face.center in world coordinates

Coding Python Support
1

I want to create a point light for each face of a mesh with a specific material.

My first attempt looked something like this:

for face in obj.data.faces:
    mat = bpy.data.materials[face.material_index]
    if mat.name == 'Lamp'
        bpy.ops.object.lamp_add(location=face.center)

Besides the fact that the script did not create any point lights, the locations all would have been wrong anyway (i guess face.center is in local coordinates?). I actually found a small article about getting the world coordinates of a vertex on a bezier curve, and tried the proposed solution in my script:

wmtx = obj.matrix_world

for face in obj.data.faces:
    mat = bpy.data.materials[face.material_index]
    if mat.name == 'Lamp'
        loc = face.center * wmtx
        bpy.ops.object.lamp_add(location=loc)

Yet the calculated locations are still nowhere near any face of my mesh :confused:

2

Can you toss us your example .blend with the code you have so far (and instructions if the undesired/desired effects are not obvious)?
-rking

3

hi, try matrix * vector, I mean: loc = wmtx * face.center

edit: and for the script I think you are mixing indices and names…
this should do the trick:


import bpy
obj = bpy.context.object

i = obj.material_slots.find('Lamp')
if i != -1:

    wmtx = obj.matrix_world
    for face in obj.data.faces:
        if face.material_index == i:
            loc = wmtx * face.center
            bpy.ops.object.lamp_add(location=loc)
4

It works! :cool:

my code now looks like this:

for obj in bpy.data.objects:
    if obj.type == 'MESH':
        obj.name = obj.data.name
        wmtx = mathutils.Matrix(obj.matrix_world)
        wmtx.invert()
        
        face_list = [f for f in obj.data.faces if obj.material_slots[f.material_index].name == 'Torch']
        
        for face in face_list:
            norm = mathutils.Vector(face.normal)
            if norm.dot((0,0,1)) == 1:
                bpy.ops.object.add(type='LAMP',location=face.center*wmtx)

There were actually two problems:

  1. it seems that the world matrix needs to be inverted (i guess that’s because my objects are rotated around the Z axis!?)

  2. bpy.data.materials is not in the same order as obj.material_slots, so my script always selected faces with the wrong material. I also tried “material_slots.find()” but blender doesn’t know that method, so i came up with the solution above.

There is probably a much easier way to do this, but since those are my very first steps in python i’m just happy that it works.

5

ohoh, in the bmesh blender (the new one!) the obj.data.faces is empty( for the default cube) there are loops etc.
meaning the Api has changed considerably!!!
Maybe like this: obj.data.polygons[0].normal where the polygon replaces the normals?? At least the command works, and the cube
has 6 polygons … looks good the normal-vectors are indeed (rounding errors!) the six different normals :wink:

By the way, I would write wmtx = mathutils.Matrix(obj.matrix_world)
wmtx = obj.matrix_world.copy()
using the copy
and in old Blender face.normal is already a Vector so you could write at once
face.normal.dot((0,0,1))
:wink: