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Technical Perspective: Motion Fields For Interactive Character Animation


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Computer graphics has long sought to create realistic depictions of humans, both in appearance and in their movement. A good starting point for animation has been to begin by collecting data from the real world about how people move. The use of prerecorded motion clips has been a standard building block for producing the motion of virtual characters for use in film, games, and interactive simulations. Of course, it is impossible to capture every motion a person might perform, and so animation methods attempt to do as much as possible with the limited movements that one might be able to record using either a motion capture setup or handmade animation.

With motion clips in hand, the basic idea for creating new motions is simple: play sequences of the prerecorded clips while being careful to only switch to a new clip when this can be done without causing any noticeable artifacts. Smoothly blending between motions during the transition period can further help to increase the number of feasible transitions between clips. For interactive settings such as games and simulations, control can be added by making decisions about which clip to play next based on the user's goals, such as a particular desired walking direction or speed. A set of motion clips and their feasible clip transitions naturally form a "motion graph," which is a directed graph of motions with demarcated transition points where motions can join or split off. Modern game engines exploit various extensions of these ideas, such as the further use of blending to interpolate between motions, for example, walks of different speeds or turn rates, and the use of independent motion clips for the upper body and lower body when this can be done without introducing artifacts. However, the ongoing motion is still constructed from an underlying set of motions that are following their preset paths.


 

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