slice_mesh_plane no longer preserves masked faces

[ramtingh]

Hi,

I've noticed a change in behaviour of slice_mesh_plane sometime between version 3.15 and 3.20. I believe the intent originally (#1369) was that if a boolean mask is provided, only those cases are cut and the remaining faces are preserved. But in 3.20 they are no longer preserved.

Here's simple example using this sphere mesh, where I attempt to cut the mesh along the xy plane but only modify the right half of the mesh:

mesh = trimesh.load("Sphere.stl")
mask = np.any((mesh.vertices[:,0]>0)[mesh.faces], axis=1)
mesh = trimesh.intersections.slice_mesh_plane(mesh, np.array([0,0,1]), np.array([0,0,0]), face_index=mask)

3.15 result:

3.20 result:

[mikedh]

Thanks for the report and reproducible test! Looks like this was probably broken in https://github.com/mikedh/trimesh/commit/e295de9cf1ffeaf74336e2afe8ef564ee664cf65 which was a fix for #1745. I may not be able to get to this all that soon but PR's welcome!