comparing z axis of normals

Question

I would like to sort objects faces with the Python API based on their normals, and more specifically, the orientation of the Z axis of the normals. What would be the easiest way to do so? Comparing the normals directly does not work because I want to overlook the rotation about the z axis (that is to say, I am only interested in whether the face normal has its z axis parallel or perpendicular to the z axis of the world frame).

Thank you for helping

Answer 1

Here's an example that shows you what normal is parallel or perpendicular.

(I'm not sorting the list, cause you say sorting, but then only care for parallel/ perp. ?)

• check the object type and if has faces
• list normals, including obj world rotation + scale
• compare with vector.dot so that 1/-1 are parallel, 0 is perpendicular

Note the > < 0.0001 comparison rather than == cause of the float precision issues.

from mathutils import Vector

orientZ = []

if Object is not None and getattr(Object, 'type', '') == 'MESH':
    if Object.data.polygons:
        worldZ = Vector([0, 0, 1])

        mat = Object.matrix_world
        loc = mat.to_translation()
        normals = [(mat * p.normal - loc).normalized() for p in Object.data.polygons]

        # if dot is 0, is perpendicular, 1/-1 are up, down
        for n in normals:
            dotN = n.dot(worldZ)

            if   abs(dotN) > 0.9999: orientZ.append(1)
            elif abs(dotN) < 0.0001: orientZ.append(0)
            else:
                orientZ.append(-1)

and gives a list of 1, 0, -1 for Z, perp, others

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Another example is this How can I compare normals in AN with python, doing the same, by distance compare to world XYZ axes.