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### Terms and Concepts

If you're new to rigging, you may find some of the concepts a little difficult to grasp until you actually try them out. To start out, though, you still need a basic understanding of a few terms.

#### Coordinate Systems

When you move, rotate, or scale an object, you use a set of XYZ axes that appear in the viewports. The directions in which these axes point represent the current reference coordinate system, which is simply a method of referring to locations and directions in 3D space. These axes exist solely to make it easier for you to transform (move, rotate, scale) the objects in your scene.

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The place where the X, Y, and Z axes meet is referred to as the axes' origin point, or 0,0,0 point.

In 3ds max, a number of coordinate systems are available for you to use at any time. Each one has a different way of determining which way the selected objects' axes will point. Changing the coordinate system doesn't change the objects in the scene in any way—it simply changes the axes displayed when you select an object.

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Each transform can have its own reference coordinate system. When you select a coordinate system, it is selected for the current transform only.

By default, the reference coordinate system is set to the View method. This method orients the XYZ axes depending on the active viewport. The X and Y axes are always perpendicular to the active viewport, except in Perspective or User viewports.

You can see the available coordinate systems on the Reference Coordinate System drop-down menu, which is located just to the right of the Select and Uniform Scale button on the main toolbar.

#### Local Axes

When you create an object, 3ds max assigns it a set of local coordinates or local axes. These axes rotate with the object.

To see an object's local axes, rotate it slightly, click Select and Move, and choose Local as the Reference Coordinate System. The directions of the local axes appear in viewports.

#### IK Chains

Recall that inverse kinematics (IK) allows you to control an entire chain by moving a child in the chain. For example, you'll be able to control the thigh and calf by moving one IK chain at the heel. An IK chain is a control object that allows you to manipulate bones with the IK method.

The IK chain is created as an object called IK Chain01 that appears as a set of crosshairs and a line connecting the starting and ending bones. You can select an IK chain like any other object by clicking the crosshairs, but the IK chain doesn't render.

An IK chain has special properties. When you select an IK chain, the crosshairs are the part that's really selected. The line that appears between the starting and ending bones is just a reference, so you can see where the chain starts and ends.

The IK chain isn't “linked” to the bones in the usual sense of linking, nor are the bones linked to the IK chain. The movement of the bones, however, depends on the animation of the IK chain.

You can create an IK chain by selecting the first bone you want to use in the chain, choosing Animation > IK Solvers > HI Solver, then clicking the last bone for the chain.

An IK chain is different from any other object in 3ds max. It's not like a box or a sphere, or even like bones. If you select an IK chain and go to the Modify panel, there's nothing there to modify. All the controls for an IK chain are on the Motion panel.

#### PRACTICE I: Creating an IK Chain

 1 Reset 3ds max. 2 Create a quick set of leg bones to use for practice. 3 Select the root of the chain, which is the thigh bone in this case. 4 Choose Animation > IK Solver > HI Solver, then click the foot bone. Just before you click, a dotted line appears from the thigh to the foot. After you click, the IK chain is created. You can see it as a line extending from the top of the thigh to the heel, and a set of crosshairs at the heel. 5 Use Select and Move to move the crosshairs. The foot moves with the crosshairs, and the knee bends to accommodate the motion. 6 Turn on Auto Key, and set keys for the crosshairs on different frames. Fun, isn't it? You could probably spend all day playing with this one leg. But there are more adventures in store, so let's move along. 7 Save your work as Practice_IkChain01.max.

Now that you have an idea of how an IK chain works, let's go over a few more of the concepts that go with it.

#### IK Solvers

When you moved the IK chain's crosshairs around, 3ds max actually moved the foot first, then calculated how the thigh and calf should rotate to accommodate the movement. It had to decide whether to bend the knee forward, backward, or sideways, and it had to figure out by how much to bend the knee to make the foot go where you placed it.

In other words, the IK chain had to “solve” the problem of how the knee should bend. These calculations are figured out by an IK solver. The result is called an IK solution.

The IK chain's crosshairs represent a goal for the IK solution. When you move the goal, 3ds max comes up with a solution for how to move the chain of bones so the end of the last bone in the IK chain can meet the goal.

#### The HI Solver

Recall that you set up the IK chain by choosing Animation > IK Solver > HI Solver. “IK solver” is a general name for the different types of IK chains you can set up and use.

The HI (history-independent) solver is the type of IK solver you'll use in this book. You can find a detailed explanation of its history and usage in the 3ds max documentation, accessible from the Help menu. Here, I'll simply state that it's the most versatile and useful one for character animation.

Part of the IK solution involves figuring out which way the bend will point. If you create the bones in a “bent” state, the HI solver will consider that the default bending direction. However, you can make the bend point in a different direction by changing the IK solver's swivel angle. To change the swivel angle, select the IK chain and go to the Motion panel. The Swivel Angle parameter is on the IK Solver Properties rollout.

To gain better control over a chain of bones, you can create more than one HI solver on a chain of bones. Multiple HI solvers on a single chain are necessary for most character rigs.

#### PRACTICE J: Creating Multiple IK Chains

Here, you'll practice creating multiple IK chains on the same bone chain.

