[vtkusers] how to use PolyData::GetPolys()?

[John Biddiscombe]

Dominik
> Thanks a lot for the hints, but after your previous hints I was not 
> copying anything already. There were two things:
>
> 1) To use e.g.
>      const double* p1 = 
> (double*)(polydata->GetPoints()->GetData())->GetVoidPointer(3*id1);
>
> instead
>      const double* p1 = 
> vtkDoubleArray::SafeDownCast(polydata->GetPoints()->GetData())->GetPointer(3*id1); 
>
>
> Somehow this works twice faster for me, maybe tricks g++ does to -Ox 
> switches
The SafeDownCast method uses string comparisons from the 
vtkXXX::GetClassName() and queries a lot of unnecessary stuff, but is in 
general safer

You should only ever call it once at the start of an execution. If you 
are executing many times on small data then this would be a performance 
hit and the (double*) or static_cast<double*> would be a better option. 
Note that vtkXXX::SafeDownCast should internally be replaced with 
somthing using dynamic_cast<...> but when it was first included not all 
compilers supported dynamic_cast (and it is possible to turn off rtti in 
the compiler which would break it I expect).

JB