Normalized Weighting
Here is another buzzword to add to your word of the day calendar:
normalized. Normalized weighting refers to the idea that no matter
how many joints influence the movement of any vertex, their influ
ence must always add up to 1.0.
So hypothetically we have a vertex that sits at the middle of the
elbow. This vertex would be infuenced by both the upper arm joint
as well as the elbow or lower arm joint. Since it resides right in the
middle, the two joints would more than likely have equal influence:
0.5 for the upper arm joint and 0.5 for the lower arm joint. So if you
do the math, and this is an easy one, 0.5 + 0.5 = 1.0.
There is really no limit to the number of joints that can influ
ence a particular vertex. I have found that on average three joints is
usually the norm. This really depends on the type of joint you are
dealing with. For example, the joints around the shoulder move con-
siderably more than those on a joint that merely pivots, such as the
knee joint, so more joints would be involved in weighting the shoul-
der area.
Like I mentioned in Chapter 13, it is very difficult to get real
human motion on a character regardless of how good the modeling,
rigging, and skinning may be. There are a few ways to compensate
for problems in these areas, though. The method I prefer is using
blend shapes to correct any unwanted deformations of the mesh. In
this process you can use driven keys to drive the blend shape. This
can really come in handy for problem areas such as the shoulders.
We also have the ability to mirror weighting. This speeds up the
skinning process quite a bit. In a similar fashion to the modeling
process, with mirror weighting you concentrate the majority of your
efforts on one side of the model only.
Chapter 14: The Weighting Is the Hardest Part

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