  Transparency is a key factor in improving the performance of human-robot
interaction. A transparent interface allows humans to be aware of the state of
a robot and to assess the progress of the tasks at hand. When multi-robot
systems are involved, transparency is an even greater challenge, due to the
larger number of variables affecting the behavior of the robots as a whole.
Significant effort has been devoted to studying transparency when single
operators interact with multiple robots. However, studies on transparency that
focus on multiple human operators interacting with a multi-robot systems are
limited. This paper aims to fill this gap by presenting a human-swarm
interaction interface with graphical elements that can be enabled and disabled.
Through this interface, we study which graphical elements are contribute to
transparency by comparing four "transparency modes": (i) no transparency (no
operator receives information from the robots), (ii) central transparency (the
operators receive information only relevant to their personal task), (iii)
peripheral transparency (the operators share information on each others'
tasks), and (iv) mixed transparency (both central and peripheral). We report
the results in terms of awareness, trust, and workload of a user study
involving 18 participants engaged in a complex multi-robot task.
