In Figure 6.3, you can see the difference between an IK Move operation on a wrist joint using two bones (the forearm and upper arm) and the same IK Move operation performed with an IK set that goes all the way to the root:

Figure 6.3 Left, IK Move using two bones; right, IK Move using several bones

To perform an IK operation, you must specify which bones can move in which direction by creating an IK set. In the figure on the right in Figure 6.3, for example, the IK move used an IK set that allowed the chest bone to rotate around the Z axis.

Without a meaningful IK set, IK operations are not very useful. IK sets take a little time to set up, but are purposely left "open" to give you complete control over the behavior of the skeleton.

: Note. It's a good idea to save skeletons for which you've defined appropriate DOF limits and IK sets. That way, you can use the same skeleton over and over in different animations without having to define new DOF limits and IK sets each time.

Defining an IK Set

To perform an IK operation, you must define an IK set. An IK set is nothing more than a list of which bones can be rotated around which axes to complete an IK move.

As shown in Figure 6.2, there are different ways to pose a skeleton; when you drag a wrist joint, should the skeleton move only his forearm? Forearm and upperarm? Or can he move the torso as well? Around which axes? These questions are answered by setting up one or more IK sets to govern the possible solutions for an IK operation.

IK sets are very powerful; they control how a skeleton can be posed or animated using IK operations.

Try this:

1. Load the skeleton primitive.

2. (CLICK-L) on points in the element sensitivity menu.

3. (SHIFT-L) on the wrist joint of the skeleton.

4. (CLICK-L) on IK Move.

The following dialog box appears:

Figure 6.4 The IK set dialog box

There are three sections in the IK set dialog box:

On the left is a list of bones between the selected joint and the root.

In the middle is a grid showing which axes the named bone can rotate in to perform the IK operation.

The right section shows the order in which the axes are rotated to execute the IK operation. (This is most significant when animating a skeleton using a dynamic IK operation.)

5. In the middle section, (DRAG-L) the mouse over the X, Y, and Z box next to the TopHip bone and the X and Z box next to the Chest bone:

You can (CLICK-L) and drag the mouse to toggle the state of individual boxes on the dialog box, or (CLICK-L) on them individually. The boxes named above should be grayed out:

Figure 6.5 Changing the degrees of freedom for the IK set

6. (CLICK-L) on Done.

The following dialog box appears:

Figure 6.6 Saving the IK set

The SAS generates a default name for the IK set, which includes the joint for which it was defined.
How far the bone can rotate around each of those axes is determined by the DOF limits defined for each of those bones. You can view the DOF limits using the DOF Editor.

7. (CLICK-L) on Done to save the IK set.

8. Move the mouse around.

You may find the skeleton a little "jumpy" or hard to control. That's OK for now; notice, however, that only the bones included in the IK set move. This is most obvious if you look at the chest bone, which only rotates around its own Y axis, regardless of where you move the mouse.

Figure 6.7 An IK Move using a "loose" IK set makes it hard to pose the skeleton

The IK set is too "loose;" that is, it contains too many degrees of freedom to pose the skeleton with any level of control.

Improving the IK Set

To make the IK Move operation work better, you need to define a better IK set:

9. (CLICK-L) on bodies in the element sensitivity menu.

10. (CLICK-L) on the skeleton.

11. (CLICK-L) on Base.

The following dialog box appears:

Figure 6.8 Return the skeleton to its base position

12. (CLICK-L) on Done.

This returns the skeleton to its base position.

13. (CLICK-L) on points in the element sensitivity menu.

14. (SHIFT-L) on the wrist joint again.

15. (CLICK-L) on IK Set.

A menu listing any IK sets defined for the left wrist appears:

Figure 6.9 Selecting a IK set to edit

This lets you select an IK set to edit.

: Note. You can have more than one IK set associated with any given joint; however, you must specify which set is to be used to perform an IK operation, either in N-Geometry or N-Dynamics.

16. (CLICK-L) on the IK set you saved in step 6 above.

The IK set dialog box opens, showing the selected IK set:

Figure 6.10 Editing an existing IK set

Let's remove some additional degrees of freedom to make the skeleton behave better.

Turn off the Y rotation for the LeftUpperArm.
Turn off all rotations for the LeftCollar bone.

Your IK set should look like Figure 6.11 after you've made these changes:

Figure 6.11 An improved IK set

17. (CLICK-L) on Done to save the modified IK set.

18. (SHIFT-L) on the wrist joint again.

19. (CLICK-L) on IK Move and pose the skeleton again.

