polygon `select` property is not updating correctly

Question

I have created an operator to calculate the surface area of the currently selected faces. To enable selection across objects, it loops through context.selected_objects. Then for each of these, it reads the polygons with select=true.

The problem is, when I change my face selection manually in the viewport and re-run the operator, the select attribute doesn't seem to be changing in real-time. There's some delay, and as a result my plugin is reporting an inaccurate result.

Code is as follows:

class Utils():
    def get_selected_object_faces(object):
        mesh = object.data
        polys = mesh.polygons
        return [poly for poly in polys if poly.select]
def sq_meters_to_sq_feet(sq_meters, precision):
    return round(sq_meters * 10.7639, precision);


class Calc_Selected_Face_Area(bpy.types.Operator):
    """Calculates Selected Face Area"""
    bl_idname = "interior.calc_selected_face_area"
    bl_label = ""
def execute(self, context):
    selected_polys = []
    for selected_object in context.selected_objects:
      print(selected_object)
      selected_polys += Utils.get_selected_object_faces(selected_object)

    poly_areas = [Utils.sq_meters_to_sq_feet(poly.area, precision=0) for poly in selected_polys]
    print(poly_areas)
    context.scene.shared_props.areas = ", ".join([str(int(poly)) for poly in poly_areas])

    return {'FINISHED'}

See specifically in the get_selected_object_faces method. The poly.select attirbute I'm reading in there, doesn't seem to be updating immediately when I change my face selection in the UI.

Any tips would be appreciated. New to Python and Blender.

The full script if you want to try it out.

Answer 1

Numpy version.

Changes made in edit mode are not reflected in mesh data until changing back to object mode . How can I check face selection in edit mode If you do not wish to toggle the mode, consider using an edit mode bmesh, especially if the areas are going to be used to update the mesh.

As an example thought I'd add a numpy version, as well as demonstrate use of Object.update_from_editmode() Equiv would be a toggle mode operator if in edit mode, to update selection

EDIT: For working on an evaluated mesh it will require the edit mode toggle via operator. Otherwise the objects are not evaluated correctly whilst in edit mode.

For an unmodified untransformed method can update the mesh via

if context.mode == 'EDIT_MESH':
    for ob in context.selected_objects:
        if ob.mode == 'EDIT':
            ob.update_from_editmode()

Quick rundown of code.

• For selected objects in edit mode update their meshes
• Get the modified and transformed mesh of each selected object from depsgraph
• Calculate the area, add to total area
• Make all the calculations in native unit, can convert the result(s) when required. ie $c * a + c * b = c * (a + b)$

Test code.

import bpy
import numpy as np
from bpy import context
toggle = context.mode == 'EDIT_MESH'
if toggle:
    bpy.ops.object.editmode_toggle()
print("-" * 44)
dg = context.evaluated_depsgraph_get()
all_areas = []

for ob in context.selected_objects:
    me = dg.objects[ob.name].data
    me.transform(ob.matrix_world) # globalize coords
    data = np.empty(len(me.polygons))
    me.polygons.foreach_get("select", data)
    selection = data.astype(bool)
    me.polygons.foreach_get("area", data)
    areas = data
print(ob.name)
print("Total ob area", np.sum(areas))
print(np.count_nonzero(selection), "/", len(selection))
all_areas.append(np.sum(areas[selection]))
print("Selected area", all_areas[-1], "sq bu")
#bpy.data.meshes.remove(me)
print()


print("Total Area Selected")
print(sum(all_areas), "sq bu")
if toggle:
    bpy.ops.object.editmode_toggle()

Test Run.

Default cube copied 3 times. One scaled double, another scaled half. Each given a subsurf modifier. One face selected on each.

---

Cube
Total ob area 8.114266633987427
4 / 24
Selected area 1.3523776829242706 sq bu
DoubleCube
Total ob area 32.457067370414734
4 / 24
Selected area 5.409510254859924 sq bu
HalfCube
Total ob area 2.0285658836364746
4 / 24
Selected area 0.3380943015217781 sq bu
Total Area Selected
7.099982239305973 sq bu