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A mathematical model is presented for comparing geometric and image-based simplification methods. Geometric simplification reduces the number of polygons in the virtual object and image-based simplification replaces the object with an image. Our model integrates and extrapolates existing accuracy estimates, enabling the comparison of different simplification methods in order to choose the most efficient method in a given situation. The model compares data transfer and rendering load of the methods. Byte size and expected lifetime of simplifications are calculated as a function of the desired visual quality and the position and movement of the viewer. An example result is that, in typical viewing and rendering conditions and for objects with a radius in the order of one meter, imposter techniques can be used at viewing distances above 15 meters. Below that, simplified polygon objects are required and, below one meter distance, the full-resolution virtual object has to be rendered. An electronic version of the model is available on the web.

In large ubiquitous computing environments it is hard for users to identify and activate the electronic services that match their needs. This user study compares the newly developed service matcher system with a conventional system for identifying and selecting appropriate services. The study addresses human factors issues such as usability, trust and service awareness. With the conventional system users have to browse a hierarchical list of currently available services and activate the service that they think satisfies their current needs. With the service matcher users just enter their current need using natural language, after which a wizard, emulating an existing service matcher algorithm, searches for and activates a matching service based on the given need and the users' location and gaze direction. This study shows that with the hierarchical list, only 66% of the tasks are solved correctly, and females score significantly worse than males. With the service matcher, the performance increases significantly to 84% correctly performed tasks and the gender difference disappears.

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