[vtkusers] vtkPointInTriangle - bug?

[David Gobbi]

Hi Dov,

The PointInTriangle method checks whether the point, after being
projected onto the plane of the triangle, lies within the triangle.

  David

On Tue, Feb 18, 2014 at 1:11 PM, Dov Grobgeld <dov.grobgeld at gmail.com> wrote:
> The following is a comparison of testing whether a point p falls within the
> plane of a triangle a,b,c by basic python code vs by using vtk.
>
> #!/usr/bin/python
>
> from numpy import cross, dot, sqrt
> from numpy import array as P
> import vtk
>
> def norm(v):
>   mag = sqrt((v**2).sum())
>   return v/mag
>
> a,b,c = (P((0,0,0)),
>          P((10,0,0)),
>          P((10,10,0)))
>
> p = P((5,1,1))
>
> # Test for linearity
> #
> #       b
> #      / \
> #     /   \
> #    /  p  \
> #   a-------c
> #
> N1 = norm(cross(b-a, p-a))
> N2 = norm(cross(p-c, b-c))
> N1dotN2 = dot(N1,N2)
> print 'By norms: ', N1dotN2 > 0.99
>
> # Same test with vtk
> print 'vtkInTriangle = ', vtk.vtkTriangle.PointInTriangle(p, a,b,c, 1e-10)
>
> The result is:
>
> By norms:  False
> vtkInTriangle =  1
>
> It is trivial that the point p does not fall in the triangle. But still vtk
> sais so. Is this a bug in vtk or am I misunderstanding what
> vtkTriangle.PointInTriangle() is doing?
>
> Thanks!
> Dov