[vtk-developers] Adding ComputeArea() to vtkTriangle

Tue Dec 22 16:21:09 EST 2009

1) I would modify the current test for triangle.
2) When you add the TestPolygon test, check vtkPolygon's coverage to
see if you can exercise some uncovered code. There are 193 uncovered
lines of code:
http://www.cdash.org/CDash/viewCoverageFile.php?buildid=498540&fileid=10908473

Bill

On Tue, Dec 22, 2009 at 4:03 PM, David Doria <daviddoria+vtk at gmail.com> wrote:
>
> On Tue, Dec 22, 2009 at 3:31 PM, John Platt <jcplatt at dsl.pipex.com> wrote:
>>
>> Hi,
>>
>> Have you considered computing the cell size numerically? I seem to recall
>> that Gaussian quadrature will give the exact area/volume for polynomial
>> edges provided the integration order is set appropriately. This can be done
>> with one function for triangles & quads, another for pentahedrons & hex's
>> and possibly a 3rd for tetrahedrons & pyramids (including the nonlinear
>> variants). You should expect the size to be negative if the cell is
>> inside-out, or severely distorted with the determinant of the Jacobian
>> changing sign between integration points.
>>
>> Just a thought.
>>
>> John.
>>
>> ----- Original Message ----- From: "Bill Lorensen"
>> <bill.lorensen at gmail.com>
>> To: "Berk Geveci" <berk.geveci at kitware.com>
>> Cc: "VTK Developers" <vtk-developers at vtk.org>; "Moreland,Kenneth"
>> <kmorel at sandia.gov>
>> Sent: Tuesday, December 22, 2009 5:24 PM
>> Subject: Re: [vtk-developers] Adding ComputeArea() to vtkTriangle
>>
>>
>> I agree with Berk. ComputeArea() for triangle is enough. Maybe also
>> for vtkPolygon. There is already a TriangleArea method in vtkTriangle
>> and a ComputeArea() in vtkPolygon.  David D's ComputeArea() is a good
>> convenience addition.
>>
>> On Tue, Dec 22, 2009 at 12:12 PM, Berk Geveci <berk.geveci at kitware.com>
>> wrote:
>>>
>>> I don't think that this is a good idea. Users will expect a method
>>> that computes "size" in vtkCell to do it "right". However, unless you
>>> are subdividing adaptively, simple subdivision is likely to be very
>>> inaccurate for higher order cells. Having a default implementation
>>> that may be inaccurate for bunch of subclasses is a bad ideas unless
>>> you have the time and resources to go to all subclasses and override
>>> the default implementation.
>>>
>>> We started with a very simple request: add ComputeArea() to
>>> vtkTriangle. Now, we are talking about going down a path that is long
>>> and somewhat researchy - without resources to go all the way. Unless
>>> David is willing to spend some significant amount of time figuring out
>>> how to compute the volume of higher order elements accurately, I'd
>>> suggest going back to the original ComputeArea() and skipping the
>>> ComputeSize() idea.
>>>
>>> -berk
>>>
>>> On Tue, Dec 22, 2009 at 10:03 AM, Moreland, Kenneth <kmorel at sandia.gov>
>>> wrote:
>>>>
>>>> Why not have a default implementation that breaks the cell into
>>>> simplicia
>>>> (line segments, triangles, or tetrahedra)? That’s what vtkCellIntegrator
>>>> (attached) does.
>>>>
>>>> -Ken
>>>>
>>>>
>>>> On 12/22/09 7:53 AM, "David Doria" <daviddoria+vtk at gmail.com> wrote:
>>>>
>>>>
>>>> On Mon, Dec 21, 2009 at 10:53 AM, Moreland, Kenneth <kmorel at sandia.gov>
>>>> wrote:
>>>>
>>>> Yes, but the vtkCell classes (of which vtkTriangle is one) is not really
>>>> like the vtkDataObject objects. These classes are designed to hold
>>>> methods
>>>> for common computations. For example, they already hold methods for
>>>> computing interpolation, derivatives, contours, and intersections. In
>>>> this
>>>> regard, a method to compute the length/area/volume of the cell is wholly
>>>> appropriate.
>>>>
>>>> My only criticism (everyone has to have an opinion, right?) is that it
>>>> would
>>>> be helpful if there was a pure virtual method in vtkCell, perhaps called
>>>> ComputeSize, that returned the length, area, or volume (depending on the
>>>> dimensionality of the cell). That way you would not have to down cast
>>>> the
>>>> vtkCell to some known subclass and hope the implementation is there.
>>>>
>>>> Now that I write this, it occurs to me that somewhere there is a
>>>> vtkCellIntegrator in ParaView that does something similar. Perhaps the
>>>> implementation of that could be duplicated in vtkCell.
>>>>
>>>> -Ken
>>>>
>>>>
>>>>
>>>> On 12/18/09 12:15 PM, "Berk Geveci" <berk.geveci at kitware.com
>>>> <http://berk.geveci@kitware.com> > wrote:
>>>>
>>>> I think Mathieu brings up an important point. I don't think this has
>>>> any direct impact on the issue here but it is still a good one. VTK
