chop_module.py

import bmesh
from sverchok.utils.sv_bmesh_utils import pydata_from_bmesh, bmesh_from_pydata

def average(verts):
    vx, vy, vz = 0, 0, 0
    for v in verts:
        vx += v[0]
        vy += v[1]
        vz += v[2]
    numverts = len(verts) 
    return vx/numverts, vy/numverts, vz/numverts


def chop(mesh, normal, point):
    '''
    not an optimal implementation, but simple enough to be educational.s

    input:
        takes a (what it assumes is) a closed mesh as input
    outputs: 
        a collection of vert, edge, face lists, that hold 2 sublists each
        one for each section / bisection.

    '''
    verts, edges, faces = mesh
    verts_list, edges_list, faces_list = [], [], []

    def inject_mesh(bm):
        v, e, f = pydata_from_bmesh(bm)
        verts_list.append(v)
        edges_list.append(e)
        faces_list.append(f)

    def wrev(n):
        d = n[0]*2, n[1]*2, n[2]*2
        return n[0]-d[0], n[1]-d[1], n[2]-d[2]

    for n in [normal, wrev(normal)]:
        bm = bmesh_from_pydata(verts, edges, faces)
        geom_in = bm.verts[:] + bm.edges[:] + bm.faces[:]
        res = bmesh.ops.bisect_plane(
            bm, geom=geom_in, dist=0.00001,
            plane_co=point, plane_no=n, use_snap_center=False,
            clear_outer=True, clear_inner=False
        )

        # lifted from bisect
        surround = [e for e in res['geom_cut'] if isinstance(e, bmesh.types.BMEdge)]
        fres = bmesh.ops.edgenet_prepare(bm, edges=surround)
        bmesh.ops.edgeloop_fill(bm, edges=fres['edges'])        

        inject_mesh(bm)
        del bm

    return verts_list, edges_list, faces_list