Game Development Reference
In-Depth Information
i (left) to global coordinates x i (right)
using the positions of its joint j i and the joint's parent j p .
rel
Figure 14.9. Transforming a relative coordinate x
where q is the rotation quaternion and j i is the position of the joint. Quaternions
are explained in the introductory chapter (Chapter 1), since they are quite use-
ful for the representation of rotations in computer graphics—especially character
animation.
The vertex positions are relative to its supporting joint and have to be trans-
formed into global space (se e Figure 14.9 ) .
The vertex structure is extended by a vector called relPos , which is the
position of a vertex relative to its supporting joint. This is the only coordinate
the designer has to supply; the restPos is calculated from this coordinate using
the above formula.
for each joint j {
v = j.pos
j.parent.pos;
normalize(v);
j
orient = rotationQuaternion(u, v);
for each vertex v {
j = v.joint;
tempPos = rotateVector(v.relPos, j.orient)
v.goalPos = tempPos + j.pos;
−>
}
In the first loop, we calculate the rotation quaternion of the joint as described
above. In the second loop, we use the calculated quaternion to transform our
relPos coordinates into global space ( restPos ). These are again the positions
that are used by all the force calculations we discussed before.
Calculating the force. We need a force that maintains the distance to the bone
for each vertex. For this, we compare the actual distance to the desired distance.
We have to calculate the distance from the bone to the actual positions x i and
the distance to the rest positions x i .
First, the unit vector in the direction of the joint's parent is obtained by
 
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