The Virtual Hand Lab (VHL) is an augmented reality environment for conducting experiments in human perception and motor performance that involve grasping, manipulation, and other complex 3D tasks that people perform with their hands. Our system supports a wide range of experiments and is used by (non-programmer) experimenters. Our system co-locates the hand and the manipulated objects (whether physical or virtual) in the same visual space. Spatial and temporal accuracy are maintained by using a high precision tracker and an efficient implementation of the software architecture which carefully synchronizes the timing equipment and software. There are many issues which influence architectural design; two of which are modularization and performance. We balance these concerns by creating a layered set of modules upon which the application is built and an animation control loop which cuts across module boundaries to control the timing of equipment and application. Augmented objects are composed of both physical and graphical components. The graphical object inheritance hierarchy has several unusual features. First, we provide a mechanism to decouple the movement of the graphical component of an augmented object from its physical object counterpart. This provides flexibility in the types of experiments supported by the testbed. Second, we create subclasses based on properties of the physical component of the augmented objects before creating subclasses based on the virtual components. Specifically, we categorize physical objects as either rigid or flexible based on their level of deformation. This allows us to efficiently implement many of the manipulation techniques. Third, after subclasses based on the physical objects have been created, the implementation of concrete virtual object classes is driven by the goal of creating an easy interface for the experimenters. This was based on our user centered design approach.
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