Virtual environments involve the user in an interactive three- dimensional computer generated environment. The methods of interaction typically involve direct manipulation of virtual objects via three-dimensional trackers. The tracking signal may be degraded in various ways, impacting the ability of the user to perform various tasks. This presentation will address the impact of two types of degradation in the tracking signal, lag (transport delay) and low frame rate. These degradations are common in existing virtual reality systems. While the impact of lag on human performance is comparatively well studied, the impact of low frame rate has not been widely studied. The impact of lag and low frame rate on two tasks will be compared and studied: Pursuit tracking and placing. The tasks will be studied in a two-dimensional context, eliminating ambiguities due to three-dimensional perception and display. Simple conclusions will be drawn that can serve as guidelines for developers designing interactive virtual environments. The relationship between these conclusions and theories of human performance will be briefly addressed.110
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