Overview on skeletons

Skeletons enable you to create complete character animation sequences before you have to create any of the geometry for the character.

Use Animation > IK > New Skeleton to create the skeleton, then Animation > IK > Add IK Handle, Animation > Tools > Create Constraint, and Animation > Keyframe > Set Keyframe or Animation > Keyframe > Auto Keyframe to animate your character in its rotation and translation parameters.

You can create the hierarchical geometry for the character independently from the animation, and you can useAnimation > Editors > Skeletons to turn DAG nodes in this hierarchy into joint DAG nodes. You can then useAnimation > Edit > Overlay Skeleton to overlay the animation and other skeleton attributes onto the corresponding joint nodes in the model.

To duplicate a skeleton

1. Create a skeleton using Animation > IK > New Skeleton. Animate the skeleton using inverse kinematics (use Animation > IK > Add IK Handle) or any of the standard animation tools.
2. Create another skeleton with Animation > IK > New Skeleton, or use the Skeleton editor (Animation > Editors > Skeletons) on an existing model that has the same skeleton topology as the first skeleton.
3. Choose Animation > Edit > Overlay Skeleton. The system prompts:

   Select the root of the skeleton that is to be copied.
   

4. Pick the skeleton to copy by selecting the joint node that defines the root of the skeleton. You can pick this joint from the SBD window or select a joint in the modeling window.

Choose Animation > Tools > Bone Style to specify bone style.

The following illustrations demonstrate the various bone styles.

1. Once the joint node is picked, the system prompts:

   Select the root of the skeleton to be copied to.
   Pick the root node of the destination skeleton that is to receive the same animation and skeleton attributes as the source skeleton.
   

   

   After you select the destination skeleton, the translation and rotation animation of each of the joint nodes in the source skeleton are copied to the corresponding joint nodes in the destination skeleton.

   See Animation > Editors > Skeletons for a complete description of the joint information.

   In addition, the joint limits as well as other joint information is copied to the corresponding joint nodes in the destination skeleton.

Topological skeleton matching

Two skeletons have the same topology if each joint node of Skeleton 1 has the same number of direct descendants joint nodes as Skeleton 2. In the following SBD window diagrams, Skeleton 1 and Skeleton 2 are topologically equivalent.

These diagrams show that skeletons with the same topology can have non-joint DAG nodes grouped anywhere above, below, or as siblings to joint nodes. In determining the skeleton from the DAG node hierarchy, the non-joint DAG nodes are ignored.

Sometimes two skeletons can have the same topology, but differ in the significance of their topology. For instance, two skeletons could have two branches of three grouped joint nodes, each branch representing the hip, knee, and ankle joint of a leg. The difference is that in one skeleton, the first branch represents the right leg, and in another, the first branch represents the left leg.

If Overlay skeleton is used to copy animation from one skeleton to the other, the animation of the left leg is copied to the right leg, and vice versa. To make sure that the animation is copied to the appropriate respective branches, use the SBD swap icon (located in the SBD window title bar) or an -arrow (Windows) or -arrow (Mac) combination to swap the positions of the branches in the SBD window.

Draw a skeleton

UseAnimation > IK > New Skeleton to create a skeleton that is defined by pivot points or joint positions.

• Refresh your memory with the definitions of Skeletons and Inverse kinematics terms before proceeding.
• Decide how complex your skeleton will be, and if your intention is a simple animation, choose the single-chain solver option.
• If your animation involves rotations of joints in all directions, use the multi-chain solver option.

To create joint nodes

Use Animation > IK > New Skeleton to creates a skeleton.

As you create each joint, a DAG node is added to the Scene Block Diagram (SBD) and a bone is drawn in the modeling windows connecting this joint to the previous joint.

Once it is drawn, you can copy and mirror the skeleton using Edit > Duplicate > Mirror, creating an inverse duplicate of the original.

You can also turn existing DAG nodes into joint nodes or make joint nodes into regular DAG nodes, by toggling their joint state in the skeleton editor (Animation > Editors > Skeletons).

Each time you click in the modeling window when using New skeleton, you create a new joint node that is a child of the active joint node, and a bone is drawn between the two joints.

Each time you create a joint DAG node in this way, the pivot position of the joint DAG node is placed where you last clicked in the window. (You can also create a new joint DAG node by entering the pivot position on the keyboard.)

To create bones

Bones are drawn between joint nodes and their nearest ancestor joint node by connecting the rotation pivot points of the two nodes. You can change the shape of the bone (see Animation > Tools > Bone Style ) or turn off the display of the bones completely (see Animation > Show > Skeletons ).

The actual size of a 3D bone is defined by the distance between pivot points. The larger end of the bone shows the location of the rotation pivot point for the joint DAG node above the bone — the smaller end of the bone points to the rotation pivot point for the joint DAG node below the bone. Because the bone is defined by the pivot points, you can useTransform > Local > Set Pivot to change the length of a bone.

You can create a chain of joints by clicking several times in an application window. To create a hierarchy , use the key with one of the four arrow keys on the keypad to move around the joint node hierarchy.

Display skeleton bones

How to display of skeleton bones and their associated constraints or IK handles in application windows on or off.

1. Create a skeleton using Animation > IK > New Skeleton or the skeleton editor (Animation > Editors > Skeletons). The skeleton is displayed with bone-like geometry.
2. Choose Animation > Show > Skeletons to turn off the display of these bones. All the skeletons in all the windows are not displayed. To turn the display back on, select the function again.

Add IK handles to your skeleton

Use inverse kinematics IK handles to control the movement of different parts of your skeleton without disturbing the rest of the skeleton.

Animation > IK > Add IK Handle is a continuous-action tool in which you add IK handles one at a time to a skeleton. To add single-chain handles, choose Single-chain in the option box for this tool. Then, select the root joint, and finally the end effector joint. The tool is still active after this operation, so you can continue to select other parts of the skeleton to put other IK handles on.

To add IK handles

1. Choose Animation > IK > Add IK Handle or click its icon.
2. The system prompts:

   Pick a joint as the root of a single-chain (multi-chain or spline) handle.
   Pick a root joint to start the handle. (You can drag a pick box around a joint to select it.)
   

3. The system prompts:

   Pick a descendant joint as the end-effector of the single chain (multi-chain or spline) handle.
   Pick an end joint.
   

4. If you created a spline handle and you did not select Create Curve to create a new curve, the system prompts:

   Pick a curve node as the target of the spline handle.
   Pick the curve.
   

5. After you create a handle, the system prompts you to create another.
6. To view, change, or turn off the parameters of a handle, open the Information window and select the handle.
7. Limits and joint information about a handle can be edited in the Skeleton Editor. To see handle information, choose Handles from the List Type pop-up menu in the Skeleton Editor (Animation > Editors > Skeletons).

Limitations

• If you select Single-Chain solver or Spline-handle solver from the Add IK Handles Options box, the handle is not created if it will overlap any existing handle. However, it is acceptable for a Single-Chain handle to share its root/end-effector with a Multi-Chain handle end-effector/root.
• Joints can be the end-effector for a maximum of one handle. This restriction prevents handles from overlapping in the modeling windows, thereby allowing you to pick a particular handle. If you need two handles at the same point in the skeleton, you can add additional joints rooted at the same parent.
• Joints with handles are represented in the SBD window by a handle icon.
• A handle is always drawn from the joint it controls as the end-effector. If you transform the skeleton chain, the handle snaps to maintain its position at the joint.
• If a handle is animated, has an expression, or is constrained, then it will still be drawn at the end-effector joint, but its “goal” position for inverse kinematics purposes is the animated/expressioned/constrained position.
• Although handles are not displayed in the SBD window, you can still select them. Handle names appear in the Information Window, Action Window, or any other lister that displays names.
• Multi-chain or single-chain handles can be both position goals and orientation goals. A position goal handle puts the end-effector joint at the handle’s rotate pivot point position. An orientation goal handle orients the end-effector joint to match the orientation of the handle.

