Character Rigging – Shoulders are the Pits
by Brian Immel
Rigid Binding With Flexors
To make a joint location to have better surface deformations, try using a flexor on that joint. Note: When a flexor is created, it automatically becomes part of the hierarchy of the skeleton. There is no need to parent of constrain the flexor (unless of course you want double-transformations).
1. Select the shoulder joint.
2. Go to Skin > Edit Rigid Skin > Create Flexor
3. In the Create Flexor Dialogue box, make sure the following is set according to the following list:
- Flexor Type = lattice. Creates the flexor out a lattice. You can also choose Sculpt and jointCluster as well.
- Joints = At Selected Joint(s). This will place the flexor at the selected joint
- Bones = nothing
- S Divisions = 2 (depends on type of bending involved but this is Maya’s default)
- T Divisions = 5 (depends on type of bending involved but this is Maya’s default). This value is important because it normally follows the longer length of the geometry. So if you have seven divisions in the geometry that follows the YZ plane, you may want to make the number of divisions of the geometry with the number of divisions in the flexor on the T Division.
- U Divisions = 2 (depends on type of bending involved but this is Maya’s default). The S, T, and U divisions can be thought of as values for creating a lattice.
- Turn on Position the Flexor option. This will place and orientate the flexor according to the selected joint.
4. Hit the Create button and then close the Create Flexor window.
5. While the flexor is still selected after it has been created, you can adjust the rotation, position and scale without affecting the geometry.
As soon as you drop the selection, the flexor will now influence the geometry. I normally rescale the flexor so that it better fits the surface, as it needs to bend. For example, if a joint bends along the Z-axis, the geometry will need to be able to flex better along the Z bend axis. So I would scale the flexor to make it cover more surface along the X-axis of the flexor like in Figure 6.
Note: It is not normally a good idea to move the flexor from its original creation location around the selected joint. Reason for this is that the bend of the flexor location will be different than that of the joint itself.
6. Next try bending the joint to see how the flexor affects the surrounding vertices of the surface. If you find a trouble area, leave the rotated joint were it is and select the flexor.
7. Select the flexor and in the Channel Box, adjust the Creasing, Rounding, Length In, Length Out, Width Left and Width Right to make that troubled bend look smoother and to carry the displacement of the mass better (Figure 7 & Figure 8).
8. After you have adjusted the attributes of jointFlexor1, select the shoulder joint and rotate it back to the original position. You will notice that the flexor will return to its normal shape, as the joint gets closer to its original bind position. Maya has sent up a Set Driven Key lattice driven by all the rotations of the shoulder joint and blending between the attributes (shapes) listed under jointFlexor1. Although this is a great feature, it has very little control over how the surface is bent when the joint rotates.
9. You should name the flexor so that you won’t confuse your self or your co-workers when they work with it.