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Skin Wrap Modifier

A character is often modeled body first, with clothing added later as separate geometry. You may want to reuse that model with different clothing, so you need a flexible workflow that lets you animate one character with several choices of clothing, without the hassle of setting up new Envelopes and Weights within the Skin Modifier for each piece of clothing. 3ds max 7's new Skin Wrap modifier simplifies the usual animation workflow for objects that wrap a character.

With Skin Wrap, you can animate a character (or any other object), remodel or clothe it, and use the original character as a proxy to control the remodeled one. This is great in a studio production situation because the animators no longer need to wait for the modelers to finish modifying a model for which an original exists—animation can move right along using the original model, or a low-polygon version, as a proxy. The modified model is swapped in for the proxy at a later time.

The Skin Wrap modifier is applied to the character (or any other objects that you want to control) with the motions of the original model. For example, let's say you want to add a hat and gloves to a character that has already been animated; from the Modifier's list, you would apply a Skin Wrap modifier to both the hat and the gloves and link the Skin Wrap to the original character model. Both the hat and the gloves would need to be properly positioned relative to the positions of the head and hands that they cover. This can be demonstrated simply by positioning some primitives on a plane.

The Skin Wrap modifier has a sub-object level called Control Vertices. Each control vertex is derived from the vertices of the animated control object and has a spherical influence, similar to an envelope from the Skin modifier. The vertices of the Skin Wrap object are influenced according to this envelope. The size of the envelope is determined by the Distance Influence parameter of the Skin Wrap modifier (Figure 4.13).

Figure 4.13. The Skin Wrap modifier is applied to each object that sits on this plane. The plane is added to the list. The Distance Influence setting determines the radius of the Skin Wrap spherical envelope of influence.

Set the Distance Influence too low, or position the Skin Wrap object too far away from the control vertices, and the animated control object will not have any influence. Set the Distance Influence too high, and the animation of each control vertex will bleed over to unwanted vertices of the Skin Wrap object. Multiple control vertices may influence individual vertices of the Skin Wrap object, producing unexpected and probably unusable results.

The value of the Distance Influence is globally assigned to all the control vertices, even though each control vertex is likely to need its own Distance. Use the Local Scale Multiplier parameter to affect individual control vertices and avoid influence bleed-over. At the same time, you have to make sure that each control vertex envelope is large enough to influence the desired vertices of the Skin Wrap object. Some trial and error may be required to find the right values.

Once envelopes are properly sized, the original object can be deformed; this causes the Skin Wrapped object to deform also (Figure 4.14).

Figure 4.14. Moving some of the plane vertices upward carries along the objects within the Skin Wrap spherical influence envelopes.


Be sure that the scale of the original object is set to 100 percent for X, Y, and Z. Otherwise, the shape of the area that is influenced by the object's control vertices will not be spherical and the vertices will influence the Skin Wrap model unpredictably. Use the Reset XForm modifier from the Utilities panel to fix the improper scale of a model.

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