How to select concave quads

Question

how can i select all concave quads, is there such a tool?

Answer 1

This may be a naive answer, I'm no coder, but is there anything wrong with letting BMLoop.is_convex do this for you?

(Starting from Edit Mode):

import bpy
import bmesh
obj = bpy.context.edit_object
me = obj.data
bm = bmesh.from_edit_mesh(me)
bm.faces.active = None
for face in bm.faces:
    face.select_set(False)
    for loop in face.loops:
        if not loop.is_convex:
            face.select_set(True)
            break
bmesh.update_edit_mesh(me, False)

Answer 2

ok, so I made a modification of this script: Face edges angle - python

I simply putted some selection commands at the end. it will do the work (you need to select the faces you want to check)

# based on the script by batFINGER: https://blender.stackexchange.com/questions/203249/face-edges-angle-python
import bpy
from mathutils import Matrix, Vector
from bpy import context
from math import degrees, atan2, pi
import bmesh
# project into XY plane, 
up = Vector((0, 0, 1))
ob = context.object
me = ob.data
bm = bmesh.from_edit_mesh(me)
def edge_angle(e1, e2, face_normal):
    b = set(e1.verts).intersection(e2.verts).pop()
    a = e1.other_vert(b).co - b.co
    c = e2.other_vert(b).co - b.co
    a.negate()

    axis = a.cross(c).normalized()
    if axis.length < 1e-5:
        return pi # inline vert
if axis.dot(face_normal) &lt; 0:
    axis.negate()
M = axis.rotation_difference(up).to_matrix().to_4x4()  

a = (M @ a).xy.normalized()
c = (M @ c).xy.normalized()

return pi - atan2(a.cross(c), a.dot(c))


selected_faces = [f for f in bm.faces if f.select]
bpy.ops.mesh.select_all(action='TOGGLE')
for f in selected_faces:
    edges = f.edges[:]
    #print("Face", f.index, "Edges:", [e.index for e in edges])
    edges.append(f.edges[0])
for e1, e2 in zip(edges, edges[1:]):

    angle = edge_angle(e1, e2, f.normal)

    #print(&quot;Edge Corner&quot;, e1.index, e2.index, &quot;Angle:&quot;, degrees(angle))
    if degrees(angle) &gt;= 180:
        f.select_set(True)

Answer 3

I see I'm late, though I ended up with simpler code:

import bpy
tolerance = 0  # increase to something like .01 or .1 to ignore small concavities
bpy.ops.object.mode_set(mode = 'EDIT') 
bpy.ops.mesh.select_mode(type="FACE")
bpy.ops.mesh.select_all(action = 'DESELECT')
bpy.ops.object.mode_set(mode = 'OBJECT')
me = bpy.context.active_object.data
me.calc_loop_triangles()
poly_i_to_tris = {}
for tri in me.loop_triangles:
    poly_i_to_tris.setdefault(tri.polygon_index, []).append(tri)
for i, tris in poly_i_to_tris.items():
    try:
        t1, t2 = tris
    except ValueError:
        continue  # not a quad
    dist = (t1.center - t2.center).length
    tolerance = dist  # makes tolerance relative
    ray_len = dist/2  # making sure to not overshoot, but I don't think it would be possible
    test1 = t1.center + t1.normalray_len
    test2 = t2.center + t2.normal*ray_len
    test_dist = (test1 - test2).length
    if test_dist < dist - tolerance:
        # concave
        me.polygons[i].select = True
bpy.ops.object.mode_set(mode = 'EDIT')