[vtk-developers] howto report a bug?

[Henning Meyer]

Thank you all for helping me understanding the issue. For now I'll
implement my own version of vtkSphere and vtkCylinder - I think the
algorithm they use could be switched depending on a certain radius
threshold (somewhere around 50). I'll try and produce some patches.
Am 05.10.2012 20:11 schrieb "David Gobbi" <david.gobbi at gmail.com>:

> Several other implicit functions return squared distance instead
> of linear distance.  Of course there's the caveat that clipping is
> approximate for non-planar implicit surfaces, so people using
> these functions should only expect approximate results.
>
> vtkCylinder
> vtkQuadric
> vtkSuperquadric
> vtkImplicitHalo
>
> The last two classes, vtkSuperquadric and vtkImplicitHalo, aren't
> even complete, they don't compute the function gradient.  And
> vtkImplicitHalo has a math error in EvaluateFunction, and will only
> give correct results if Radius=1.0.  So it's a very good thing that
> these days we have gerrit to enforce code review.
>
> I'll see about posting a bug report to report my findings.  If anyone
> wants to send patches, I'll be glad to review them.
>
>  - David
>
>
>
>
> On Fri, Oct 5, 2012 at 10:42 AM, Bill Lorensen <bill.lorensen at gmail.com>
> wrote:
> > Will,
> >
> > Sounds like a good solution.
> >
> > Bill
> >
> >
> > On Fri, Oct 5, 2012 at 12:40 PM, Will Schroeder <
> will.schroeder at kitware.com>
> > wrote:
> >>
> >> 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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> >> Clifton Park, NY 12065
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