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for controlling rigid motion, but they don't help much when it comes to creating muscle bulges or the fluid motion of skin moving over muscles. CAT has solutions for these cases also in the form of two different types of muscles. Muscle strands are composed of a set of spheres that squash and stretch when their end points are linked to bones. The second muscle type is a series of rectangular panels that flow and move together.

Both muscle strands and CAT muscle objects are created using their respective buttons in the CAT Objects subcategory of the Helpers panel in the Command Panel.

Note

Generally, muscle strands are used on the arms and legs, and CAT muscles are used for the abdomen, chest, and shoulders. •

Using muscle strands

Muscle strand objects are created by clicking in the viewport to place control handles. A single muscle strand has four unique handles, divided into two sets. Click and drag to create the first set and repeat to create the second set. The first and last handles mark the end points of the muscle strand, and the middle two handles can be moved to affect the position of the center line of the muscle. The handles can be made visible or hidden, or you can change their size using the settings in the Muscle Strand rollout.

Using the options in the Muscle Strand rollout, you can set the muscle to replace an existing bone in the rig with the Bones option, or you can include the muscle strand with the bone and use the Skin Wrap modifier to drape the skin mesh over the muscle with the Mesh option.

The Muscle Strand rollout also includes buttons for copying and pasting properties between muscles and for setting the muscle as a left, middle, or right side of the body. You also can set a mirror axis for the muscle.

The shape of the muscle is determined by the number of spheres that gradually get larger and then smaller again as you traverse the muscle length. You can change the number of spheres included in the muscle or set the size of each individual muscle sphere. The Show Profile Curve button displays the profile curve for the muscle in a window that you can edit.

Once the muscle is in place, you can link its end points to the nearby bones using the standard Link tool found on the main toolbar. Within linked end points, the muscle compresses and stretches as the bones are moved. You also can enable the Squash/Stretch feature in the Muscle Strand rollout to add this behavior to the muscle. The midpoint for the squash and stretch is set using the Set Relaxed State button.

Figure 39.16 shows the Ape rig preset with two muscle strands inserts for the rig's upper arm bones. Notice how the right arm bulges when the forearm is brought forward.

FIGURE 39.16

Muscle strands provide the bulging effect of muscles.

Using CAT muscles

CAT muscle objects are composed of sheets of rectangles that move and slide relative to each other. They are created by dragging out a rectangle area much like a plane object. Each corner of the CAT muscle has a handle, and several interior handles are also available for deforming the muscle. You can hide the corner handles or the middle handles, or you can set their size using the settings in the Command Panel.

Just like muscle strands, you can specify that the CAT muscles are either Bones or Mesh objects. The resolution of the muscle plane is set using the U and V Segs, which can be increased for greater resolution. Once the CAT muscle is in place, you can link its corners to the rig bones, and the plane deforms as the attached bones are moved.

CAT muscles include another really nice feature: collision detection. This lets you place objects underneath the CAT muscle and add them to a list of objects for collision detection. The result is that the muscle is molded to fit the underlying object. The collision object can be any standard object or even a muscle strand object. Figure 39.17 shows a CAT muscle that is being molded by an elongated sphere. For each collision object, you also can set the Hardness and Distortion values.

FIGURE 39.17

CAT muscles provide a way