>>>> usually separates computation from the data structures. There is a
>>>> good reason behind this. For example, when you are computing the
>>>> volume of a polyhedron or performing some other numerical integration
>>>> over it, there may be more than one way to do it numerically. Some
>>>> more accurate but more costly, others less accurate but faster. This
>>>> is why you don't find a lot of computational geometry related methods
>>>> on vtkPolyData.
>>>>
>>>> On Fri, Dec 18, 2009 at 11:09 AM, Mathieu Malaterre
>>>> <mathieu.malaterre at gmail.com <http://mathieu.malaterre@gmail.com> >
>>>> wrote:
>>>>>
>>>>> Ok, my bad. I thought this was introducing a new framework to compute
>>>>> 2D area which seem difficult to extend to 3d cell.
>>>>>
>>>>> thanks for clarification.
>>>>>
>>>>> On Fri, Dec 18, 2009 at 5:05 PM, Bill Lorensen <bill.lorensen at gmail.com
>>>>> <http://bill.lorensen@gmail.com> > wrote:
>>>>>>
>>>>>> Mathieu,
>>>>>>
>>>>>> vtkTriangle is a cell. It is not filter. It currently has a
>>>>>> TriangleArea() method, but it is cumbersome to use. David is proposing
>>>>>> a convenience method to do the computation. It calls the
>>>>>> TriangleArea() method but save the user from setting up the parameters
>>>>>> for that call.
>>>>>>
>>>>>> Bill
>>>>>>
>>>>>> On Fri, Dec 18, 2009 at 10:54 AM, David Doria
>>>>>> <daviddoria+vtk at gmail.com
>>>>>> <http://daviddoria+vtk@gmail.com> > wrote:
>>>>>>>
>>>>>>> On Fri, Dec 18, 2009 at 10:51 AM, Mathieu Malaterre
>>>>>>> <mathieu.malaterre at gmail.com <http://mathieu.malaterre@gmail.com> >
>>>>>>> wrote:
>>>>>>>>
>>>>>>>> Hi David,
>>>>>>>>
>>>>>>>> I do not have a strong opinion on this particular case.
>>>>>>>> However here is a couple of thoughts, I have from my modest
>>>>>>>> understanding of VTK.
>>>>>>>> 1. I would really separate source/sink from filter. You are writing
>>>>>>>> a
>>>>>>>> partial filter (aka algorithm) in a data object.
>>>>>>>> 2. You are duplicating (possibly) code from vtkMeshQuality.
>>>>>>>>
>>>>>>>> 2cts
>>>>>>>> -Mathieu
>>>>>>>
>>>>>>> 1) I'm not sure I follow. Isn't vtkTriangle an "object", not a
>>>>>>> "source"
>>>>>>> or
>>>>>>> "filter"?
>>>>>>> 2) That is likely the case and that is exactly my point here -
>>>>>>> vtkMeshQuality (or anything else) should not have to implement this
>>>>>>> code, it
>>>>>>> should just be able to call ComputeArea() on a triangle object.
>>>>>>> Thanks,
>>>>>>>
>>>>>>> David
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>>>>>>>
>>>>>>>
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>>>>>>
>>>>>
>>>>>
>>>>>
>>>>> --
>>>>> Mathieu
>>>>> _______________________________________________
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>>>>>
>>>>>
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>>>>
>>>>
>>>>
>>>>
>>>>
>>>> Per Ken's suggestion, I am trying to implement a virtual double
>>>> ComputeSize() function for vtkCell. There are a lot of cells! Many of
>>>> which
>>>> I have no idea how to get their size. Is it acceptable to somehow commit
>>>> these changes without having all of them implemented? Something like a
>>>> filler "return 0" and a comment "this function has not yet been
>>>> implemented"?
>>>>
>>>> The idea would be that then the framework is setup, so when a developer
>>>> who
>>>> is familiar with the particular cell subclass sees the missing
>>>> implementation, they could easily fill it in.
>>>>
>>>> Thoughts?
>>>>
>>>> Thanks,
>>>>
>>>> David
>>>>
>>>>
>>>>
>>>> **** Kenneth Moreland
>>>> *** Sandia National Laboratories
>>>> ***********
>>>> *** *** *** email: kmorel at sandia.gov
>>>> ** *** ** phone: (505) 844-8919
>>>> *** web: http://www.cs.unm.edu/~kmorel
>>>>
>>>>
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>>>>
>>>>
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>>
>
> This is indeed a big project to implement correctly. I'll stick with the two
> functions that I can handle well and leave the rest for a larger group
> effort at a later date.
> I have added ComputeArea() for vtkTriangle and vtkPolygon. Before I commit
> them, I was going to add tests. There is an existing TestTriangle.cxx -
> should I put the area test in there, or create a separate
> TestTriangleComputeArea.cxx ?
> There is no TestPolygon.cxx, so I was going to create one.
> I was also going to add ComputeArea for vtkQuad, but I'm a bit confused. Why
> does vtkQuad derive from vtkCell instead of vtkPolygon. It seems like both
> vtkQuad and vtkTriangle are special cases of vtkPolygon. Is there a benefit
> to the current structure?
> David
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