An envelope is an object or hierarchy that is assigned as a “skin” to a set of deformers such as IK chains. Envelopes move and deform in response to the movements of their deformers. In this way, for example, a character moves as you animate its skeleton.
Every point in an envelope is assigned to one or more deformers. For each point, weights control the relative influence of its deformers.
The process of setting up an envelope is sometimes called skinning or boning.
Deformers are the objects that control the shape of the envelope. You pick the deformers when you first create an envelope deformation. You can still add and remove deformers later if necessary.
Although it’s common to use skeletons as deformers, you can actually use any object. The geometry of a deformer does not matter because the points of the envelope are assigned to its center. (Bones are the exception — points are deformed by the whole length of a bone.) Whether you use a cube, a sphere, or a null, all transformations of the surrounding envelope are relative in size, orientation, and position to the deformer’s center, not its shape. Nulls and implicit objects are good choices because they do not render.
Each point on an envelope has a total weight of 100, which is divided between the deformers to which it is assigned. For example, if a point is weighted by 75 to deformer A and 25 to deformer B, then A pulls on the point three times more strongly than B.
You have several levels of control over how points are assigned and weighted:
• Deformers and weights are initially assigned to points when you apply an envelope deformation. There are several options for controlling how this initial assignment is performed.
• You can use bounding volumes to fine-tune the automatic assignment of deformers.
• You can then select points and reassign them to specific deformers or to no deformers at all.
• You can edit the weighting of individual points by painting or editing numerically.
When you make changes that cause the initial weight assignment to be recalculated—for example, if you add deformers, or reset the reference pose—all modifications you have made to weights are preserved.
Envelope Weight Operator Stack
When you modify the weighting, your changes are added to an operator stack, similar to the geometry operator stack. Once you are satisfied with your envelope, you can freeze the weights to improve performance as described in Freezing Envelope Weights.
After you have frozen the weights, you can still make modifications, but Softimage cannot recalculate the initial assignment.
Autodesk Softimage v.7.5