[vtkusers] Tolerance of InsertNextPoint in python

[Brett Tully]

I am not sure if this is a bug, by design, or just an intricacy of Python
wrappers, but I have found a difference between VTK points depending on if
they are created by passing in x, y, z coordinates, or by passing a 3
element array [x, y, z]. The first method gives a true double precision
point as requested by SetDataTypeToDouble, where as the second method still
results in a single precision point. I have fixed our code to ensure double
precision, but I thought I would post it here for future reference.

Brett

import vtk
vtk_points = vtk.vtkPoints()
vtk_points.SetDataTypeToDouble()
e = [0.0, 0.0065, 0.0]

# pass in the point using its individual components
e1_id = vtk_points.InsertNextPoint(e[0], e[1], e[2])
p_e1 = vtk_points.GetPoint(e1_id)

# pass in the point as an array
e2_id = vtk_points.InsertNextPoint(e)
p_e2 = vtk_points.GetPoint(e2_id)

# check the difference between the two methods
print p_e1, p_e2
print (p_e1[0] - p_e2[0]), (p_e1[1] - p_e2[1]), (p_e1[2] - p_e2[2])

# gives
# (0.0, 0.0065, 0.0) (0.0, 0.006500000134110451, 0.0)
# 0.0 -1.34110451043e-10 0.0

On Tue, Apr 3, 2012 at 1:43 PM, David Gobbi <david.gobbi at gmail.com> wrote:

> Hi Brett,
>
> The default type of vtkPoints is "float".  If you need more precision,
> then you can change the precision to "double":
>
> vtk_points = vtk.vtkPoints()
> vtk_points.SetDataTypeToDouble()
>
> This is true in both C++ and Python.
>
>  - David
>
>
> On Tue, Apr 3, 2012 at 6:22 AM, Brett Tully <brett.tully at oxyntix.com>
> wrote:
> > Dear all,
> >
> > I am finding that vtkPoints.InsertNextPoint when called from python does
> not
> > return a point where I want it -- it seems to be out by ~1e-10 -- is
> there a
> > way to set the tolerance of this function or improve its location to be
> more
> > precise than 1e-10? Or is this due to python floats?
> >
> > Thanks,
> > Brett.
> >
> > I.e. the following python:
> >
> > import vtk
> > vtk_points = vtk.vtkPoints()
> > e = [0.0, 0.0065, 0.0]
> > e_id = vtk_points.InsertNextPoint(*e)
> > p_e = vtk_points.GetPoint(e_id)
> > print p_e, e
> > print (e[0] - p_e[0]), (e[1] - p_e[1]), (e[2] - p_e[2])
> >
> > # gives
> > # (0.0, 0.006500000134110451, 0.0) [0.0, 0.0065, 0.0]
> > # 0.0 -1.34110451043e-10 0.0
>
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