Select the skin geometry you would like to deform or displace. This will be set in the Group parameter. Select the muscles you would like to affect the deformation in the Muscle Group parameter. The given skin geometry will be projected away from any points provided in the input muscle geometry belonging to the Muscle Center Group until they hit the surface of the muscle. This projection can be controlled with a number of parameters given below.

Sometimes you may want to project the skin geometry to the muscles using the normal direction on the skin geometry (these normals will be computed inside Muscle Deform so no need to pre-compute them ahead of time). To achieve this, simply turn off the Project from Muscle Centers toggle, or alternatively slide the Normal Projection Mixing to 1 if you still prefer to use the Center Projection Control options.

Performance may be improved at the cost of quality and limited control. Turning off the Project from Muscle Centers toggle will improve performance significantly, although it may not produce desirable results for non-convex skin geometries. Setting Blend Radius to zero will achieve a similar performance increase however the quality of the projection will heavily rely on the placement and frequency of muscle centers throughout the animation. If some center points fall outside of the skin geometry, disturbing artifacts will dominate the resulting mesh. Increasing Blend Radius will help eliminate these artifacts by considering neighboring point centers when computing the position to project the skin from. Furthermore, the Min Surf Distance parameter will project muscle centers back into the skin geometry if they happen to come within the specified distance or if they protrude through the skin. However these tools are often non sufficient for providing truly artifact free result, so care must always be taken when placing muscle center points, ensuring they fall well inside the simulated skin.

In cases where the skin geometry is concave, as is the case with most character models, extra care must be taken when using this tool. More specifically, the projection rays used to place the skin geometry over the given muscles may hit a muscle belonging to a different part of the character resulting in disturbing artifacts. To avoid these issues you must group different muscle groups under a unique ID attribute and specify this attribute in the Muscle ID Attribute parameter. Note that this attribute must exist on muscle primitives as well as muscle centers. This allows Muscle Deform to consider each muscle group individually, and combine the results of each projection at a later stage. To control displacement where different muscle groups overlap, use the Overlap Exponent parameter.

This SOP may also be used with the Muscle Capture SOP, which assigns weights to individual muscle groups (specified by the unique Muscle ID Attribute you provide). Muscle capture weights can be used to control where the displacement should occur. If Attenuate with Weights is turned on, the capture weights will act as a multiplier for the resulting displacement. For more details see the documentation for Muscle Capture.