Tips for using the spline handles

You can control the spline handles in the Modeling window in several ways:

If you pick a root handle, either:

• UseTransform > Move and the to move the chain along the curve.
• ChooseTransform > Rotate and the to roll the chain.

If you pick a master handle, use Transform > Rotate and the to twist the chain.

To control the shape of the skeleton

You may want to use spline IK to control the shape of the skeleton, but not its position in space. This is useful if you want one curve to control the shape of a number of similar objects in different locations. To do this:

1. Constrain the root joint of the skeleton to the desired object.
2. Create a spline handle.

Creating a neutral pose for IK skeletons

Use Animation > IK > Set Rest Pose to define a neutral pose.

The single-chain IK solver uses a rest pose from which all its unique solutions initiate. The rest pose is a set of values for each joint that provide an initial value for the IK solution.

Each skeleton joint “remembers” a rest value for its translation and rotation, X, Y, and Z values. The rest pose can be copied and mirrored onto a similar skeleton. For information on reverting to a rest pose, see Returning joints and IK skeletons to a rest pose.

To create a neutral pose for an IK skeleton

1. Create a skeleton using Animation > IK > New Skeleton.
2. Position the skeleton as required, using Animation > Pick > Joint and the necessary Transform tools.
3. Choose Animation > IK > Set Rest Pose.

Alternatively, you can pick an IK handle to set a rest pose for all joints on the IK chain.

Returning joints and IK skeletons to a rest pose

Using Animation > IK > Assume Rest Pose to return a skeleton to its neutral position.

Each skeleton joint can assume a rest value for its translation, rotation, X, Y, and Z values. You can also return a joint or a regular node back to its neutral pose (that is, having its translation, rotation, X, Y, and Z values set to be zero).

How to return a skeleton to a neutral or rest pose

1. Create a rest pose using Animation > IK > Set Rest Pose.
2. Reposition the skeleton as required using Animation > Pick > Joint and Transform tools.
3. Choose Animation > IK > Assume Rest Pose to return the skeleton to its rest pose.

   You can also pick an IK handle for all joints on the IK chain to assume a rest pose.

Find information on IK handles

Use the Windows > Information > Information Window menu item to display the details.

To find information on IK handles

1. Select the IK handle.
2. Choose Windows > Information > Information Window.
   

Name

The name of the IK handle. You can edit this field.

Bounding Box

Turns off the display of the IK handle’s bounding box.

Invisible

Turns off the display of the IK handle.

Translate

The handle’s translation coordinates. These are the only editable coordinates at this time because under Control type, TRANSLATION ONLY is chosen. The rotation and scale are not used to control the handle’s behavior.

Rotate Pivot

The handle’s rotation pivot coordinates.

Local Axes

The handle’s local axes coordinates.

Scale Pivot

The handle’s scaling pivot coordinates.

Bounding Box Min/Max/Size

The limits placed on the handles’s bounding box, which is made up of the Limb axis and Plane indicator.

Bounding Box Size

The size of the bounding box. Size = Max - Min.

Handle Type

The type of the IK handle: single, multi, or spline.

On/Off

Click these buttons to turn the IK handle on or off. When the handle is turned off, it is still visible but has no effect on the IK chain. To hide it completely, click Invisible at the top of the IK handle section.

root

The top-most joint in the skeleton hierarchy affected by the IK handle.

end-effector

The bottom-most joint in the skeleton hierarchy affected by the IK handle.

Control type

A single-chain IK handle has three levels of control accessed from the Control type pop-up menu.

• TRANSLATION ONLY — this is the default control setting, suitable for most basic IK use. TRANSLATION ONLY means that you can only move or animate the IK handle’s translation channels.
• PLANE ROTATION — this setting gives you more in-depth control of the IK handle. It lets you rotate the skeleton plane, the plane in which the joints contained in the IK handle lie (if all the joints of the IK handle do not lie in one plane, a “best” fit plane is chosen). When you choose this option, you can rotate the IK handle using the value in the Plane rotate box, letting you re-orient the plane indicator, and subsequently the skeleton plane, to create more complex motion in the chain.
• You can also control the rotation of the plane axis by picking the IK handle and using the with the Rotate tool (Transform > Rotate).
  
