What Makes Up a Skeleton?

 
 
 

Skeletons in Softimage are made up of bones that are linked together by articulated joints that can rotate. The combination of bones and joints is referred to generically as a chain in Softimage because you can use chains for animating any type of object, not just humans or creatures.

Chains have four basic elements: a root, bones, joints, and an effector. There is no limit to the number of bones a chain can have, but there must be at least one bone in a chain.

You can also create chains using only implicit bones to create non-IK rigs - see Drawing Implicit Bone Chains.

TipYou can change the visual display of the chain's bones, roots, and effectors — see Changing the Chain Element Display.

A: Root

The chain root is a null that acts as the starting point on the chain. It is the parent of all other elements in the chain. Because the first joint is local to the root, the root's position and rotation determine the position and rotation of the rest of the chain. In a 2D chain, the entire chain extends on the root's (and first bone's) XY plane, called the resolution plane (see The Chain's Resolution Plane). A chain root can be the child of another object, such as another chain root or an effector.

TipYou can make any joint in a chain act like a root by creating a subchain — see Creating Subchains for Independent Movement for more information.

B: Bones

The bone is the piece that connects roots, joints, and effectors. A bone always rotates about its joint, which is at its top. The first bone in a chain rotates around the root. The first bone in the chain is a child of the chain root, and all other bones are children of the previous bones in the chain.

Keying the rotation of bones is how you animate with forward kinematics (FK).

NoteThe first bone in the chain is position-constrained to the root. As a result, the bone's position constraint to another object will be superseded by its constraint to the root.

C: Joints

A joint is the connection between two elements in a chain: there is a joint between the root and the first bone, between bones in the chain, and between the last bone and the effector. The first joint is between the root and the first bone.

NoteBy default, joints are not displayed: to view them, see Displaying the Joint Circles.

A joint can be either 2D or 3D and acts differently for each (see 2D and 3D Chains for a description of the types of chains):

D: Effectors

The effector is a special null that is the last part of a chain. Moving the effector invokes IK, which modifies the angles of all the joints in the chain between the root and the effector.

By default, when you create a chain in Softimage, the effector is a child of the chain root. This allows you to place the effector anywhere in the chain's hierarchy or even make it the child of an object outside of the chain. As well, you can animate the effector in local space as opposed to global.

For more information about animating the effector, see Animating the Effector.

2D and 3D Chains

Chains can be either 2D or 3D. So what's the difference?

The first joint connecting the root to the first bone on either a 2D or 3D chain is always a ball joint, and it can rotate on any axis.

Finding Skeleton Properties in the Explorer

The following shows where different skeleton elements can be found in the explorer (or using the Selection button on the Select panel to open an explorer specific to the object). By clicking on their icons, you can open their property editors:

However, to change only effector display and shadow properties in the Chain End Effector property editor, select the effector and press Enter or click the effector's icon in the explorer (not the Chain End Effector icon — the one for the node above it).

Getting the Chain's Properties

When you're working with chains, you often need to open the Kinematic Chain property editor. This property editor contains most of the parameters that affect how an IK chain is solved (especially effector properties).

To open the Kinematic Chain property editor

Do any of the following:

  • Select any chain element from the same or different chains and choose Create Skeleton Inspect Chain Properties or press Ctrl+R.

    or

  • Select the first bone of the chain and press Enter.

The Kinematic Chain property editor for the selected chain is opened. If you had elements selected from several chains, the property editors open in multi-mode.