Various techniques for object selection in virtual environments have been proposed over the years. Among them, the virtual pointer or ray-casting is one of the most popular method for object selection because it is easy and intuitive to use and allows the user to select objects that are far away. Variants of the virtual pointer metaphor include the Aperture [5], Flashlight [3], and Image plane method [fl as categorized as such by [10]. In a monoscopic environment, these methods are essentially 2D interaction techniques, as the selection is made effectively on the image plane. Such a 2D based selection (or more generally, interaction) method has an added advantage in that it can find many good uses in 3D environments ranging from a simple 2D oriented subtask (object selection on a constrained surface, menu selection) to a siutation where a whole 2D application (e.g. sketching tool, desktop manager) is embedded in the 3D environment. In this paper, we experimentally compare the performance of four different virtual pointer implementations, namely, the direct image plane selection, head-directed pointer, hand directed pointer and head-hand directed pointer. The experimental results revealed that the direct image plane selection produced the best performance among the four in terms ofboth task completion time and the pixel-level pointing error.

The development and maintenance of a virtual reality (VR) system requires indepth knowledge and understanding in many different disciplines. Three major features that distinguish VR systems are real-time performance while maintaining acceptable realism and presence, objects with two clearly distinct yet inter-related aspects like geometry/structure and function/behavior, and the still experimental nature of multi-modal interaction design. Until now, little attention has been paid to methods and tools for the structured development of VR software that addresses these features. Many VR application development projects proceed by modeling needed objects on conventional CAD systems, then programming the system using simulation packages. Usually, these activities are carried out without much planning, which may be acceptable for only small-scale or noncritical demonstration systems. However, for VR to be taken seriously as a media technology, a structural approach to developing VR applications is required for the construction of large-scale VR worlds, and this will undoubtedly involve and require complex resource management, abstractions for basic system/object functionalities and interaction tasks, and integration and easy plug-ins of different input and output methods. In this paper, we assembled a comprehensive structured methodology for building VR systems, called CLEVR (Concurrent and LEvel by Level Development of VR System), which combines several conventional and new concepts. For instance, we employ concepts such as the simultaneous consideration of form, function, and behavior, hierarchical modeling and top-down creation of LODs (levels of detail), incremental execution and performance tuning, user task and interaction modeling, and compositional reuse of VR objects. The basic underlying modeling approach is to design VR objects (and the scenes they compose) hierarchically and incrementally, considering their realism, presence, behavioral correctness, performance, and even usability in a spiral manner. To support this modeling strategy, we developed a collection of computeraided tools called P-VoT (POSTECH-Virtual reality system development Tool). We demonstrate our approach by illustrating a step-by-step design of a virtual ship simulator using the CLEVR/P-VoT, and demonstrate the effectiveness of our method in terms of the quality (performance and correctness) of the resulting software and reduced effort in its development and maintenance.

For real-time performance, virtual reality systems often employ various performance optimization techniques. One of the most popular methods is using geometric models with different "Levels of Detail (LOD)". In a previous paper by this author [l], we have proposed to use software engineering principles such as the concept of hierarchical and incremental modeling, and simultaneous consideration of form, function and behavior for modeling VR objects. Each refinement stage driven by such a modeling philosophy produces step-by-step form, function and behavior specifications of a VR object. We can make good use of these by-products as LOD for adaptive display and simulation by additionally specifying conditions for LOD switching. These specifications can be simulated and analyzed in advance for an estimation of performance for a given VR execution environment. Such an engineering process deals with behavior and geometry together; different geometric LOD may possess different behaviors and vice versa. A certain function or behavior might dictate an inclusion of a particular geometric feature that may not be possible to preserve, if the geometric LOD were to created in a bottom-up fashion (e.g. using the mesh simplification algorithms). We demonstrate our approach by modeling an automobile object with three levels of geometric and behavior detail in a top-down manner, simulate their instances in a small virtual town, and based on the simulation result, make predictions to the maximum allowable number of vehicles that will maintain an acceptable frame rate if executed in a faster simulation environment. We believe that our approach combines the idea of hierarchical refinement of virtual objects and the use of LOD in a very natural and intuitive manner.

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