[vtk-developers] Implicit functions - Ability to specify if it is evaluable

[David Gobbi]

Hi Bill,

That's not quite the same thing.  That paper is about clipping
polydata with polydata, which is quite a difficult problem.
Cutting polydata with polydata, on the other hand, should be
a relatively simple thing to do.

 - David


On Mon, Feb 14, 2011 at 1:35 PM, Bill Lorensen <bill.lorensen at gmail.com> wrote:
> There is some excellent work by Bryn Lloyd, Computer Vision
> Laboratory, ETH Zurich.
> http://www.insight-journal.org/browse/publication/726
>
> The only limitation is that it uses the GNU Triangulated Surface
> Library GTS. GTS is LGPL'ed which is not compatible with vtk's
> license.
>
> Rather than reinvent the wheel, we should figure out a way to make
> contributions like this available to VTK users. Now that we've
> switched to git, I think we have the tools to do this.
>
> It's on my back burner...
>
> Bill
>
>
> On Mon, Feb 14, 2011 at 2:58 PM, David Gobbi <david.gobbi at gmail.com> wrote:
>> Hi Karthik,
>>
>> It seems that a better solution would be to add a new filter to VTK
>> that can cut one polydata with another polydata.  Since the contouring
>> operations are built into the cell classes, this could be done with
>> almost no code overlap with vtkCutter.
>>
>>  - David
>>
>> On Mon, Feb 14, 2011 at 11:23 AM, Karthik Krishnan
>> <karthik.krishnan at kitware.com> wrote:
>>> Thanks for the replies. Perhaps a screenshot would help clarify.
>>>
>>> The first screenshot is CutAfterModification-vtkCutter.png. The implicit
>>> function here is a polyplane, (function at a given point evaluates to the
>>> signed distance to the plane). It the point is beyond the "extent" of the
>>> plane, its returns an "outside" value.
>>>
>>> - If one extends the function to infinity, as David suggests, one cuts
>>> through the entire dataset. This is not what I want.
>>> - If one returns any arbitrary value, say a large positve or negative value,
>>> outside the extents of the plane, one ends up with artefacts when contouring
>>> with vtkCutter as seen in the second screenshot.
>>>
>>> To summarize:
>>> - I agree, I does change the definition a bit, but one can't do something as
>>> simple as cut a dataset with finite plane.
>>> - Adding one boolean evaluation is negligible compared to the function
>>> evaluation. It may even be faster, if one doesn't have to evaluate points if
>>> the function isn't defined.
>>>
>>> I did modify the code already, in my repository..
>>> Its the modification to vtkCutter that I am proposing, to ignore contouring
>>> cells that contain points with a certain value/attribute. There'd be no
>>> modification to vtkImplicitFunction then.
>>>
>>> I guess we'll do it another way then..
>>>
>>> Thanks
>>> --
>>> karthik
>>> (sorry for the repost.. my reply bounced with the image attachemtns)
>>>
>>> On Mon, Feb 14, 2011 at 10:56 PM, Will Schroeder
>>> <will.schroeder at kitware.com> wrote:
>>>>
>>>> An implicit function is f(x,y,z) = constant. If it doesn't fit that IMO
>>>> it's not an implicit function. The beauty of these beasts is that they are
>>>> simple, now you want to complexify the matter by checking validity on every
>>>> point, it would likely slow everything down, not to mention require
>>>> modifying a lot of code to do it right.
>>>> I think you need a new design not a new hack ;-)
>>>> W
>>>>
>>>> On Mon, Feb 14, 2011 at 12:04 PM, Karthik Krishnan
>>>> <karthik.krishnan at kitware.com> wrote:
>>>>>
>>>>> Hello:
>>>>>
>>>>> I need to cut a dataset with a polyplane. So I wrote a vtkPolyPlane class
>>>>> that derives from vtkImplicitFunction; so one could use it in combination
>>>>> with a vtkCutter.
>>>>>
>>>>> It does not quite work, unless the polyplane is infinite in extent. What
>>>>> I need is a way for the implicit function to specify that it cannot be
>>>>> defined at a certain point. The hack I have now is for the Evaluate method
>>>>> to return VTK_DOUBLE_MAX and a modification to vtkCutter to avoid contouring
>>>>> any cells which contain a point where the evaluated function value is
>>>>> VTK_DOUBLE_MAX.
>>>>>
>>>>> I propose
>>>>> - Adding a IsDefined method to vtkImplicitFunction that returns true by
>>>>> default (subclasses may override it)
>>>>> - Modifying vtkCutter to query IsDefined() on points in a cell before
>>>>> attempting to contour a cell.
>>>>>
>>>>> Any thoughts.
>>>>>
>>>>> Thanks
>>>>> --
>>>>> karthik
>>>>>
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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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