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The Animator provided by VolVis allows the user to specify transformations to be applied to objects within the scene. Unlike the Navigator, which is used to apply a single transformation at one instance, the Animator provides the user with the capability of specifying a sequence of transformations to produce an animation. Preview of the animation is achieved using one of the fast rendering options within the Navigator. Finally, a high-quality animation is created off-line with the volumetric ray tracing technique. Alternatively, using the Navigator, several key transformations representing a "flight path" of the view can be interactively specified, which can then be passed to the Animator and rendered to create a complex flythrough animation along the colon.
Even in an extensive interactive visualization system like VolVis, however, specifying a sequence of key frames to generate a high-quality flythrough animation along the colon still involves a high level of expertise and may require a lengthy trial-and-error process. Especially for computer illiterate users, this can become an extremely challenging problem, and may prohibit the wide acceptance of our proposed method. The scenario that we want to support in this research is the following. Once the colonic CT data set is scanned during the day, the physician uses the Navigator of VolVis to get a first impression of the colon. Then, with some input from the physician, a high-quality flythrough animation along the colon can be automatically generated off-line with minimum user-interaction during the evening, and the physician can thus watch the animation the next morning for further analysis.
Generally speaking, desired properties of a high-quality automatically-generated flythrough animation include: the animation should not be too jerky; the eye position should stay, if possible, in the center of the colon; and the viewing direction should point to the end of the current visible colon segment to provide the user with the feeling of what is coming next. To achieve these requirements, we have developed a technique and incorporated it into VolVis for automatic generation of flythrough animations. This technique is described in the next section.