtkProgramableFilter can be used ? It is a
> > class that lets you fill in the execute method for a filter etc, and
> > the execute method can be written in Tcl, Python, Java. I think there
> > is a test of it in VTK/Graphics/Testing/Tcl
> >
> > Ken
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