I am trying to understand the axis/angle representation.
So far, I have understood that the main idea is that every rotation can be represented as a rotation around an arbitrary axis of an angle $\theta $.
So, for example, if I have a rotation matrix $R$ I can find an axis $r$ and an angle $\theta$.
what I have not clear is the following:
Suppose I have a rotation matrix which represents the orientation of a frame, $R ^{i}_0$ for where $i$ is used to express the fact that this is an initial frame.
Now, suppose I want to change reference frame, and so I can obtain a final reference frame, for example applying Roll Pitch Yaw sequence, and so I obtain $R_{f}^{i}$, where $f$ stands for final frame. So I end up with the orientation of a final frame with respect to the initial frame.
What I don't understand is: In which frame is the unit vector around which the rotation happens expressed?