Ray-tracing (or casting) between rays and STL file

Question

I have a 3D shape (a bone) as a triangular mesh in STL format and I'm working with Python language.

I then have a ray with its origin and direction and I need to know:

which face of the mesh collides with this ray
coordinates of intersection point
the angle between my ray and the normal vector of the collided face

I know 'pycaster' ( https://pyscience.wordpress.com/2014/09/21/ray-casting-with-python-and-vtk-intersecting-linesrays-with-surface-meshes/ ) but it only works with Python 2.x and I need to use Python 3.x.

I know 'trimeshgeom' class ( http://cgkit.sourceforge.net/doc2/_sources/trimeshgeom.txt ) from cgkit but PyCharm can't install it as an interpreter (can't understand why).

Does anybody know how I could do what I'm trying to do?

Thanks, Cheers,

Davide

Joe Joe · Accepted Answer · 2018-07-12T07:52:40

We had a similar task once and ended up implementing the rather simple Möller-Trumbore-Algorithm.

The code can be stolen from printrun:

def ray_triangle_intersection(ray_near, ray_dir, v123):
    """
    Möller–Trumbore intersection algorithm in pure python
    Based on http://en.wikipedia.org/wiki/M%C3%B6ller%E2%80%93Trumbore_intersection_algorithm
    """
    v1, v2, v3 = v123
    eps = 0.000001
    edge1 = v2 - v1
    edge2 = v3 - v1
    pvec = numpy.cross(ray_dir, edge2)
    det = edge1.dot(pvec)
    if abs(det) < eps:
        return False, None
    inv_det = 1. / det
    tvec = ray_near - v1
    u = tvec.dot(pvec) * inv_det
    if u < 0. or u > 1.:
        return False, None
    qvec = numpy.cross(tvec, edge1)
    v = ray_dir.dot(qvec) * inv_det
    if v < 0. or u + v > 1.:
        return False, None

    t = edge2.dot(qvec) * inv_det
    if t < eps:
        return False, None

return True, t

For the angle calculation you can use e.g. angle_between_vectors() from transformations.py

Ray-tracing (or casting) between rays and STL file

1 Answers