kindaaa... crazy

You can construct a Transform from an abstract transformation, like this:

Transform t(AngleAxis(angle,axis));

or like this:

Transform t;

t = AngleAxis(angle,axis);

But note that unfortunately, because of how C++ works, you can not do this:

Transform t = AngleAxis(angle,axis);

Explanation: In the C++ language, this would require Transform to have a non-explicit conversion constructor from AngleAxis, but we really don't want to allow implicit casting here.

source : http://eigen.tuxfamily.org/dox-devel/group__TutorialGeometry.html