Building the Skeleton

by Brian Immel

General rules about creating skeletons for biped characters	Building the Skeleton	Joint Orientation	Inverse Kinematic Systems	Main
Torso	Legs	Arms	Hands	Head/neck

Torso

1. Starting with the Root joint, create 3-4 joints for the belly region and one joint for the upper chest region.

2. When creating these ellipsoidal joints, you want to start with the root location and work in an upward direction.

3. We use 3-4 joints for the belly region because the majority of the torso movement comes from this area due to the physics of the rib cage and lack of bones in the lower torso region. Since the upper torso does not move all that much (just skin pulling over the rib cage) we only really need one joint to skin these vertices.

4. You should add one more joint above the chest1 joint to serve as the shoulder and neck base joint.

Adding a ‘shoulder base’ will help you later on when skinning the lower neck and inner shoulder regions.

5. Use a good naming convention for the torso joints such as belly1, belly2, belly3, and chest1. Figure 3 suggests where and what you should name your joints.

Legs

1. Start the joint creation process from the side view of the character. Place the joints of the legs according to Figure 4. The main reason you want to work in the side view is to keep the joints in a start alignment. Angling (not straight up and down or side to side) the joints is usually not a good idea. Alignment for the leg is important because the FK joints will be connected by an IK system. We’ll cover that later on in the section called IK Legs.

2. When creating the joints for the leg, start with the hip and work in a downward direction.

First joint will be the hip. This ball joint should be placed in the center of the geometry where the leg meets the lower groin region. This joint handles the major rotations of the entire leg.
Second joint is the knee. Place directly center of the geometry for the knee region. This hinge joint handles the forward and backward rotations of the knee.
The third joint is the ankle. Place this hinge joint in the center of the geometry for the ankle region. This joint should not be confused with the heel joint.
The fourth joint is the heel joint. This hinge joint is placed at the rear of the foot geometry. I use this joint to handle the rotation of the foot when performing a heel pivoting motion. Some animators skip over this joint and go straight from the ankle to the ball joint. This is fine as long you don’t plan on using a more complex system for animating the foot.
The fifth joint is the ball joint. Place this hinge joint in the center of the foot (exact averaged distance from toe to heel). This joint is responsible for bending the foot upper when picking the foot up.
The final joint is the toe joint. Place this non-moving joint at the very tip of the foot where the toes are. This joint has no bending responsibilities but it is useful in some situations for better skin weighting of the foot.

3. After you have created this joints, select the Hip joint and move it to the relative center of the leg geometry. Its okay to move some of the joints to be center of the geometry but if you angle the joints too much, be warned that this may cause problems later on.

4. After you have created the leg from the hip down, create another joint directly below the Root joint. Name this joint center or hip center or something like this idea. Although, as humans, we don’t have a center hip joint, this joint will serve as the center of rotation for the hips. Without this ‘central joint’, your character will not be able to swivel his/her hips.

5. Name your joints accordingly from the hip down:

Hip
Knee
Ankle
Ball
Toe
Keep in mind you will need to add the suffix left (_L) or right (_R) according to which side of the character you are working with.

6. Create the ‘central joint’. Attach the Hip joint to the Central joint.

7. The Central joint is attached to the Root joint.

8. Instead of repeating all these steps to create the other leg, we can mirror what we have created and automatically rename the joints with the proper suffix.

To mirror a set of joints, select the joint branch that you wish to mirror. In this case, select the hip joint.
Go to Skeleton > Mirror Joint Option Box.
In the Mirror Across options, select YZ (if you have set up your character in the fore mentioned position).
Set Mirror Function to Behavior.
In the section Replacement names for duplicated joints: Search for: should list the suffix, which you used on the current leg and Replace With: should list the suffix, which did not use on the leg we are about to create.
Hit the Mirror button and watch the magic.

Arms

1. The arm branch is typically built out of five joints: clavicle, shoulder, elbow, forearm, and wrist. See Figure 6 for a typical layout. This layout will easily support both FK and/or IK setups in any version of Maya.

2. Clavicle

Start by placing the first joint of the arms near the base of the neck. This first joint will be the Clavicle joint.
The clavicle helps the shoulder with motions that raise the arm, moving the arm forward and backward, and adds the possibility of shrugging.
This ellipsoid joint should be place near the front of the geometry rather than the center of the inner shoulder region.

3. Shoulder

The next joint will be the shoulder joint. Center this ball joint at the base of the geometry where the arm meets the shoulder.
This joint has the primary responsibility of rotating the entire arm. This rotating motion is coupled with the movements of the clavicle.

4. Elbow

Down from the shoulder joint is the elbow joint. This hinge joint has the primary responsibility of bending the geometry around the elbow region. Place this joint in the center of the geometry.
This joint will have limited movement because it acts a lot like a hinge.
When drawing this joint, don’t place it straight out from the shoulder. Place that so that there is a slight bend backwards in the entire arm when you finish drawing the joints. This will help with usage of IK joints later on.

5. Forearm

Next is the forearm joint. Although we humans don’t have a ‘forearm’ joint, we do have to bones that orbit one another to give us the ability to rotate the forearm region. This joint is a pivot type joint.
Lock the rotations of all the axis of the forearm joint except for the axis that allows the twisting motion in the forearm region. If you started creating your character in a standard position (Z front, X right/left, Y up), then you will want to lock the Y and Z Rotate attributes of this joint. The main reason you want to lock these rotation attributes to help control the joint from bending if you use an IK joint on the arms.

