]]>Thanks

]]>depends on what you use the quaternions for. quaternions with only imaginary part can be used to represent vectors, and then, quaternion addition results in vector concatenation, a.k.a. translation.

]]>I believe quaternion addition is similar to vector space translation (in 4D), but have no geometric meaning.

]]>I know that if q1 and q2 indicate rotations, then q2q1

represents the composite rotation of q1 followed by q2.but what about addition?