##### Create a Toe IK Chain
 1 Load the file Practice_IkChain01.max that you created earlier, if it's not still on your screen. You can also load this file from the Practice folder on the CD. 2 In the Left viewport, zoom in on the leg structure so you can easily see all the bones. 3 Select the foot bone. 4 Choose Animation > IK Solvers > HI Solver, and click the nub at the end of the toe. Now there are two IK chains on the leg.
##### Move the IK Chains
 1 Use Select and Move to select the IK chain at the heel, and move it around. The toe stays put while the heel moves. It's OK if the toe bends in the wrong direction—we're just practicing right now. 2 Undo the movement of the heel that you just made. 3 Select the IK chain at the toe, and move it around. The toe moves all by itself. 4 Undo the movement of the toe that you just made.
##### Make the Entire Foot Move at Once

The way the IK chains are set up now, you'd have to select both IK chains to make the entire foot move at once. However, you can simplify things by linking one IK chain to another, so you only have to move one chain to move the entire foot.

 1 Select the heel IK chain. 2 On the main toolbar, click Select and Link. Click and drag from the heel IK chain to the toe IK chain. The heel IK chain is now linked to the toe IK chain. 3 Select and move the toe IK chain. Now the entire foot moves when you move the toe. 4 Undo any movements you just made. 5 Select and move the heel IK chain. The heel still moves independently of the toe. 6 Undo any movements you've made before continuing.
##### Control the Foot Roll

This is a better rig, but it still has a problem. Every time you move the heel, the toe bends or flexes. Animators call this type of bend a foot roll, since it's the type of “rolling” action a back foot makes just before coming off the ground to take a step. Foot roll is necessary when you're animating a walk or a run, but only when the foot is about to come off the ground. The foot usually doesn't make this motion when it is in the air.

You can still get some animation out of the rig you've just created, but it would make things unnecessarily difficult. Every time you wanted to raise the heel but not roll the foot, you'd have to be very careful to always keep the heel far enough away from the toe to prevent a mid-air foot roll.

Fortunately, there is a solution. You can prevent unwanted foot roll by adding yet another IK chain.

 1 Select the toe IK chain, and delete it. 2 Select the foot bone. Choose Animation > IK Solvers > HI Solver, and click the toe bone (not the nub, but the toe itself). You might wonder why you're making an IK chain that simply goes from one bone to the next. The answer is, because it makes it easier to control each part of the foot. Let's make another one. 3 Select the toe bone. Choose Animation > IK Solvers > HI Solver, and click the toe nub bone. If you try moving any of these IK chains, the foot motion won't be very good. You'll need to link them together before you can get any decent results. For the purposes of explanation, we'll call the IK chains the heel, middle, and toe IK chains, in that order. 4 Use Select and Link to link the heel IK chain to the middle IK chain. 5 Link the middle IK chain to the toe IK chain. 6 Select and move the toe IK chain. The entire foot moves. 7 Select the middle IK chain, and rotate it. The heel comes up off the ground, and the foot rolls only exactly as much as you want it to. TIP If you make a mess of the linking, select all the IK chains and click Unlink Selection. Then try linking again. This is an added bonus of linking IK chains together—you can rotate the child IK chain by rotating the parent. 8 Save the scene as Practice_IkChain02.max.

#### TUTORIAL R5: Creating IK Chains for the Legs

Now you're ready to create the IK chains for your character rig. You'll use the same setup you created in the previous practice exercise.

##### Load and Hide the Character Mesh
 1 Load the file CharRig06.max that you created in the previous chapter, or load it from the Rigs folder on the CD. You won't need the mesh for this part of the rigging process, so you can hide it. First you'll have to unfreeze it. 2 Right-click any viewport to display the Quad menu, and choose Unfreeze All. 3 From the Named Selection Sets drop-down menu, choose the Mesh set. 4 Right-click any viewport, and choose Hide Selection from the Quad menu. 5 Select and hide all the bones for the upper body, leaving just the legs and feet.
##### Create the IK Chains
 1 If your lower-right viewport displays a Perspective view, change it to a User view. You can do this quickly by activating the Perspective viewport and pressing [U] on the keyboard. 2 In the User viewport, use Arc Rotate to adjust the viewport to an angle where you can clearly see and select all the bones in the both legs. Zoom into the viewport if necessary. 3 Select the right calf. In the Left viewport, rotate the calf so it bends like a normal human knee. This bend will help the IK chain figure out which way the knee should bend. If you leave the bones straight when you create the IK chain, the knee might bend in any direction. 4 Select the right thigh bone. Choose Animation > IK Solvers > HI Solver and click the right foot bone. 5 Use the new IK chain to move the leg back into its original position. You can use the Left viewport to help you match the legs' positions. 6 Create two more IK chains: one from the right foot to the toe, and one from the toe to the nub. 7 Rotate the left calf to bend the knee, and repeat the same steps for the left leg to create three IK chains for that leg.
##### Name the IK Chains

IK chains can be named like any other object. Simply select the IK chain by clicking the crosshairs, then change the name on the Modify panel.

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Naming IK chains is just as important as naming bones.

 1 Go to the Modify panel. 2 Select and rename each IK chain to the names shown in the diagram. The middle IK chain is named IKBallL or IKBallR for the ball of the foot, which is the name for the pad just below the toes.