• PLANE/POLE ROTATION — this setting provides advanced, specialized control of the IK handle with a new set of options. In some cases, the combination of translation and plane rotation might cause the IK solution to flip uncontrollably. Set the two Pole rotate box values to alter the plane of the skeleton, while keeping the pole in the same orientation relative to the skeleton plane.
• Use the middle and right mouse buttons to rotate the IK solution’s Pole Axis and its projection.
  
  

The Pole order field is used to determine which two degrees of freedom are to be used to rotate the Pole Axis. In the earlier example illustrating plane rotation, the skeleton was drawn in the XY plane. Thus the Pole Axis is automatically positioned along the Z-axis (that is, perpendicular to the skeleton plane) when the IK handle is created, and the pole order is ZXY. This means that the Pole Axis is created parallel to the IK handle’s Z-axis (and thus given the blue Z-axis color), and its orientation is adjusted by applying a local X rotation, then a Y rotation, to the Pole Axis.

Orientation

The IK handle solution orients itself to a coordinate system to work. The coordinate system matches either the joint (LOCAL) or scene (WORLD) orientation.

When to use plane rotation settings for IK handles

If the IK chain never needs to change orientation from its rest position, the default Translation Only setting will be all you need to animate the skeleton. For example, if the legs of a simple biped character in a walk cycle are oriented in a constant direction (that is, the knees always point forward), the IK handles for the legs will only need translation.

However, if that same biped character needs to have a “bow-legged” or “knock-kneed” appearance or if the character is dancing, you might want to rotate the skeleton plane of both legs to achieve this effect. In this case, choose the Plane Rotation setting for those handles in the Information Window, and adjust the Plane Rotate fields accordingly.

Using the plane/pole rotation setting

You will need to adjust the rotation of the Pole Axis, through the “Plane/Pole Rotation” setting, only if the skeleton chain is to be animated through an extremely wide range of motion in the scene. Specifically, if the limb axis of the chain comes near the Pole Axis, an undesirable “flipping” of the chain may occur; generally, you will want to move/animate the Pole Axis so that the handle’s limb axis does not come too close to it during a given scene.

When you create a single-chain IK handle on a chain, a Pole Axis is created perpendicular to the skeleton plane in its rest position. As long as the chain is not contorted very far from this position, and the limb axis of the chain doesn’t approach the Pole Axis, this default position of the Pole Axis is suitable for animating the skeleton. Otherwise, adjusting or animating the Pole Axis may be necessary to avoid flipping in the chain.

Complex single-chain example

Below is an example of two torsos and how different types of arm movements are achieved through POLE/PLANE ROTATION.

In this example, the two torsos are identical and both have IK handles on the left arms. The default value for Plane rotate is 90 degrees. The default value for Pole rotate Y and Z is zero degrees. The left torso requires a drinking motion, while the right torso will perform a jumping motion.

1. For the drinker, no pole rotation is required, so Pole rotate Y and Pole rotate Z are zero. Plane rotate is kept at 90 degrees.
2. For the jumping torso, change the Plane rotate to zero, the Pole rotate Y to -90 and the Pole rotate Z to zero.
3. With the IK handles picked, choose Transform > Move to drag the arms up to simulate a drinking motion.
   
4. Open the Information window.
   
5. Continue to move the arms up to create the jumping motion for the right torso. At some point, you’ll see the left arm “flip” a little, when the arm’s limb axis crosses over the Pole axis. The right arm doesn’t flip because the Pole Axis points forward, and therefore the arm’s limb axis never approaches it.
   

Limitations

• World orientation does not work on a Single Chain handle with non-proportional scale.

  If the root joint of a Single Chain handle inherits a non-proportional scale, Worldspace Orientation for the plane will not work correctly.

