Unfortunately the Euler angles are currently not exposed in the API of the rotation class. This is something we should maybe add.
In the mean time you can use the method
RotationSpace3D.rotMatrixToEulerAngles as a workaround.
Something like this should work:
val sourcePoints: IndexedSeq[Point[_3D]] = ???
val targetPoints: IndexedSeq[Point[_3D]] = ???
val rigidTransformation = LandmarkRegistration.rigid3DLandmarkRegistration(sourcePoints.zip(targetPoints), center = Point3D(0, 0, 0))
val (phi, theta, psi) = RotationSpace3D.rotMatrixToEulerAngles(rigidTransformation.rotation.rotationMatrix)