[vtk-developers] Pyramid and QuadraticPyramid formulation issues

[Andy Bauer]

Hi Bill,

It looks to me like the apex of vtkPyramid is at {.5, .5, 1}. I used this
test code to play around with it:
from vtk import *
ugrid = vtkUnstructuredGrid()
pts = vtkPoints()
pts.InsertNextPoint(0, 0, 0)
pts.InsertNextPoint(1, 0, 0)
pts.InsertNextPoint(1, 1, 0)
pts.InsertNextPoint(0, 1, 0)
pts.InsertNextPoint(.5, .5, 1)
ugrid.SetPoints(pts)
ugrid.Allocate(10)
p = [0, 1, 2, 3, 4]
ugrid.InsertNextCell(VTK_PYRAMID, 5, p)
c = ugrid.GetCell(0)
pcoords = [0, 0, 0]
x = [0, 0, 0]
w = [0, 0, 0, 0, 0]
subid = vtk.mutable(0)
# EvaluateLocation() is from parametric to global coordinate
c.EvaluateLocation(subid, pcoords, x, w)
print x
pcoords = [.5, .5, 1]
c.EvaluateLocation(subid, pcoords, x, w)

# EvaluatePosition() is from global to parametric coordinate
x = [.5, .5, 1]
closestpoint = [-1, -1, -1]
dist2 = vtk.mutable(0)
result = c.EvaluatePosition(x, closestpoint, subid, pcoords, dist2, w)
print result, pcoords





I didn't test vtkQuadraticPyramid though. Looking at the vtkPyramid.h
though I see the following which looks wrong to me:
//----------------------------------------------------------------------------
inline int vtkPyramid::GetParametricCenter(double pcoords[3])
{
  pcoords[0] = pcoords[1] = 0.4;
  pcoords[2] = 0.2;
  return 0;
}


I would have thought that pcoords[0] and pcoords[1] should be 0.5 instead
of 0.4. Since the volume of the above cell is 1/3 I would also think that
pcoords[2] should be 1/3 instead of 0.2, but I may be wrong on that. It's
also kind of weird that in vtkQuadraticPyramid.h that method is:
//----------------------------------------------------------------------------
// Return the center of the quadratic pyramid in parametric coordinates.
//
inline int vtkQuadraticPyramid::GetParametricCenter(double pcoords[3])
{
  pcoords[0] = pcoords[1] = 6./13;
  pcoords[2] = 3./13;
  return 0;
}

Just my 2 cents on the matter though since I haven't gone through the code
that closely.

Cheers,
Andy

On Mon, Jun 22, 2015 at 6:29 AM, Will Schroeder <will.schroeder at kitware.com>
wrote:

> Bill it could be that the vtkPyramid functions were originally written (a
> long time ago) as degenerate hex shape functions which might explain the
> apex parametric coordinates. I believe Mathieu found and implemented an
> alternative formulation and also added the quadratic version; I'm not sure
> where to find that reference.
> W
>
> On Sat, Jun 20, 2015 at 2:31 PM, Bill Lorensen <bill.lorensen at gmail.com>
> wrote:
>
>> Folks,
>>
>> As I continue to work through a comprehensive unit test for vtk cells,
>> I have found an issue(s) with vtkPyramid and vtkQuadraticPyramid.
>>
>> For example, the parametric coordinates for the Pyramid (and Quadratic
>> Pyramid) place the apex at 0, 0, 1. I believe it should be centered on
>> the base (which would be .5, .5, 1).
>>
>> Changing that causes other issues with the shape functions.
>>
>> I am seeking a copy of the reference quoted in the QuadraticPyramid
>> code so that I can verify the implementation.
>> // The shape functions and derivatives could be implemented thanks to
>> // the report Pyramid Solid Elements Linear and Quadratic Iso-P Models
>> // From Center For Aerospace Structures
>>
>
>
>
> --
> 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
>
> _______________________________________________
> Powered by www.kitware.com
>
> Visit other Kitware open-source projects at
> http://www.kitware.com/opensource/opensource.html
>
> Search the list archives at: http://markmail.org/search/?q=vtk-developers
>
> Follow this link to subscribe/unsubscribe:
> http://public.kitware.com/mailman/listinfo/vtk-developers
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://public.kitware.com/pipermail/vtk-developers/attachments/20150622/eef15803/attachment.html>