[vtk-developers] howto report a bug?

[Will Schroeder]

One approach is to just add an option to the class which indicates whether
the take the sqrt or not. Leave the default behavior alone or you'll be
driving a lot of people nuts :-)
W

On Fri, Oct 5, 2012 at 12:09 PM, David Gobbi <david.gobbi at gmail.com> wrote:

> I think the root of the problem is vtkSphere.  Here is the function
> that it uses:
>
>   return ( ((x[0] - this->Center[0]) * (x[0] - this->Center[0]) +
>               (x[1] - this->Center[1]) * (x[1] - this->Center[1]) +
>               (x[2] - this->Center[2]) * (x[2] - this->Center[2])) -
>               this->Radius*this->Radius );
>
> It is computing the squared distance from the surface of the sphere.
> This is different from vtkPlane, which computes the linear distance
> from the plane.  This is definitely going to cause scaling problems
> when you combine these two functions with vtkImplicitBoolean.
>
> VTK's clipping and contouring operations work by doing linearly
> interpolating the scalar value along each edge or line segment that is
> clipped.  In order for the clipping to be done accurately, the
> implicit functions must return a signed linear distance from the
> surface that the implicit function defines.  Because vtkSphere is
> returning a squared distance, I have to declare that vtkSphere is
> broken.
>
> It should be using a formula like this:
>
>   double d2 = (dx*dx + dy*dy + dz*dz - r*r);
>   return (d2 < 0 ? -sqrt(-d2) : sqrt(d2));
>
> This will make vtkSphere a bit slower, but it will make it give better
> results.
>
>  - David
>
>
> On Thu, Oct 4, 2012 at 5:05 PM, Sebastien Jourdain
> <sebastien.jourdain at kitware.com> wrote:
> > Ok then, could you save the 2 output of the vtkSampleFunction to a
> > binary vtk file format and upload them to the bug tracker, so I can
> > try to clip the dataset inside ParaView.
> >
> > Thanks,
> >
> > Seb
> >
> > On Thu, Oct 4, 2012 at 6:51 PM, Henning Meyer <tutmann at gmail.com> wrote:
> >>> But I don't think the issue come from the contour filter. Did you
> >>> tried to just render the SampleFunction ?
> >> It looks almost the same in both cases - besides the 1000 * value range
> in
> >> the second example.
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-- 
William J. Schroeder, PhD
Kitware, Inc.
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