Making the Choice between IK and FK
Your decision as to whether to use IK or FK should be determined by how you like to work, and by the requirements of the performance you’re trying to create. As a general rule, we always use IK for legs, unless the character will not be using his or her feet to contact the ground. As for arms, this comes down to a personal decision. We know many great animators who use FK for arms, and many others who use IK. As another general rule, though, if the character’s hand is going to come in contact with an object in the scene, it will be much more efficient to use IK. More often than not, we do have a character interacting with something in the scene. Therefore, we tend to use IK for the arms more than FK. Go with what works best for you.

It is possible to switch between IK and FK during an animation. We’ll discuss how to do so in Chapter 16.

[Figure 21] Setting preferred angle through the right-click menu.

Types of IK Solvers
Maya includes three main types of IK handles:

Rotation pole (RP). The RP has the ability to control the twisting direction of the chain that it is controlling. This solver is good for a chain such as thearm, where the elbow joint needs to be positioned (pointed) when you move the wrist. You’ll be using this later in the book for the arms, legs, and head of a character rig.

Single chain (SC). The SC solver does not have twisting control, and is more useful for simple connections, such as what you will use later to control the lower areas of the foot, which doesn’t need the twisting controls.

Spline. The spline IK solver uses a curve to control the joint chain. This can be useful when working with a tail or a spine. In Chapter 15, you will be using a spline IK solver to control the spine of the character.

When you build a chain that will be controlled with IK, the IK handle must be told in which direction to bend a joint. If you were to build an arm with the joints in a straight line and then place an IK handle, the joint would be rigid because the IKhandle would not know in which direction you wanted it to bend. There are two solutions for this problem. If your joints must be in a straight line at the default pose (when the IK is placed), you can define the direction of the preferred angle to bend. In order to do this, bend the joint in the direction you want, right-click on the joint, choose Set Preferred Angle (see Figure 21), and then return the joint to the previous (straight) angle of rotation. This will give the IK a direction to choose. The other solution is to put a small bend in the joint chain at the default pose. The IK handle will not allow the joint to bend backward with an RP or SC solver, no matter which method you use.

We will go into detail on the spline IK later in Chapter 13, when we’re building the spine for our character rig. One important limitation to be aware of is that you cannot set any limits on a joint controlled by a spline IK handle.

[Figure 22] Two images of a skeletal rig with iconic controllers and just joints.

Control Systems
You have a variety of options available to you when deciding how a character’s nodes will be selected. The two main methods on which we’ll focus later in the book are direct selection of the individual nodes and indirect selection of control curves (see Figure 22). These control curves will give you an intuitive layout of icons attached to your character rigs, allowing you to, say, select a curve near the shoulder and control the shoulder. These iconic controllers can be a huge boost to workflow when animating.

Another system that will help with the general workflow is creating layers to easily hide and unhide groups of objects. Creating attributes to control visibility or resolution of objects will also be presented later in the book (in Chapters 15 and 16). The main reason to create control systems is to make the animator’s job easier by reducing the amount of hunting it takes to find, say, the second knuckle of the left index finger, when he wants to set a key. Your goal is to make it easy, intuitive, and fast for the animators to do their jobs.

Summary
Working with joints, bones, and FK and IK can be a bit confusing at first. Once you experiment with your software you’ll begin to see the mechanical logic of the system. The information presented in this chapter is meant to be read while working on the computer and experimenting with the tools. You may find that after you have completed the exercises that appear later in the book, returning to this chapter will have great benefits.

To learn more about skeletons and kinematics, Smooth Skinning deformers, the process of analyzing storyboards and other topics of interest to animators, check out Inspired 3D Character Setup by Michael Ford and Alan Lehman, series edited by Kyle Clark and Michael Ford. Boston, MA: Premier Press, 2002. 268 pages with illustrations. ISBN: 1-931841-51-9 ($59.99). Read more about all four titles in the Inspired series and check back to VFXWorld frequently to read new excerpts.

Author Alan Lehman (left), series editor and author Michael Ford (center) and series editor Kyle Clark (right).

Author Alan Lehman, an alumnus of the Architecture School at Pratt Institute, is currently a technical animator at Sony Pictures Imageworks, as well as a directed studies advisor in the Animation Studies Program at USC's School of Cinema-Television.

Series editor and author Michael Ford is a senior technical animator at Sony Pictures Imageworks and co-founder of Animation Foundation. A graduate of UCLA’s School of Design, he has since worked on numerous feature and commercial projects at ILM, Centropolis FX and Digital Magic. He has lectured at the UCLA School of Design, USC, DeAnza College and San Francisco Academy of Art College.

Series editor Kyle Clark is a lead animator at Microsoft's Digital Anvil Studios and co-founder of Animation Foundation. He majored in Film, Video and Computer Animation at USC and has since worked on a number of feature, commercial and game projects. He has also taught at various schools including San Francisco Academy of Art College, San Francisco State University, UCLA School of Design and Texas A&M University.