10.19.2007
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12:02
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This paper has presented a number of anatomy-based muscle modelsappropriate for simulating the behavior of skeletal muscles inhumans. Each muscle model allows the extremities of muscles tobe specified relative to different underlying bones, whether adjacentor not, and automatically adjusts the dimensions of the musclewhen the extremities are moved closer together or further apart.The models are implemented in classes with consistent interfaces,thereby creating reusable components which may be used in contexts other than in human figure modeling, such as in 3D characteranimation and the animation of other animals with endoskeletons.The muscle models manage the deformation of muscles due toisotonic contraction. These deformations are inherent in the models,completely automatic, and functionally dependent on the configuration(or pose) of the underlying articulated skeleton. To allowfor isometric muscle contraction, we introduced a tension parameterto control the ratio of a muscle’s height to its width, independentof the current pose. The muscle models take the muscle’s tension asan instance parameter and deform the muscle accordingly. By bindingthe tension of individual muscles to articulation variables, usershave complete control over the deformations of individual muscles.We used a procedural modeling language to describe all ouranatomy-based models. A language-based definition of complexhierarchical models is elegant and intuitive, and affords the creationof functional dependencies between different components. Interactivecontrol is supported through the use of articulation variables,which may be used either directly, or in expressions, to modifycomponents of the hierarchical model. Cooperating tools can bemade available to give nontechnical users interactive control overthe complex models.We adopted an approach to modeling which parallels the onetaken in the discipline of artistic anatomy. By analyzing the relationshipbetween exterior form and the structures responsible forcreating it, surface form and shape change may be understood best.We identified three general anatomical structures responsible forcreating surface form and described one of these, the musculature,in some detail. Application of knowledge of the human anatomy tothe development of human figure models is necessary if we hope toachieve a high degree of realism.We are currently investigating anatomy-based models for generatingskin surfaces based on the influence of underlying deformablestructures. The capability of implicit functions to blend individualprimitives together is exploited in the generation of surfaces torepresent the skin. Initial results look promising.Implicit versions of the simple geometric modeling primitives areused to adjust the control points of bicubic patch meshes representingthe skin. This technique also allows us to model fatty tissuebetween the muscles and the skin—adjusting the radius of influenceof the implicit functions allows different thicknesses of fattytissue deposits to be modeled.Future research could analyze the structure and function of musclesfurther to enable a more automated approach to their creationthan the one used here. If the origin, insertion, volume, and generalshape of a muscle could be determined heuristically, perhapsbased on the type of joint(s) being acted upon, or the desired actionof the muscle, the creation of human figure models may be greatlysimplified. Used in conjunction with a method for generating articulatedskeletons automatically, this approach has great potential increating new or fictional articulated figures for 3D animation applications.
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