When working with the Muscles & Tissue system, your character will progress through three separate simulation passes leading to the final deformation of its renderable surface. At the end of each simulation pass, you will have the opportunity to save a geometry cache to disk before incorporating it into subsequent passes. The simulation passes are as follows: the muscles pass, the tissue pass, and the skin pass. The final stage of the Muscles & Tissue simulation creation process does not involve a simulation pass, but rather it requires that you use a geometry deformer to transfer your character’s final simulation to its high-resolution renderable mesh.

Each simulation pass does the following:

• Operates on incoming solid softbody dynamic objects (objects comprised of tetrahedrons) and interprets attributes found on those objects to configure their physical properties and Vellum constraints. You can consider the majority of the Muscle & Tissue workflow as attribute manipulation that ultimately drives the constraint creation within the solvers.

• Uses the output of the previous pass as its input so that it has animated static geometry to constrain to and to collide with. For example, in the first simulation pass—the muscles pass—the constraint geometry is supplied by the animated anatomical bones. This bone animation can be derived from a number of sources like a KineFX animation or an Alembic cache. It doesn’t matter how the bone animation was generated. The only requirement is that a static t-pose of the bones is available in conjunction with the animated bones. All muscle and tissue surfaces should always be created relative to their bones' static t-pose.

As you move your character through each of these passes, you will encounter three major concepts for the Muscle & Tissue system:

• Simulation geometry

• Attachments

• Forces