[vtk-developers] Find center of polydata

[Laurent Chauvin]

Hello Kenneth,

Thank you for your explanations.
However, when I meant on the surface, I was not taking about the center.

My goal is not to have the center of the complete polydata, because if it's
a U shape or S shape for example, the center could be in empty space
(between the two | of the U).
For example, let's say the U shape is a tube with a U shape. I don't want
the center of the U shape, but the center of the tube, all along the U
shape.

I don't know if it's clearer.

If it's not, let me know I will try to make a 3D drawing.

Thank you very much.
-Laurent


On Mon, Mar 11, 2013 at 6:20 PM, Laurent Chauvin
<lchauvin at bwh.harvard.edu>wrote:

> Hello Kenneth,
>
> Thank you for your explanations.
> However, when I meant on the surface, I was not taking about the center.
>
> My goal is not to have the center of the complete polydata, because if
> it's a U shape or S shape for example, the center could be in empty space
> (between the two | of the U).
> For example, let's say the U shape is a tube with a U shape. I don't want
> the center of the U shape, but the center of the tube, all along the U
> shape.
>
> I don't know if it's clearer.
>
> If it's not, let me know I will try to make a 3D drawing.
>
> Thank you very much.
> -Laurent
>
>
>
> On Mon, Mar 11, 2013 at 2:37 PM, Moreland, Kenneth <kmorel at sandia.gov>wrote:
>
>>   Laurent,
>>
>>  (This discussion is probably more appropriate to VTK users, so I'm
>> forwarding it there.)
>>
>>  Although, I don't know for sure, but I suspect that the short answer to
>> your question is no.
>>
>>  The problem is that you have not given a clear definition of what
>> "center" means.  Center of mass is well defined even if, as you stated,
>> there is no guarantee that it in the mass or on the surface.  I'm not sure
>> what you mean by "center… close to the fiducial".  Center, by any
>> definition, is absolute relative to an object.  It is either close or not
>> close.
>>
>>  I can think of two approaches that might satisfy what you are asking
>> for.  First, you can find the polygon cell close to or under a selected
>> position and find the center of that polygon.  As long as the polygon is
>> planar and concave, this center will lie on that polygon.
>>
>>  Otherwise, perhaps you mean the center relative to part of the object
>> within some region around the selected point.  You could then extract those
>> cells within the region and take the center of that.  Be wary, though, that
>> the center is still not guaranteed to be located on the surface (and is
>> most likely not going to be if the surface is not in a plane).
>>
>>  -Ken
>>
>>   From: Laurent Chauvin <lchauvin at bwh.harvard.edu>
>> Date: Monday, March 11, 2013 10:43 AM
>> To: VTK Developers Developers <vtk-developers at vtk.org>
>> Subject: [EXTERNAL] [vtk-developers] Find center of polydata
>>
>>   Hello,
>>
>>  I would like to know if there is a vtk object that can find the center
>> of the vtkPolyData.
>> I've seen there is vtkCenterOfMass but I'm not sure it's gonna fit.
>>
>>  My concern is if I use a U shape dataset (or S shape, or another
>> strange shape), I feel like the center returned will be in the middle of
>> the U.
>> What I would like is a "local" center, like this (the green one, not the
>> red):
>>
>>  I would like to do this without having to define a ROI.
>> I would like for example to place a fiducial on model, and then, find the
>> center of the model close to the fiducial I placed.
>>
>>  Is there any tool like this ?
>>
>>  Thank you very much.
>>      -Laurent
>>
>>
>>
>>
>
>
> --
> Laurent Chauvin, MS
> Surgical Navigation and Robotics Laboratory, Radiology
> Brigham And Women's Hospital, Harvard Medical School
> http://wiki.ncigt.org/index.php/User:Lchauvin
>



-- 
Laurent Chauvin, MS
Surgical Navigation and Robotics Laboratory, Radiology
Brigham And Women's Hospital, Harvard Medical School
http://wiki.ncigt.org/index.php/User:Lchauvin
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