6. Wrist

The next joint we need to create for the arm is the wrist joint. This joint has the primarily responsibility of rotating the hand in every rotation except for a twisting motion (last time I checked, I couldn’t twist my wrist). The wrist joint is not a hinge or an ellipsoid joint but rather a double hinge type because it can rotate on two axis only.
The wrist joint can move up, down, forward, and backward but with some limitations according to human anatomy. Since our biped is based on human standard, you can either choose to ignore the wrist’s limitations or use a forward/backward rotational limitation of about 45 degrees.
Use the Limit Information options under the Attribute Editor for the wrist joint to set the rotational limits.
The wrist joint should be placed in the center of the geometry where the palm meets the lower region of the arm.

Hands

1. My typical handset consists of 16 joints: one palm joint and three joints for each finger (thumb, index, middle, ring, and pinky). In Figure 7, I used a cartoon hand (four fingers instead of five) and added an extra joint for closing each finger branch. I know this illustration is mixing reality, but the fingers have the right number of joints according to our real anatomy.

2. Palm joint

As human beings, we have a quite a few bones and muscles in the palm region of our hands. To simplify the process of creating skeletons for our hands, we will use just one joint for the palm.
The palm joint is a non-moving joint. This joint has two purposes: 1- to help skin weight as needed for the wrist and knuckle joints. 2- Add an extra location for future IK solutions for arm controls.
The palm joint should be placed relatively center of the palm geometry.

3. From the Palm joint, place joints according to the bend locations of the geometry for each finger. The following set up uses only three joints instead of four, as we humans would normally have. Why not use four? More work to skin, another joint to animate and the results would be the same (in this case) with three to close the fingers.

4. All finger joints are ellipsoid joints. I usually set up my finger joints with the ability to rotate in (to close the hand) and out (to open beyond the straight position). The reason the finger joints are ellipsoids is because you can spread your knuckle joints in and out to widen or narrow the shape of the hand. However, you may not want to include the spreading ability of your fingers because the animation does not call for this type of animation possibilities. In which case, the finger joints will act like simple hinges.

5. Every finger branch we will create will have three joints: 1-knuckle, 2-middle, 3-tip.

The knuckle joint is placed at the base of the finger where the palm meets the lower finger region. This joint is responsible for the upper finger bends.
The middle joint is located in the middle of the finger geometry. This joint is responsible for the middle finger bending.
The tip joint is placed at the very end of the finger geometry. This joint has to rotation responsibilities but it does play a small roll in making skinning the finger a little easier.
Sometimes, you may have more than three joints for your fingers. Therefore, you will need to name that extra joint accordingly. Again, use anatomy as a reference for where to place and what you should name these joints.

6. Under normal anatomical situations, we have five fingers: Thumb, Index, Middle, Ring, and Pinky. If you wish to follow the rules of cartoons, then you should not use the Ring branch for the fingers.

7. Thumb

The thumb joint knuckle is an ellipsoid joint type. The thumb is sometimes a rather difficult joint to setup because how it moves. It more or less has the motion of a ball joint but it cannot rotate completely backwards.
The thumb joint is placed on the inside of the body (points inward). When the hands are placed side-by-side flat on a table, the thumbs will point towards each, not away from one another. See Figure 8.

8. Index

The index branch is also referred as the pointing finger.
This branch is the first set in on the palm.
This branch sits between the thumb and middle branches.

9. Middle

This branch usually has the greatest distance between each joint due to the length of the middle finger geometry.
This branch is placed next to the index branch away from the thumb branch.

10. Ring

This branch sits next to the middle branch away from the index branch.

11. Pinky

This branch sits at the end of the palm next to the ring branch.

12. Once we have the entire arm and hand completed, we can use the Mirror Joint tool save ourselves some time.

Select the Clavicle joint and hit Skeleton > Mirror Joint. Use the same rules we did before for mirroring the legs (suffix naming and mirroring options).

Head/Neck

1. For Oscar Jr. as illustrated in Figure 9, I only created three joints to handle the motion of the head: Neck joint, top of head joint, and a chin joint.

2. Create at least one joint for the neck. I like using two joint just to give the neck region a little more flexible but that again depends on the length of the neck and intended range of motion.

3. Name these joints Neck1 and Neck2.

4. Neck1 and Neck2 joints are ellipsoid type joints.

5. Place another joint next the top rear of the head. This ‘top head’ joint serve as the major weighting joint for the head geometry. Note: this 3-4 joint system for the head does not support facial animation very well. If you want a system that supports facial animation, you should consider what regions move and how they move and do some R&D design work with the skeleton system and weighting system for facial animation systems. There is so many ways to set this and it would take forever to talk about but we are just concerned with creating a system that will allow us to animate the character without facial animation at this point in this lesson.

6. Create another joint near the chin region. Some people may argue that this joint isn’t necessary and they may be right but I like having this joint there because I can weight the vertices of the chin region to give just a little extra emotional reaction to the character by making it appear to drop his jaw when he/she is surprised or scared. This joint is not 100% necessary in the grand scheme of the skeleton design.