• If the joint angle animation output for Single Chain IK jumps from -180 or 180 degrees (or from 180 to -180 degrees) from one frame to the next, motion blur will not work properly.

  Use the motion blur compensation option in Run IK. This will insert additional keyframes at the motion blur sample points near the jumps.

• Models with constraints or expression animation on joints with IK might play back with inconsistent motion.

Turn IK handle display on or off

How to control which IK handles are displayed.

To display IK handles

Use Animation > Show > IK Handles to turn the IK Handle display on or off.

To enable or disable IK handles

Turns on or off IK handles so that they become enabled or disabled.

You can do the following:

• toggle states of all IK handles without picking any of them.
• set a group of active IK handles to be the same state.
• change only the selected types of IK handles (for example, only single-chain).
• change only the IK handles of your picked skeletons.

When an IK handle is OFF, it behaves as if it does not exist. You can not select it. An individual IK handle can be turned ON or OFF from the Information window.

1. Pick the IK handles (if you're using the Active option).
2. Choose Animation > IK > IK Handles On/Off to set their states (ON or OFF according to the options' setup.)

Making a mirror copy

Edit > Duplicate > Mirror enables you to create a symmetrical copy of hierarchies according to an axis plane.

This works for skeletons as well as ordinary hierarchies and mirrors IK handles, constraints, rest poses, limits, animation, and selection handles.

With this function, you can do the following:

• Create real nodes for mirrored hierarchies, including the geometry.

  (By contrast, Layers > Symmetry makes the symmetry by instances, so the geometry is not really duplicated.)

• Create a mirror copy starting from any level of a hierarchy, so that not only geometry, but also the complete hierarchy, animation controlling IK handles and constraints, and joint attributes are mirrored across the specified axis plane.

  (By contrast, while Layers > Symmetry lets you use an arbitrary plane of symmetry, only the geometry is made symmetric-that is, only the leaf level is affected.

  Tip

  To make the best use of Mirror, create your character at the origin and then transform the character (and do any cluster attachment) only after mirroring is complete.

Select Edit > Duplicate > Mirror to open the Mirror Options box. From there, you can select two types of mirroring and three mirroring planes.

Mirror Type: Duplicate

Lets you pick a branch as the source branch. The tool duplicates the branch and mirrors its nodes on the new branch.

1. Choose Edit > Duplicate > Mirror.
2. Pick a branch of a hierarchy.

   A new branch is created as the sibling of the picked one. The new branch is mirrored across the plane defined in the option box (Mirror Across settings). All IK handles, constraints, animations, and skeletons on the branch are also duplicated and mirrored.

   Note

   You can also pick the branch and then select the Mirror item.

Mirror Type: Reshape

Lets you pick an existing branch as the destination (rather than have the tool create it). Geometries are not mirrored for this option.

1. Choose Edit > Duplicate > Mirror.
2. Pick the branch that you want to reshape.
3. Pick another branch that you want to be the source of the mirror.

   The first branch is reshaped as a mirror of the second branch. All IK handles, constraints, animations, and skeletons on the first branch are replaced by new objects copied and mirrored according to the second branch.

Mirror Across

Mirror provides three mirroring planes under world space. This option specifies the plane to use for later mirror operations.

1. Choose Edit > Duplicate > Mirror to open the Mirror Options box.
2. Choose the plane to use: XY, YZ or XZ.

Run IK to render your animation

Animation > IK > Run IK creates an animation using multi-chain IK handles attached to a skeleton so that the animation may be rendered.

1. Create a skeleton using Animation > IK > New Skeleton.
2. Choose Animation > IK > Add IK Handle to attach IK handles to various joints of the skeleton.
3. Animate the IK handles using any of the Alias animation tools.
4. Set up your skeleton to be in the configuration that you want it to be in for the first frame of animation.
5. Pick the root of the skeleton and choose Animation > IK > Run IK.

You can view the skeleton animation through all the frames of the constraint animation. At the end of this operation, the skeleton is animated using its IK handles.