# pytransform3d.rotations.compact_axis_angle¶

pytransform3d.rotations.compact_axis_angle(a)[source]

Compute 3-dimensional axis-angle from a 4-dimensional one.

In a 3-dimensional axis-angle, the 4th dimension (the rotation) is represented by the norm of the rotation axis vector, which means we transform to .

We usually assume active rotations.

Parameters
aarray-like, shape (4,)

Axis of rotation and rotation angle: (x, y, z, angle).

Returns
aarray-like, shape (3,)

Axis of rotation and rotation angle: angle * (x, y, z) (compact representation).