A relevant issue for any Virtual Environment (VE) is the navigational support provided to users who are exploring it. Semitransparency is sometimes exploited as a means to see through occluding surfaces with the aim of improving user navigation abilities and awareness of the VE structure. Designers who make this choice assume that it is useful, especially in the case of VEs with many levels of occluding surfaces, e.g. virtual buildings or cities. This paper is devoted to investigate this assumption with a proper experimental evaluation on users. First, we discuss possible ways for improving navigation, and focus on implementation choices for semitransparency as a navigation aid. Then, we present and discuss the experimental evaluation we carried out. We compared subjects' performance in three conditions: local exploitation of semitransparency inside the VE, a more global exploitation provided by a bird's-eye-view, and a control condition where neither of the two features was available.
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