Mirror/flip transform on triangular mesh

[seblars]

Hi,
I am trying to mirror transform the z-axis of a mesh. The transform works but I am getting weird lighting effect when I compute the vertex normals after the transform. It appears that the mesh is inside-out so the inside surface is reflective rather than the outside. I tried transforming the vertex normals without success. Is there another way? Example code below using version 0.17:

import open3d as o3d
import copy
import numpy as np

# mesh = o3d.geometry.TriangleMesh.create_coordinate_frame()

knot_mesh = o3d.data.KnotMesh()
mesh = o3d.io.read_triangle_mesh(knot_mesh.path)

I = np.eye(4) 
I[2,2] = -1
mesh_mirror = copy.deepcopy(mesh).transform(I)
mesh_mirror.compute_vertex_normals()
o3d.visualization.draw_geometries([mesh_mirror])

[benjaminum]

The mirroring causes the triangles orientation to flip. What you see is the inside of the mesh. You can try mesh_mirror.orient_triangles() or change the vertex index order of the triangles by manipulating mesh_mirror.triangles.

[seblars]

Thanks, reversing the order of mesh_mirror.triangles fixed the issue

[wmcnCG]

@seblars @benjaminum reversing the mesh triangles with mesh.triangles = mesh.triangles[::-1] gave me a random pattern. orient_triangles() did not work either. Am I missing something?

[cwreynolds]

Probably not directly to the questions of @seblars or @wmcnCG but just in case someone else working in c++ finds this Discussion, this is the code I use to “evert” (turn inside-out) a mesh. I use it for meshes meant to be seen from the inside:

// Flip orientation of each tri in a triangle mesh (destructively modifies).
void evertTriangleMesh(open3d::geometry::TriangleMesh& tri_mesh)
{
    for (auto& triangle : tri_mesh.triangles_)
    {
        std::reverse(std::begin(triangle), std::end(triangle));
    }
}

[wmcnCG]

Thank you. This python equivalent worked.

for i in range(len(mesh.triangles)):
        mesh.triangles[i] = mesh.triangles[i][::-1]

[seblars]

Agree - it is a useful workaround to make things transparent. If your mesh is large you can use a vectorised method such as:
mesh.triangles = o3d.utility.Vector3iVector(np.flip(np.asarray(mesh.triangles), axis=1))

[wmcnCG]

@seblars excellent!