[vtk-developers] Class design for spline visualizations

[David Thompson]

> I forgot one thing, the vtkPoints can not hold a 4-dimension vtkDataArray so I cannot store the control points in the vtkStructuredGrid. Currently I stored it separately in a vtkDataArray for test. Do I need to implement a vtkPoints subclass likt vtkControlPoints to allow it store 4-dimension coordinates? 

Hi Lin,

I would not create a new class. There are two alternatives and different spline software packages use them both:

+ Do as you have done, keeping point coordinates separate from the homogenous coordinate. This approach increases the number of things that have to get passed around between functions.

+ Pass the points around as just a vtkDataArray with 4 components per tuple. This approach makes it hard to use just the first three coordinates as points when you know the homogeneous coordinate is 1 for all of the values you are dealing with.

My mild preference is the latter (i.e., make the arguments to your functions take a vtkDataArray with 4 components per tuple), but either will be fine.

I want to avoid writing a 4-D point subclass because then algorithms that generate patches as output will need a new dataset type and it will be hard to use existing filters (such as the vtkArrayCalculator) on them.

	David

> 
> On Tue, Jun 30, 2015 at 12:42 PM, David Thompson <david.thompson at kitware.com> wrote:
> Hi Lin,
> 
> > I have finished the conversion from NURBS patch to Bezier patch and generation of surface meshes.
> >
> > Now we can load some real nurbs shapes to test. :-)
> 
> Great! I'm working on 2 sources of data.
> 
> + [CGM](https://bitbucket.org/fathomteam/cgm) provides a nice interface to OpenCascade's surface definitions, including these methods:
> 
> https://bitbucket.org/fathomteam/cgm/src/1aab72dc7c4ca6d0333a88df7b857e49d661db43/geom/Surface.hpp?at=master#cl-370
> https://bitbucket.org/fathomteam/cgm/src/1aab72dc7c4ca6d0333a88df7b857e49d661db43/geom/Curve.hpp?at=master#cl-364
> 
> CGM has a few CAD models for testing in the "test" directory of the source repo.
> 
> + [PetIGA](https://bitbucket.org/dalcinl/petiga) has 3D NURBS including simulation results but is in a binary file format that is undocumented (except in the source).
> 
>         David
>