Weighting - Weighting in 3D Modeling and Animation

Weighting in 3D Modeling and Animation

Weighting in the context of 3D modeling and animation refers to how closely the components of a soft body follow their "target," "guide," "goal," or "controller." Components (usually vertices) with higher weights follow (or "conform" to) their guide quite closely, while those with lower weights do not. Take the following instances for example:

Skeleton Structure
Often, when an object is rigged with a skeleton, weights are applied to the vertices near the joints. The vertices closer to the joint will usually have a lower weight assigned; the reason for this is so that during deformation, the geometry of the skin does not fold in on itself. Weighting in this situation will most of the time be done automatically using skinning techniques, but is often done by hand in order to fine-tune the skeleton's deformation effects.

Another example of using weights with a skeleton structure would be to actually apply weights to vertices that are not part of a character's skin. An appropriate situation for this method would be the case when an elephant's trunk dangles freely as it walks. The elephant's trunk is rigged with a skeleton and inverse-kinematic (IK) curve. Then a goal curve is created so that the IK curve has something to "aim for" and thus the trunk has a certain shape to which it conforms. Closer to the end of the trunk, the vertices of the IK curve are given lower weights. This way, say if the goal curve was parented to the elephant and a gravity field was in effect, if the elephant stopped walking, the end of the IK curve and thus the end of the trunk would continue to move, dangle back and forth, and ultimately come to a rest in the position determined by the goal curve.

Cloth
Weights are used with cloth as well. Say for instance a character was being outfitted with a dress. Around the waist, the cloth should stay attached to the character, but farther down, its movements should be governed more by effects directly induced by the neighboring cloth vertices rather than the hip movements of the character. In this situation, higher weights are applied to the cloth vertices close to the hip so that they hug the character quite closely compared to the cloth farther down the dress. In order to make the dress more realistic, it is even possible to assign cloth weights of 0 to much of the dress because springs hold the cloth together; one must keep in mind though that doing this for the entire dress would effectively cancel the attachment of the dress to the character.