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In a character workflow with muscles and skin, this tool allows you to push skin geometry outward until it encapsulates all the given muscles.
Although this tool is designed for skin and muscles, it may be used in other contexts where a quick draping effect or a somewhat more sophisticated ray cast is desired.
Using Muscle Deform
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.
Subset of points to be deformed.
Subset of muscles to be considered for deformation.
Muscle Center Group
Subset of points at the second (or third if specified) input to be considered as projection sources. Points on the skin will be the projected away from these center points if the Project from Muscle Centers option is on.
Muscle ID Attribute
Name of the attribute specifying unique groups of muscles. Skin points being projected from centers of one muscle group will only be projected to the muscle primitives in the same group. This avoids different muscle groups affecting each other in non-convex areas of the model.
Weight attribute which acts as a global multiplier for displacements produced by this tool.
Pin Border Points
Fix the positions of points adjacent to unshared edges on the input skin geometry.
Bulge to Max Weight
Use the input capture weights to determine which muscle to project to in areas of overlap between different muscle groups. Otherwise the skin will be projected to the farthest muscle.
Attenuate with Weights
Scale the displacement of points for each muscle group by the corresponding capture weights (if any).
Only consider points with weights greater than this cutoff when computing the point displacement.
Cache Time Independent Topology
Turn this on if the geometry in the input remains topologically the same throughout the animation, for a minor performance boost.
When Cache Time Independent Topolgy is enabled, this parameter specifies the time (frame) to generate the cache. Use this parameter if you have a geometry sequence to deform that has a start frame other than frame 1.
Project from Muscle Centers
When enabled, skin points will be projected away from the Muscle Center points given on the second (or third if provided) input. This option also enables all Center Projection Control options.
This set of controls allows the user to smooth out any harsh projection artifacts making the result look more organic. Since this node does not compute output normals, ensure that normals are computed when visualizing the output (with the Normal SOP for instance) before making a judgment on how the surface looks.
Set the method for weighing neighbor contributions on position and normal smoothing options.
Smooth evenly regardless of their distance from one another.
Smooth accounting for the fact that some points may be closer than others. This option preserves relative distances between points, which is especially useful when points on the input are unevenly distributed.
The number of times the input geometry is smoothed. This works as in Delta Mush where only the displacement is smoothed, preserving initial detail in the skin mesh.
Smooth the area of skin where the skin drapes away from the peak displacement. In contrast to Global Iterations this type of smoothing will not reduce the amount of displacement.
Smooth the point normals used for projection. Note that this will have no effect if displacement is computed purely from muscle centers (when Normal Projection Mixing is set to 0).
This controls the size of each smoothing step for all types of smoothing iterations.
A constant point offset amount along the point normal.
Reduces the amount of offset amount in areas of maximum displacement.
The maximum distance to which the skin geometry will be projected.
Center Projection Control
Determines how closely the skin drapes around the muscle. Higher values make the skin drape tighter around the muscle.
Normal Projection Mixing
Mix projection direction between the normal direction (at 1) and the direction from muscle centers (at 0).
The radius away from the closest muscle center, which includes all muscle centers to be considered when picking a point to project from.
This slider controls how much distant muscle centers contribute to the computation of the point to project from. Note that only muscle centers within the Blend Radius from the closest muscle center are considered. The higher the value, the more weight is given to points closest to the closest muscle center.
Min Surf Distance
When enabled, projects all muscle centers that fall outside of the skin geometry (as determined by the normal of the closest primitive) to the interior of the skin. If a center is inside the skin but closer than the allowed distance, it will also be pushed further away from the skin.
This parameter controls how the displacements from different muscle groups are combined to produce the final displacement. When disabled, each consecutive muscle group will displace the skin geometry one after the other in some arbitrary order. When enabled, the displacement magnitudes are raised to the given exponent, added and then raised to the reciprocal of the given exponent. In effect, an exponent of 1 will simply add all the magnitudes, while a larger exponent will pick the largest of all magnitudes. This is only a heuristic to combine contributions from different muscle groups.