The modified IK set now controls the wrist joint: the chest rotates around its own Y axis, the collar bone doesn't rotate at all, and the upper arm and forearm rotate in a much more manageable manner:

Figure 6.12 Posing a skeleton with a "tighter" IK set

Joint Type	Usually Controls	X DOF Limits	Y DOF Limits	Z DOF Limits
None*	No default	0 0	0 0	0 0
Spine Joint	Spine	-10 10	-7 7	-7 7
Neck Joint	Head	-15 37	-80 80	-15 15
Jaw	Jaw	-5 40	0 0	0 0
Shoulder	Upper Arm	-170 40	-90 15	-5 120
Elbow	Forearm	-150 0	0 0	0 0
Wrist	Hand	-20 20	-90 10	-50 50
Knuckle	Finger	-10 10	0 0	-10 90
Finger Joint	Finger part	0 0	0 0	0 60
Hip Joint	Femur	-120 10	-10 75	-5 45
Knee	Tibia	0 150	0 0	0 0
Ankle	Foot	-30 60	-20 20	-10 10
Heel	Heel	0 0	0 0	0 60
Ball of Foot	Toes	-10 80	0 0	0 0

* The joint type of None lets you remove a joint identification and the saved DOF limits for the selected bone.

Using the DOF Editor

In all the examples of kinematic poses above, we were able to move the individual bones without limitation. However, in order to get the maximum value out of a skeleton, one needs to be able to define certain constraints, or limits for how bones can move.

By properly defining DOF limits for a skeleton, you can better limit how a skeleton can behave during IK moves. Let's take a look at the skeleton primitive, which already has some DOF limits defined:

1. Load the skeleton primitive.

2. (CLICK-L) on bodies in the element sensitivity menu.

3. (SHIFT-L) on the skeleton.

4. (CLICK-L) on DOF Editor.

The DOF Editor appears:

Figure 6.15 The DOF Editor

The DOF editor lets you define the limits for any bone on the skeleton. For each bone you can define:

rotation order (used when animating)
the minimum and maximum x, y, and z rotations
the minimum and maximum bone scale

(Meaningful limits may or may not be included with a skeleton read in as part of a motion capture data file.)

If you're building your own skeleton, you should define the constraints for each bone before setting up any poses or trying to perform any IK operations. There are two steps to defining limits for a bone:

defining the axes around which a bone can rotate

defining how many degrees a bone can rotate around those axes

Selecting Rotation Axes

Try this:

1. (CLICK-L) on points in the element sensitivity menu.

2. (SHIFT-L) on the skeleton's left wrist joint.

3. (CLICK-M) on IK Move 2D.

4. Move the mouse left and right.

This normally tries to perform an IK move by rotating the two bones above the wrist joint around their X axes , as shown below:

Figure 6.16 Performing the same IK Move 2D with limits for the upper arm turned on

Suppose, however, that you want to pose the skeleton without moving the upper arm bone at all? How do you lock it into position?

5. (SHIFT-L) on the selected skeleton or a single bone.

If you select the entire skeleton, you can modify the limits for one or all bones at the same time.

6. (CLICK-L) on DOF editor.

7. Locate the bone whose limits you want to set.

For this example, lets use the bone "LeftUpperArm."

8. (CLICK-L) on the text box under RotOrder next to the bone.

This shows the planes in which movement is allowed; if you loaded the default skeleton, the axes "Z X Y" should appear in this box. In the dialog box that appears, you can (CLICK-L) to highlight which planes a skeleton should be able to move in:

Figure 6.17 Selecting rotation axes

9. (CLICK-L) on the X plane.

10. (CLICK-L) on Do It.

11. (CLICK-L) on Save Changes.

This saves the changes with the skeleton.

12. (CLICK-L) on Yes when prompted to save changes:

Figure 6.18 Saving limit changes

13. (SHIFT-L) on the skeleton's left wrist joint.

14. (CLICK-M) on IK Move 2D.

15. Move the mouse left and right.

Now, when you perform the IK move, only the forearm moves, because the upper arm has been restricted from rotating around its own X axis:

Figure 6.19 Setting limits for a bone; note that the left upper arm bone does not move, compared to Figure 6.16

Limiting Range of Motion for a Selected Bone

In the previous section, we learned how to completely restrict the rotation of a bone in one plane. What if we want to partially limit rotation of the bone in one plane? Using the DOF Editor again, we can specify an appropriate range of motion for any bone in any axis.

We can use the skeleton from the previous exercise to try this out:

1. Open the DOF Editor again.

2. Locate the bone whose rotations you want to limit.

Let's use the bone "LeftUpperArm" again. If you're using the skeleton from the previous exercise (the skeleton primitive), the values -170.0 and 40.0 should appear under the X Min/Max label.

3. (CLICK-M) on the text box to clear the values.

4. Enter the following text in the box:

-20 20

This specifies the the upper arm can move only twenty degrees in either direction.

5. (SHIFT-L) on the skeleton's left wrist joint.

6. (CLICK-M) on IK Move 2D.

7. Move the mouse left and right.

Now, when you perform the IK move, both the upper arm and forearm bones move; however, note that the upper arm is severely restricted:

Figure 6.20 Limiting range of motion

You can specify a maximum range of motion for any bone; in this case, the upper arm is limited to a range of -20 to 20 degrees; no matter how you move the wrist joint, the upper arm does not move beyond 20 degrees

Locking Bones

When posing complicated parts of a skeleton (such as a hand) where you want to rotate or pose several bones at once in the same manner, you can "lock" a bone to one of its siblings.

Sibling bones come from the same joint, giving them the same relationship to each other with respect to the root.

