[vtk-developers] howto report a bug?

[David Gobbi]

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.
>> 28 Corporate Drive
>> Clifton Park, NY 12065
>> will.schroeder at kitware.com
>> http://www.kitware.com
>> (518) 881-4902
>>
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