Getting face normals from python?

Coding Python Support
1

I’m looking for to output per face normals for Blender models but from
looking at the Python docs Blender doesn’t seem to have the normal coordinates
stored in the faces…??
Am I missing something here, where exactly is Blender getting
the normals for flat shading if they’re not stored in the faces. It’s hardly using
a per frame calculation based on vertex normals… or is it?

2

well, it isn’t based on vertex normals
but they are recalculated at least every frame.

there have been other people asking how to get them, look for their threads.

3

Ok z3r0 d maybe you can anwser this question that has mystified me for a while. How can a vertex have a normal. I mean its just a point in space. If I remember from my high school geometry class, a normal is the vector that is perpendicular to a plane. Well I vertex is just a point how can it have a normal??

4

Normal vectors are (re)calculated every time they are needed.
But it is not so difficult to calculate the normal of a face if you know the coordinates of the face.

# vector substration
def vecsub(a, b):
    return [a[0] - b[0], a[1] - b[1], a[2] - b[2]]

# vector crossproduct
def veccross(x, y):
    v = [0, 0, 0]
    v[0] = x[1]*y[2] - x[2]*y[1]
    v[1] = x[2]*y[0] - x[0]*y[2]
    v[2] = x[0]*y[1] - x[1]*y[0]
    return v

# calculate normal from 3 verts
def Normal(v0, v1, v2):
    return veccross(vecsub(v0, v1),vecsub(v0, v2))

That goes good for triangular faces, but if the face is not coplanar (could happen with quads), use the following (found in the blender sourcecode):

# calculate normal from 4 verts
# found in the blender sources in arithb.c 
# (bl-blender\blender\source\blender\blenlib\intern\arithb.c)
def Normal4(v0, v1, v2, v3):
    return veccross(vecsub(v0, v2),vecsub(v1, v3))
5

Thank you very much for this tuinbels. I’m working (when I have free time) on a Select/Paint-Gradient script,
that will either:
allow me to select faces with a normal facing in a particular direction (with some room for error);
or paint it, either with a material, or vertex painting.
The plan is that I can add e.g. snow materials to the secondary valleys of a mountain ridge.

I spent quite some time working out some truely horrendous code to calculate the weighted average of a face normal from the vector normals. Now I just have to work out how to seperate valleys, saddle-points and peaks. sighs