How do I scale a specific set of vertices via the api

Question

I'm trying to programatically apply a partial scaling to a mesh. For this specific scaling, I'm trying to replicate what happens when you've selected a number of vertices of a mesh in Edit Mode and scale them by pressing 'S' to bring them closer together or farther apart.

Like so:

What I've tried is this:

bpy.ops.object.select_all(action='DESELECT')
object was originally a plane which I extruded into the z-axis, so this selects the upper half of the mesh.
upper_verts = [vert for vert in glyph_obj.data.vertices if vert.co[2] > 0]
for vert in upper_verts:
    vert.select = True
bpy.ops.transform.resize(value=(0.9, 0.9, 0.9), orient_type='GLOBAL', orient_matrix=((1, 0, 0), (0, 1, 0), (0, 0, 1)), orient_matrix_type='GLOBAL', mirror=True, use_proportional_edit=False, proportional_edit_falloff='SMOOTH', proportional_size=1, use_proportional_connected=False, use_proportional_projected=False, snap=False, snap_elements={'INCREMENT'}, use_snap_project=False, snap_target='CLOSEST', use_snap_self=True, use_snap_edit=True, use_snap_nonedit=True, use_snap_selectable=False)

Where glyph_obj is a bpy Object .

The last command, the bpy.ops.transform.resize, I copied from the info window from when I performed the operation by hand.

I can see in the viewer that my script does select all the appropriate vertices, but no transformation is performed. No errors are generated either. Because I followed programmatically all the steps I would have done by hand (I think), I don't know what I'm missing.

Operators in bpy.ops are for interactive use through the UI and should be avoided in scripts because they depend on context. Using them through scripts may yield unexpected results at best, or fail entirely at worst. Manipulate data directly from bpy.data instead. — Duarte Farrajota Ramos, Jan 26 '23 at 09:29
I would try going out of edit mode then in again, it should force the selection state to be updated — Gorgious, Jan 26 '23 at 09:44
@DuarteFarrajotaRamos Yeah I sort of knew that, but had to use bpy.ops operators for some imports and then continued using them. Think I'll stick to bmesh from now on. — Harm van den Brand, Jan 28 '23 at 19:20

score 2 · Accepted Answer · answered Jan 26 '23 at 14:27

Without an operator:

import bmesh
from bpy import context as C
from mathutils import Vector
condition = lambda v: v.co.z > 0
scale = 0.9
ob = C.object
me = ob.data
bm = bmesh.from_edit_mesh(me)
verts = [v for v in bm.verts if condition(v)]
average = sum((v.co for v in verts), start=Vector()) / len(verts)
for v in verts:
    offset = v.co - average
direction = offset.normalized()
v.co += direction * scale
v.co += offset * (scale - 1)


bmesh.update_edit_mesh(me)

How do I scale a specific set of vertices via the api

object was originally a plane which I extruded into the z-axis, so this selects the upper half of the mesh.

1 Answers1

direction = offset.normalized()

v.co += direction * scale