Mirroring of a Face does not changes its Normal?

[jannisEspelage]

Hi all,

i create a face with a normal pointing in positive z direction. Then i mirror this face by the XY plane. The expected outcome would be that the normal now points in negative z direction. However this is not the case, the normal seems to be invariant for the mirroring.

If i use an arbitrarily rotated face and mirror it at the XY plane the sign of the z-element of the normal vector switches (as expected).
Am i doing something wrong?

import cadquery as cq

dx = 5.0
dy = 3.0
dz = 3.0

rect = cq.Face.makeFromWires(cq.Workplane().rect(dx, dy).wires().vals()[0]).translate((0,0,dz))

fmt="{:.4f}, {:.4f}, {:.4f}"
print("Normal:", fmt.format(*rect.normalAt().toTuple()))
print("Centre:", fmt.format(*rect.Center().toTuple()))

# mirror at XY
rect_mir = rect.mirror("XY")

print()
print("Normal (mirrored):", fmt.format(*rect_mir.normalAt().toTuple()))
print("Centre (mirrored):", fmt.format(*rect_mir.Center().toTuple()))

# arbitrarily rotate face before mirror
rect_rot = rect.rotate((0,0,0),(1,1,1),45)

print()
print("Normal (rotated):", fmt.format(*rect_rot.normalAt().toTuple()))
print("Centre (rotated):", fmt.format(*rect_rot.Center().toTuple()))

# mirror at XY
rect_rot_mir = rect_rot.mirror("XY")

print()
print("Normal (rotated & mirrored):", fmt.format(*rect_rot_mir.normalAt().toTuple()))
print("Centre (rotated & mirrored):", fmt.format(*rect_rot_mir.Center().toTuple()))

Output

Normal: 0.0000, 0.0000, 1.0000
Centre: -0.0000, 0.0000, 3.0000

Normal (mirrored): 0.0000, 0.0000, 1.0000
Centre (mirrored): -0.0000, 0.0000, -3.0000

Normal (rotated): 0.5059, -0.3106, 0.8047
Centre (rotated): 1.5176, -0.9319, 2.4142

Normal (rotated & mirrored): -0.5059, 0.3106, 0.8047
Centre (rotated & mirrored): 1.5176, -0.9319, -2.4142