Wide muscles with complex shapes can not be modeled with thesame ease as straight fusiform muscles. Although one could usemultiple instances of fusiform muscles to approximate the shape ofa complex muscle, a better alternative would be to use a generativeapproach in which any number of muscle bellies may be positionedautomatically. The multi-belly muscle model accomplishesthis task. In order to locate and orient a number of muscle bellies automatically,we need to define the origin and insertion of the muscle tobe represented. Spline curves provide a convenient alternativeto merely enumerating the individual origin and insertion points.Relatively few control points are needed to define these curves, andby using a parametric formulation of the spline curve, points alongthe curve can be sampled simply and efficiently. Thus, instead oforigin and insertion points, the multi-belly muscle model requiresthat origin and insertion curves be specified. Locating each muscle involves finding two points of attachment on each curve for everymuscle belly, a task easily accomplished by sampling the curvesand pairing-off corresponding sample points. Orientation of individualmuscle bellies requires finding a reference vector to indicatethe ‘up-direction’ of a muscle belly. As illustrated in Figure 8, thereference vector for each pair of points (oj ; ij ) is the normal vectorof the plane through three sample points,The origin of each multi-belly muscle model resemblesthat of the fusiform muscle model. The origin of each multi-bellymuscle is represented by a list of control points defining the origincurve. Another list defines the insertion curve in a similar way. Asbefore, the origin and insertion curves may be defined in whicheverlocal coordinate system necessary; the class transforms the controlpoints (and hence, the curves) into world coordinates prior to storingthem. By default, ten muscle bellies are created between theorigin and insertion curves. This default behavior can be changed by specifying a different belly count before instantiating the muscle.
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