We'll show you what we mean using a sample object:

1. Load the following object:

/usr/local/ngc/demo/objects/hand.geo

The hand looks like this:

Figure 6.21 Sample hand bone (with sibling bones)

2. (CLICK-L) on segments in the element sensitivity menu.

Move the cursor over the various bones; note that we've named all the bones attached to the wrist joint index_1, middle_1, and so on. These are all sibling bones-they are attached to the same joint, and have the same orientation and distance from the root.

3. (SHIFT-L) on the index_1 bone.

4. (CLICK-L) on Lock Bone.

A list of sibling bones for the selected bone is displayed.

5. (SHIFT-L) on middle_1.

6. (CLICK-L) on XYZ Rotate.

Move the mouse left and righ. The bones move in unison, as if they were locked together:

Figure 6.22 Locked bones move in unison

You can use the Lock Bone command to move several bones the same amount without having to line up each bone over and over. It's possible to lock a bone to more than one other bone; however, in an example like the hand above, it's probably easier to pick one bone you want to attach the other sibling bones to rather than locking them in a chain (each to the bone next to it).

The lock bone feature is particularly powerful when used with the Pose Editor, described below.

Unlocking a Bone

To unlock a bone that has been locked:

1. (SHIFT-L) on the bone.

2. (CLICK-M) on Lock Bone.

3. Select the bone(s) you want to unlock this bone from.

Saving Poses

Once skeletons are in the position you want, you can save the skeleton as a pose. By creating a number of poses, you can generate an animation.

Let's create and save several poses for a skeleton:

1. Load the skeleton primitive.

2. (SHIFT-L) on the left upper arm.

3. (CLICK-R) on XYZ Rotate.

Raise the skeleton's arm above its head, then repeat the process with the right arm, so that it looks like this:

Figure 6.23 Pose the skeleton

4. Select bodies from the element sensitivity menu.

5. (SHIFT-L) on the skeleton.

6. (CLICK-M) on Pose.

You want to save the current position as a pose. The following dialog box appears:

Figure 6.24 Saving a pose

7. (CLICK-M) on the text edit box next to Pose name and enter a more descriptive name for the pose.

Figure 6.25 Naming a pose

8. (CLICK-L) on Done after you've named the pose.

9. (SHIFT-L) on the skeleton.

10. (CLICK-L) on Base.

The following dialog box appears:

Figure 6.26 Saving a pose

11. (CLICK-L) on Done.

The skeleton returns to its original pose.

: Note. A skeleton generated from the N-Geometry menu (or by reading in a motion capture file) has a "base state" associated with it.

12. (SHIFT-L) on the skeleton again.

13. (CLICK-L) on Pose.

A menu appears, listing any poses that have been saved with the currently selected skeleton.

Figure 6.27 Selecting a pose

14. (CLICK-L) on arms-up.

15. Drag the mouse slowly from left to right.

The skeleton goes from its current position toward the named pose; if you continue to move the mouse, the skeleton continues to move beyond the saved pose (extrapolating as you continue to move the mouse).

16. (CLICK-L) to move the skeleton to its new position, or (CLICK-R) to abort the changes.

This process can be repeated over and over to save multiple poses, which can be used later when animating the skeleton.

The various pose operations are described in more detail in the SAS Reference Manual.

Viewing Saved Poses

Moving the Skeleton to a Saved Pose

To move the skeleton to a saved pose:

1. (SHIFT-L) on the skeleton.

2. (CLICK-L) on Pose.

A menu with any saved poses for the skeleton is displayed:

Figure 6.28 Moving the skeleton to a selected pose

3. (CLICK-L) on the pose you want to move the skeleton toward.

4. Move the mouse left to right to animate the skeleton toward the selected pose.

(SHIFT-L) animates the skeleton to the selected pose but does not update the attached skins.

(CTRL-L) on Pose to go directly to a pose. Specify a value of 1.0 in the dialog box that appears to view the actual saved pose; a value between 0 and 1 specifies a point between the base state and the pose.

A Sample Skeleton with Saved Poses

A skeleton with three poses has been saved in the following location:

/usr/local/ngc/demo/objects/skeleton-3-poses.geo

If you'd like, load this skeleton and try some of the variations on the Pose command.

Aligning Bone Axes

Each bone has a local axis associated with it, and that each axis is color coded:

Table 6.2 Axis colors for bones

Axis Color
x

red

y

green

z

yellow

Axis	Color
x	red
y	green
z	yellow

When animating, it's typically desirable that all the axes on the same limb have the same local axes from the root to the end of the limb. However, it's possible to build a skeleton in such a way that some limbs have local axes aligned differently.

If you build your skeletons correctly, or if you import premade skeletons, you probably won't need to modify the rotation axes of any bones.

"Mirroring" Bones

Realigning the axes is also useful if you want to pose corresponding bones on both sides of a skeleton at the same time (for example, if you wanted to define poses for a bird where you moved the wings in unison).

: Note. Changing the rotation axes for bones on skeletons that have been read in as part of a motion capture data file can cause the motion data to be applied "incorrectly" when animated.

Try this:

1. Load the skeleton primitive.