Understanding Physique

Physique is a modifier that when applied to a mesh, allows the movements of an underlying skeleton to seamlessly move the mesh like bones and muscle under a human skin. Physique will work on any point-based objects including geometric primitives, editable meshes, patch-based objects, NURBS, and even FFD space warps. For NURBS and FFDs, physique deforms the control points, which in turn deform the model. It will attach to any skeleton structure including a biped, 3DS MAX bones, splines, or any 3DS MAX hierarchy. When you apply Physique to the skin object(s) and attach it to the skeleton, Physique determines how each component of the skeleton influences each vertex of the skin based on settings you specify.

Physique affects a mesh after the Attach to Node command on the Physique rollout is used and a root node is selected in the viewports. During the attach process, Physique works its way through all of the children in a hierarchy starting at the object you select and creates its own links with associated envelopes for each link it finds. The links created by Physique are referred to within this documentation as the Physique deformation spline. Vertices that fall within envelopes are influenced to follow the links and animate the mesh. Splines and 3DS MAX bones can also be added using the Add command in the Physique Bones rollout.

Biped and Physique

When the Physique Attach to Node command is turned on and the biped pelvis is selected in the viewports, Physique traces its way from the pelvis down the legs to the toes. From the pelvis it also traces its way up through the spine and branches at the collar to the arms, hands, and fingers and up the neck to the head. A link and associated envelopes are created for each link found. If any other objects, including 3DS MAX bones, are linked to the biped, Physique treats them similarly; it creates a new link and envelopes. Keep this in mind when you use Physique to attach a mesh to the biped; if your character has additional limbs, link 3DS MAX bones to the biped for the extra arms before using the Attach to Node command in Physique to create links and envelopes. When Physique is applied (Attach to Node is used), it creates links and envelopes for all the links in the biped as well as for the linked bones. Objects that should not deform like skin but need to be linked to the biped, like a sword, should be linked after the Physique Attach to Node command is used to link a mesh to the biped. This way a link and envelopes will not be created for the sword.

Envelopes and Weighted Vertices

Envelopes are Physique's primary tool for controlling skin deformation; tendons and bulge angles are used to fine tune mesh deformation after envelopes are adjusted. All envelopes have an inner and outer bound (boundary). Any vertices falling within the inner bound of a single link receive a full weight of 1.0 from that link. Any vertices falling outside the outer bound receive no weight from that link. Vertices falling between the inner and outer bounds receive some weight between 0 and 1. Vertices move together with the link that influences them. Where multiple envelopes encompass a vertex, that vertex receives weight from each envelope and follows each link to an average position based on these weights. This weighting from multiple links is considered blending.

It is possible that weights assigned to some vertices don't reach a total weight of 1.0 or greater. Rather than leaving these vertices behind, Physique by default normalizes them to a value of 1.0.

Adjusting falloff, overlap, scale and other envelope parameters changes vertex weight distribution across links. This, in turn, changes the way skin behaves as the biped moves. Much of the work in correcting the way skin deforms on a character is done by adjusting envelopes.

Deformable and Rigid Envelopes

There are two Envelope types per link, deformable and rigid. Deformable envelopes follow the Physique Deformation Spline that runs through the joints in the hierarchy and can be deformed using bulge angles, tendons, and link parameters. Rigid Envelopes determine vertex-link assignment based upon the linear 3DS MAX link and move in an immobile relationship to the link. Vertices in a rigid envelope, however, are deformed (blended) in the overlap area of other envelopes.

Typically you use deformable envelopes; however, game developers with game engine restrictions may want to exclusively use rigid envelopes. Both rigid and deformable envelopes can be turned on for the same link. For example, by scaling both envelopes, you could deform the shoulder with a rigid envelope and the armpit with a deformable envelope.

The Number of Links that can affect a Vertex

Any number of overlapping envelopes (N Links) can influence vertices. Normally, N Links are preferred. For special purposes such as games requirements, you can limit the number of links (envelopes) that can affect a vertex. The No Blending parameter is similar to the method used in version 1 of the software; a vertex is assigned to only one link.

Physique Workflow

Before Physique is applied, align the biped to the mesh in Figure mode. Use a pose with the arms outstretched so the hands are away from the torso. Save a figure file, so it's easy to return to this pose whenever you need. Select the mesh and choose Physique in the Modify panel. Turn on Attach to Node and select the root node in the hierarchy (biped Pelvis or root node in a bones hierarchy, not the COM). In the Physique Initialization dialog, click Initialize to create default envelopes based on the links in the hierarchy. The remainder of the work is adjusting envelopes and optionally adding bulge angles and tendons.

Envelope size, overlap, and other parameters are adjusted with the character in an animated position (turn off Figure mode); by scrubbing the time slider back and forth, problem areas can be spotted and the envelopes affecting the problem areas adjusted. In Place mode is useful to keep the character in place during envelope adjustment.

Link parameters, Bulge angles and tendons are the finishing touches. Skin sliding, the amount of twist, and crease blending as a character moves are controlled using link parameters. Bulge angles are used to bulge areas like the biceps, legs and chest relative to the angle created by a link and its child in the hierarchy. Tendons can span multiple links in the hierarchy to stretch and pull a character's skin.

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