VTK camera, how to get the right vector in world space

As the title says, how can I get the orientation vectors particularly the right vector of the VTK camera, what matrix can I use to extract this right vector? from documentation, i can see that there are ModelTransform, ModelViewTransform and ViewTransform matrix, Im not entirely sure which to use to extract the world space aligned right vector.

I have the option of cross product the focal point position with up vector though, but i want to extract the vector from the matrix instead for correctness.