[vtk-developers] howto report a bug?

[Bill Lorensen]

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
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