The Skin modifier's angle deformers control creases and wrinkles, as well as add muscle bulges and deformations. There are three kinds of angle deformers: Joint,
Bulge, and Morph.
Angle deformers work by telling vertices how to behave when a certain angle is achieved between two adjacent bones. For example,
you could use a bulge angle deformer to tell vertices in the upper arm to bulge when the elbow is bent greater than 90 degrees.
After the Skin modifier is applied, it is not uncommon for some joints to crease unrealistically when the character is animated.
Angle deformers can correct these problems.
Set up the lesson:
Scrub the time slider to play the animation.
Around frame 15, the inside of the elbow is compressed.
The arm flexes.
This unrealistic deformation can be corrected with a joint angle deformer. You'll also use a bulge angle deformer to add a
slight bulge to the upper arm when the elbow is bent.
Select the Arm mesh object.
On the Modify panel, expand the Skin listing on the stack, and select Envelope.
In the Envelope list, click Bone ForeArm to choose that bone.
This bone's rotation will control the deformer.
Add a joint angle deformer:
The process to add an angle deformer consists of two steps. First, you select the bone envelope, and then select the vertices
to be deformed. Once that is accomplished, you can add the gizmo for the deformer.
Go to frame 0.
In the Parameters rollout > Select group, turn on Vertices.
Drag a selection window around the vertices above and below the elbow.
Lower-arm vertices selected
With the bone and vertices selected, now you can add an angle deformer gizmo.
On the Gizmos rollout, make sure Joint Angle Deformer is selected, then click Add Gizmo.
The new gizmo appears in the viewport. This gizmo, called a lattice, is used to control the selected vertices.
Deformation lattice displayed around selected vertices
Set the joint angle deformer:
Move the time slider to frame 13.
Flexed arm with deformation lattice
Angle deformers use the angle between bones to determine when the deformation will take place. Regular keyframes are not part
of the deformation setup, so there is no reason to turn on Auto Key.
In the Gizmo Parameters rollout, click Edit Lattice.
Move the lattice's control points to pull out the creased vertices on the inside of the elbow.
Note: Editing the gizmo works similarly to editing an FFD lattice, where manipulating the control points affects underlying
Elbow crease corrected
Tip: You can hide the bones and deselect the vertices to concentrate on the lattice.
Move the time slider from frame 0 to 13 and back again, watching the movement of the lattice.
Move the time slider to frame 24. Edit the Lattice Points again.
Outstretched arm with elbow corrected
Click Edit Angle Keys Curve to display the Joint Graph. This is not a graph of time; rather, it is based on angle. It goes
from 0 to 360 degrees of rotation.
Move the time slider back and forth, while watching the joint graph.
You can use this joint-angle graph to remove or change lattice edits. You can also adjust the easing by using the Bezier handles
on the graph points, much as you do for key timing in the Curve Editor.
Close the Joint Graph.
Next, you'll add a Bulge Deformer to make the biped grow as the arm bends
Add a bulge angle deformer:
Turn off Edit Lattice, and move the time slider to frame 0.
With Bone ForeArm still selected, drag a horizontal selection rectangle to select a few vertices in the middle of the upper arm.
Initial vertex selection
Click Loop to highlight a loop of vertices around the arm, and then click Grow a few times until all of the upper arm's vertices
Loop button changes selection to loop around the arm.
The Loop and Grow buttons, along with Ring and Shrink, are vertex-selection tools. Ring is like Loop, but selects along the
opposite UV axis. Shrink is the complement of Grow: it reduces the size of the selection.
Grow used to select all upper-arm vertices
Once you have the correct vertices selected, choose Bulge Angle Deformer from the list on the Gizmos rollout, and click Add
The new gizmo is displayed in the viewport.
Bulge Angle lattice displayed over upper arm
Move the time slider to frame 15, where the arm is bent.
On the Deformer Parameters rollout, click Edit Lattice.
Select the gizmo control points near the bicep.
Move the lattice points to create a bulging bicep. Move points on the front and then on the back of the arm.
Bulge Angle lattice used to create a bulging bicep
Move the time slider back and forth to see the bulge angle deformer create a bicep with the bending of the arm.