[vtk-developers] Class design for spline visualizations

[Lin M]

Hi Dr. Thompson,

I think one of my problem now is that I'm not sure how patches connecting
with each other. Originally I thought patches in a shape are irrelevant
with each other. For example in the ellipse, we need 4 patches for each
quadrant and I generate four patches independently only that they share
same end points, but from your previous email I think for NURBs it's not
exactly true.

Another is that if a NURBs shape is given by a vtkStructuredGrid and a know
vector, how to infer the number of patches, degree of interpolant and
number of control points.

Best,
Lin

On Fri, Jun 19, 2015 at 7:30 PM, David Thompson <david.thompson at kitware.com>
wrote:

> Hi Lin,
>
> I've just realized that the control points for Bézier patches will not
> have the same coordinates as the NURBs control points. That's because the
> Bézier patches interpolate their corner control points. So, we cannot just
> use a mapped data array to provide access to a subset of the NURBS control
> polygon.
>
> This site:
>
>   http://www.mactech.com/articles/develop/issue_25/schneider.html
>
> describes the conversion process for curves in its "CONVERTING NURB TO
> BÉZIER CURVES" section. Basically, we need to implement knot insertion,
> which is described in Piegl's NURBS Book in Chapter 5.
>
> The adaptor class shouldn't need to change much, but it might be better
> not to use a vtkMappedDataArray for the NURBS adaptor since it will have to
> generate new control points.
>
>         David
>
>
> > On Jun 19, 2015, at 7:04 PM, David Thompson <david.thompson at kitware.com>
> wrote:
> >
> > Hi Lin,
> >
> >> Does different patches in one vtkStructuredGrid have same order of
> interpolant and same number of control points?
> >
> > No, all the patches in a NURBs control polygon should have the same
> degree and control-point topology.
> >
> >> Each patch should have its own knot vector, so number of patches =
> number of knot vectors.
> >
> > Not quite. Each patch has a range of knot values. Those ranges overlap
> more as the degree increases.
> >
> >       David
>
>
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