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The deformations of the surfaces within a deformable modeling mesh are coupled at run time by the addition of a set of link constraints. A link constraint is a form of the general curve constraint.
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Interestingly, the two surfaces can be made to be tangent-continuous across a link constraint without being position-continuous by omitting the constraint on position while using the cross-tangent constraint. The behavior of the system under these circumstances is inappropriate for modeling manifold objects, but does make an interesting modeling effect for systems which need to crack and split, like modeling sub-surface structures that show separations due to faults.
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The multi-surface deformable modeling package allows the end user to specify the behavior of the link constraint at run time. The behavior of the link constraint describes how the position and the cross-tangent properties of the two curves within the link constraint are related to one another. For position or cross-tangent properties, each curve may have one of three behaviors: linked, fixed, or open.
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A linked behavior means that a property of the curve on one side of the link constraint is equal to the same property of the curve on the other side of the link constraint. The link constraint equations shown in the last section describe the linked behavior of a link constraint. A fixed behavior means that the position or the cross-tangent property of a curve in a link constraint is constrained. Rather than being free to move so that it can be connected to the other curve, the shape of the curve is fixed and is not modified by subsequent deformations. An open behavior means that a property of one of the curves in the link constraint is not constrained at all. It is free to change as the mesh is deformed but it does not necessarily move to stay connected to the other curve in the mesh.
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The overall behavior of the link constraint is determined by the behaviors assigned to each property on each curve. The following table shows the link's behavior given the various possible behavior states for each of the link's curves. One such table exists for each property that the link constraint controls. The behavior state for each curve in the link constraint can be modified at run time.
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Table 1-1. Link Behavior for Combined Link Curve Behaviors
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