# [Bf-committers] Cursor to center of mass

bjornmose at gmx.net bjornmose at gmx.net
Thu Nov 22 02:46:56 CET 2012

```Did you check against  'non orientable surfaces'?
like
Möbius strip
Klein Bottle
I doubt the algorithm stands with that...
As far as I remember those objects have no defined volume?
As : how would you like  to make a cylinder from a Möbius strip?
You can always create a bounding object  .. like in triangulating the
circle ..
alas .. it will be getting close enough, which might be sufficient for
blender, and might be nice to have..

Well if blender refuses to create  such geometry .. bad enough
Mathematica and friends can.

Well I think you should check  the definition of  'closed' and require
'orientable' and it will work out fine.
so much for that
BM

one more .. your algorithm assumes homogeneous density .. how realistic
is that ? .. for all cases ?
Is a balloon filled with air the same as the balloon filled with water?
As the mass is on the surface versus the mass is in the volume ?

Am 21.11.2012 04:30, schrieb Ummi Nom:
> Hi!
>
> I've written some center of mass calculations in python for closed meshes,
> but there is an issue with using tessfaces in my code; I run mesh.update
> with tessfaces=True but when I loop over mesh.tessfaces there are only 6
> faces in default scene cube so the center of mass is offset.
>
> It seems to work when the mesh is triangulated
> http://www.pasteall.org/37396/python
>
> # Code
> import mathutils
> from mathutils import *
> import bpy
>
> tess_count = 0
>
> # Handle tri as a tetrahedron with fourth point at origo
> def handleTri(v1, v2, v3):
>      global tess_count
>      tess_count = tess_count + 1
>
>      temp = (v3-v1).cross(v2-v1)
>      nor = temp.normalized()
>      area = 0.5*temp.dot(nor)
>      vol = area*nor.dot(v1)/3.0
>      centroid = (v1+v2+v3)/4.0
>
>      return (centroid, vol)
>
>
> for me in bpy.data.meshes:
>      me.update(calc_tessface=True)
>
>      sumc= Vector()
>      summ = 0
>
>
>      for f in me.tessfaces:
>          v1 = Vector(me.vertices[f.vertices[0]].co)
>          v2 = Vector(me.vertices[f.vertices[1]].co)
>          v3 = Vector(me.vertices[f.vertices[2]].co)
>
>          centroid, mass = handleTri(v1, v2, v3)
>          sumc += centroid * mass
>          summ += mass
>
>      print(tess_count)
>
>      print(sumc/summ)
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```