You can create an
object and use it to influence the shape of smooth skin. The object,
called an influence object, acts like a surgical implant against which
the skin deforms. For example, you can create a sphere and use it
to simulate a muscle or bone that bulges as you pose the character
in certain positions. You can also use an influence object to smooth
deformation and maintain volume in regions that collapse while bending.
In the next steps, you’ll
use a sphere as an influence object to simulate an elbow jutting
out as Jackie’s arm bends. To use an influence object, the skeleton
must be in the bind pose—the original pose at which the
skin was bound to Jackie.
To return the skeleton to the bind pose
- Click the Select Tool and
cancel the selection of Jackie.
- Turn off
Modify > Evaluate Nodes >
you use an IK handle to pose a character, you must do this step before
you can return the skeleton to the bind pose.
- Select back_root, the root of the skeleton.
Skin > Go To Bind Pose.
- Select the ikHandleLeftArm in the Hypergraph.
Use the Move Tool to move the handle
manipulator slightly in any direction. When you release the mouse,
note that the arm does not move and that the IK handle manipulator
snaps back to the desired position at the end of the joint chain.
Repeat this step for any other IK handles you posed previously.
- Turn on
Modify > Evaluate Nodes>
You must do this step
to enable the IK solvers so you can manipulate IK handles in the
To position a sphere to be used as an
Jackie’s surface (anywhere but on the skeleton) and select Actions
> Template from the marking menu.
By making Jackie a template,
you won’t accidentally select Jackie in subsequent steps.
- Create a sphere (select
Create > Polygon Primitives > Sphere >)
and name it elbow_influence.
- Scale the sphere to be slightly smaller
than any of Jackie’s joints (see illustration below).
- Move the sphere so that its surface is positioned
where the elbow would jut out, but still within the skin. You can
optionally reshape the sphere’s curvature to resemble the tip of
an elbow jutting out from a bent arm. For example, you can scale
the sphere in one dimension to squash its shape or you can move
individual vertices. A top view of the sphere’s shape and position
The exact position and
scale of the sphere is unimportant. You can make slight position
and scale adjustments later to enhance the deformations that result
from its use.
To make the sphere an influence object
- In the Hypergraph,
select Jackie and Shift-select elbow_influence.
Skin > Edit Smooth Skin > Add Influence > . In
the Add Influence Options, select Edit
> Reset Settings, and then click the Add button.
Maya completes the operation within a few
moments. To ensure the influence object stays in the correct position
at the elbow, you will parent it to the left_shoulder joint.
- In the Hypergraph,
Shift-select elbow_influence, elbow_influenceBase, and left_shoulder
joint, in that order.
The order of selection
is important. The parent object should be selected last.
- Press p to parent the elbow nodes under
the left_shoulder joint.
The elbow_influence nodes
appear in the skeleton hierarchy below the left_shoulder joint.
elbow_influenceBase is known as a base object. It stores vertex
position information for the influence object. The reason for parenting
the base object to the left_shoulder joint is so that the influence
object’s vertices stay in the correct position at the elbow whenever
the elbow moves to a new position. Other than this step, you do
not work with the base object.
- Right-click Jackie and select Actions
> Untemplate. It is easier to see subsequent skin
deformations when Jackie is displayed with smooth shading rather
than as a template.
- From the menus at the top of the top
view, turn off Show > Joints to avoid having
the joints obstruct your view of the skin.
- Use each arm’s IK handle (ikHandle1 and
ikHandle2) to pose the arms in the following position.
The left elbow looks
more natural than the right elbow. To tune the elbow’s deformation
during bending, change the position, scale, and rotation of elbow_influence.
Do not be concerned if
the influence object pokes through the skin. It won’t be displayed
when you render the animation.