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Sunday, November 23, 2008

Maya Muscle System Basics

Maya’s Muscle Solutions are very versatile and are comprised of three main Components (Figure 4). These components are the CAPSULE, Geometry used as Bone, and Muscles (normally NURBS surfaces). Each of these components can be used separately to deform geometry or in combination with one another.

The Capsule is used to incorporate base level bone type of interaction replaces Maya’s default bone systems. Capsules control simple sliding from the capsules shape translated to the skin geometry by sliding only. Conveniently traditional Maya bone rigs can be converted directly into Capsules which carries over traditional IK/ FK functionality for animation. The use of IK relationships can’t be directly incorporated into Capsule without this conversion. A Displacement distance value known as “Fat” can be adjusted to allow for larger sliding changes to be made from the capsules location.

Geometry used as bone can replace the Capsule by using actual geometry to affect the sliding of skin. This will be of extreme value to an anatomically correct facial rig because there are only two actual moving bones in the face; the skull, and the lower jaw bone. By creating geometry that is dimensionally correct to these bones it will be possible to use geometry as the Bone objects instead of Capsules which cannot take the shape of complex geometry.

Finally the Muscle Object in Maya is extremely powerful and versatile tool that can interact with capsule or bone geometry to perform, sliding, sticking, bulging, and reactions to gravity and motion to achieve highly realistic and predictable movements below the skin geometry. The motion and bulging of muscles is determined by constraining locators at each end of the muscle to positions on Capsules or bone geometry and then morphing the muscle shape between different angles of bone movements. To determine how the skin geometry reacts to the muscle specifically, one can paint vertices to respond to the underlying muscle and bone. Attributes that one can paint into each vertex are jiggle, sliding, sticking, among many others (See Figure 5). With these three components in the Muscle Toolset a fully interacting and complex muscle system can be achieved.

Thankfully the toolset allows for “Smart Collisions” both between Muscles and Capsule/Bones, but also with the Skin and the muscles. This huge because the muscles of the face in particular overlap and intersect each other. So if one to were picture the face below the skin, the layers interacting with each other are Skull and Jaw bone, muscles attached both to the bone and to each other, fat incorporated with the bone and muscle for distance displacement between bone and skin, and then the skin polygonal geometry interacting with everything below it. That interaction is decided by the various attributes which are controlled on a 0 to 1 scale by painting the values directly onto the geometry. All of these relationships then react to the movement of the bones driven by the animator and also the gravity in the scene. Additionally, once the animation has become acceptable and the settings for the geometry reactions are satisfactory, one can “Bake” the vertex movements into a separate .txt file to speed up viewport playback and rendering. Baking references external data to speed up calculations at render and during viewport playback because the software no longer needs to recalculate each vertex position by its painted weights, muscle reaction, or bone driven animation.

All of these tools make for a very high set up time, but more importantly a toolset that is versatile enough to create an anatomically correct muscle driven rig.

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