Subtract quaternions in python?

Question

how do you subtract two quaternions, in blender scripting?

I've tried allot of things and none of them work, quaternion1 * quaternion2.inverted() does not work inverted gives some random answer, instead of the reciprocal as I had expected

On a similar note, how do you get the inverse translation by a matrix? so Vector / quaterion @ matrix would translate it by the matrix, but again Vector @ matrix.inverted() does not give the right answer

i've been trying to get this to work for two days and i'm really frustrated by blender python right now

Answer 1

Rotation Difference.

Believe what you are looking for is rotation_difference ie the minimum rotation required to rotate from one quaternion to another

Python console code

Define a couple of angles using Eulers,

>>> r1 = Euler((radians(30), radians(60), radians(45)))
>>> r2 = Euler((radians(0), radians(60), radians(45)))

As quaternions

>>> q1 = r1.to_quaternion()
>>> q2 = r2.to_quaternion()

The difference

>>> qd = q1.rotation_difference(q2)


>>> list(map(degrees, qd.to_euler()))
[-30.000000834826057, 2.481181138556882e-06, -3.316504514323493e-06]

Or other way

>>> qd = q2.rotation_difference(q1)
>>> list(map(degrees, qd.to_euler()))
[30.00000424992064, -3.4388197927083468e-06, 1.2883001108333913e-06]

And using this result to get back to q1 from q2

>>> qr = q2 @ qd
>>> qr
Quaternion((0.8223630785942078, 0.022260043770074844, 0.5319756269454956, 0.20056211948394775))

>>> q1
Quaternion((0.8223631381988525, 0.022260019555687904, 0.5319756865501404, 0.20056211948394775))

difference (within precision) is pretty much Identity Quat (1, 0, 0, 0)

>>> qr.rotation_difference(q1)
Quaternion((1.0, -1.4901161193847656e-08, 3.4458935260772705e-08, -2.8870999813079834e-08))