 # vtkPyramid / vtkQuadraticPyramid Inconsistencies

#1

Hi Folks,

I’ve got some questions regarding the pyramidal elements in vtk:

1. First of all: Why is the GetParametricCenter() different in both cells, although they span the same domain:

linear Pyra:
pcoords = pcoords = 0.4;
pcoords = 0.2;

quadr. Pyra:
pcoords = pcoords = 6.0/13.0; // approx 0.46
pcoords = 3.0/13.0; // approx 0.23

Wouldn’t I expect that both cells have the same parametric center?

1. Regarding the EvaluatePosition() method:
If I look at how the initial position is chosen

linear Pyra:
// set initial position for Newton’s method
double params = {0.3333333, 0.3333333, 0.3333333};
pcoords = pcoords = pcoords = params;

quadr. Pyra:
// set initial position for Newton’s method
pcoords = pcoords = pcoords = 0.5;
double params = {0.5, 0.5, 0.5};

I wonder even more:

• Why is the linear initial position set to 1./3. instead of the parametric center?
• Why is the quadratic initial position set to 0.5 instead of the parametric center? Here I wonder even more because it looks like this point is located on the surface instead of the interior, correct?

Thanks for the help.