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The Navigator of VolVis is responsible for the interactive manipulation and display of objects within the VolVis environment. There are several basic object types used in the system, including View, Volume, Light, and World. The number and types of properties vary according to the particular applications. The View specifies the current viewing properties. Viewing parameters such as eye position, field of view, image size, and stereo viewing option can be specified interactively by the user. By manipulating the viewing parameters, the user can interactively flythrough the scene in a manner similar to a flight simulator, employing a variety of input devices, which range from the standard mouse, an Isotrak, a Spaceball, to the relatively complex DataGlove. To provide interactive navigation speed, fast-rendering algorithms capable of providing a rough estimate of the actual image have been developed, which involve projecting reduced resolution representations of the objects in the scene. The interactive navigation speed is especially important for our study because, during the interactive flythroughs, positioning the colon volume within the scene and selecting desired viewing parameters can be a complex and time-consuming process. After the satisfactory eye positions and viewing directions have been obtained, images of the colon are then generated with a more accurate and more time-consuming volumetric ray tracing technique [7], which incorporates the global illumination effects and performs the ray-object intersection calculation more accurately.