Graphics Reference
In-Depth Information
in both a front and side view so the new bone will appear in the right place. Use Ctrl-A to add a
new bone.
Grab the tail of the bone, and pull it so that the bone approximates the span that you see in
Figure 9.39
.
Make this new bone the disconnected child of the upper arm bone, so that its head always moves along with
the upper arm. Mark the spot where the helper bone meets the lower arm bone with the 3D cursor.
Return to Pose mode. RMB select the lower arm bone, then Shift-RMB select the new helper bone.
Trigger the
Add Constraint
operation with
Shift-Ctrl-C
and choose
Stretch To
under the Tracking
column, or add a Stretch To constraint with the Add Constraint button in the Bone Constraints properties
context.
Stretch To
causes the tail of the constrained bone to stretch (grow or shrink, doesn't matter!) to meet the
head of the target bone. Of course, we don't want it to meet the head of the target bone; we want it to hit
the spot on the target bone that we specified in Edit mode (which we
very cleverly
marked with the 3D cursor
so we know where it is). Over on the Stretch To panel in the Bone Constraints context is a control called
Head/Tail
. This specifies the actual target along the length of the bone. The default is 0.0 for the head,
and 1.0 for the tail. Adjust this value until the helper bone crosses the lower arm at the location marked by
the 3D cursor. Now, hit the
Reset
button located below the Head/Tail control. This resets the bone to its
original (Edit mode) length. Rotating the lower arm bone causes the helper bone to shrink and grow as the
joint compacts.
Select the mesh again, enter Weight Painting mode, and RMB select the new helper bone. It won't have
any weighting assigned to it. Paint in some weighting on the inner side of the elbow and try bending the
lower arm. The mesh holds together better with the stretch bone in place.
You can use this technique anywhere that you have a fairly strong pinch in your armature: behind the
knees, under the arms, even in the stomach. If you look at the sample character's rig, you'll see a stretch
bone that extends from the middle of the upper spine bone to the top of the pelvis.
Figure 9.40
shows
stomach deformation with and without it. Notice how using this construction gives your character's defor-
mations more volume. Another place this technique is used is in the hands. A stretch bone is added that
runs from the base of the wrist to the first knuckles of the little finger. It follows the scaling curl of the
hand, giving a better deformation in extreme hand poses.
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