Game Development Reference
In-Depth Information
Figure 10.35
The robot model in the binding pose (left), and the bones used to animate the model (right)
When the mesh is done, 19 it is rigged, which means a hierarchy of bones
(a skeleton) is created and the skinning data is edited to associate vertices
with the appropriate bones. During this process, the rigger will bend bones
to various extreme angles to preview how well the model reacts to these
contortions. Is the weighting done properly so that joints don't collapse?
This is where the skill and experience of both the character modeler and the
rigger come into play. The point for us is that although Maya is constantly
calculating new vertex positions in response to the manipulation of the
bones, it has saved the original modeling space coordinates of each vertex
at the location it had in the binding pose, before it got attached to a
skeleton. Everything starts with that original vertex position.
So, to compute the bone-space coordinates of a vertex, we start with
the modeling-space coordinates of that vertex in the binding pose. We also
know the position and orientation of each bone in the binding pose. We
simply transform the vertex positions from modeling space into bone space
based on those positions and orientations.
That's the big picture of mesh skinning, in principle. Now let's get
to the optimization. The basic idea is to store the position of each ver-
tex only in the binding pose, rather than storing it relative to each bone
that exerts influence. Then, when rendering a mesh, rather than having a
bone-to-model transform for each bone, we have a matrix that transforms
coordinates from the original binding space to modeling space in the cur-
19 Well, this is the ideal process. In reality, changes to the mesh are often necessary
after the mesh is rigged. The mesh might require adjustments in order to make it bend
better, although an experienced character modeler can anticipate the needs of rigging.
Of course, changes are often needed for aesthetic purposes having nothing to do with
rigging—especially if executives or focus groups are involved.
 
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