How to cut a mesh into smaller pieces with python?

Question

I have a big terrain object, I'm trying to write a script that cut it in smaller pieces and then I will export the pieces to other formats. This is required by a game engine because the mesh, if not subdivided at export time, is not properly culled, resulting in slow rendering.

I tried to use bpy.ops.view3d.select_border() but it fails with message:

RuntimeError: Operator bpy.ops.view3d.select_border.poll() expected a view3d region

A search revealed that select_border must be called from 3d view, probably by clicking a button. But this is not a generalized script, it is specific to this model, I will rewrite parts of it if I need it for another model. I want to run it from text editor directly.

The only solution I can see for now is to iterate every vertex of the selected object and if they are inside the defined tile size, then duplicate, separate, export, delete, and repeat moving one tile to the left, starting at object dimensions[0] / 2 * -1, dimensions[1] / 2 * -1. Iterate vertices and select them can be done from text editor.

Is there a more convenient way of doing this?

Answer 1

You can override the context for the operator so you can still run it from the text editor.

From the API docs:

When calling an operator you may want to pass the execution context.

This determines the context thats given to the operator to run in, and weather invoke() is called or execute().

‘EXEC_DEFAULT’ is used by default but you may want the operator to take user interaction with ‘INVOKE_DEFAULT’.

The execution context is as a non keyword, string argument in: (‘INVOKE_DEFAULT’, ‘INVOKE_REGION_WIN’, ‘INVOKE_REGION_CHANNELS’, ‘INVOKE_REGION_PREVIEW’, ‘INVOKE_AREA’, ‘INVOKE_SCREEN’, ‘EXEC_DEFAULT’, ‘EXEC_REGION_WIN’, ‘EXEC_REGION_CHANNELS’, ‘EXEC_REGION_PREVIEW’, ‘EXEC_AREA’, ‘EXEC_SCREEN’)

# group add popup import bpy 
bpy.ops.object.group_instance_add('INVOKE_DEFAULT')

It is also possible to run an operator in a particular part of the user interface. For this we need to pass the window, screen, area and sometimes a region.

# maximize 3d view in all windows
import bpy

for window in bpy.context.window_manager.windows:
    screen = window.screen

    for area in screen.areas:
        if area.type == 'VIEW_3D':
            override = {'window': window, 'screen': screen, 'area': area}
            bpy.ops.screen.screen_full_area(override)
            break

E.g.:

import bpy

screen = bpy.context.window.screen
for area in screen.areas:
    if area.type == 'VIEW_3D':
        for region in area.regions:
            if region.type == 'WINDOW':
                override = {'window': bpy.context.window, 'screen': screen, 'area': area, 'region': region}
                bpy.ops.view3d.select_border(override, xmin=0, xmax=0, ymin=0, ymax=0, extend=False)
                break

Answer 2

I don't think hijacking select_border() is a good approach. You could use the new bisect_plane() bmesh operator to slice your mesh.

Here's a straightforward script that slices a mesh into an aligned grid:

import bpy, bmesh
from bpy import context as C

bpy.ops.object.mode_set(mode='EDIT')

bm = bmesh.from_edit_mesh(C.object.data)

edges = []

for i in range(-10, 10, 2):
        ret = bmesh.ops.bisect_plane(bm, geom=bm.verts[:]+bm.edges[:]+bm.faces[:], plane_co=(i,0,0), plane_no=(-1,0,0))
        bmesh.ops.split_edges(bm, edges=[e for e in ret['geom_cut'] if isinstance(e, bmesh.types.BMEdge)])

for i in range(-10, 10, 2):
        ret = bmesh.ops.bisect_plane(bm, geom=bm.verts[:]+bm.edges[:]+bm.faces[:], plane_co=(0,i,0), plane_no=(0,1,0))
        bmesh.ops.split_edges(bm, edges=[e for e in ret['geom_cut'] if isinstance(e, bmesh.types.BMEdge)])


bmesh.update_edit_mesh(C.object.data)

bpy.ops.mesh.separate(type='LOOSE')
bpy.ops.object.mode_set(mode='OBJECT')