Convert quaternion to Euler-Rodrigues vector
Determine the Euler-Rodrigues vector from the quaternion.
q = [-0.7071 0 0.7071 0] r = quat2rod( q )
q = -0.7071 0 0.7071 0 r = 0 -1.0000 0
M-by-4 array of quaternions.
its scalar number as the first column.
rod— Euler-Rodrigues vector
M-by-3 matrix containing M Euler-Rodrigues vectors.
An Euler-Rodrigues vector represents a rotation by integrating a direction cosine of a rotation axis with the tangent of half the rotation angle as follows:
are the Rodrigues parameters. Vector represents a unit vector around which the rotation is performed. Due to the tangent, the rotation vector is indeterminate when the rotation angle equals ±pi radians or ±180 deg. Values can be negative or positive.
 Dai, J.S. "Euler-Rodrigues formula variations, quaternion conjugation and intrinsic connections." Mechanism and Machine Theory, 92, 144-152. Elsevier, 2015.