IK

IK

You have probably heard the term inverse kinematics (IK) before; it has become quite a buzzword in the 3D world, particularly when it comes to character animation. The main difference from the forward kinematics (FK) is the direction in which something is animated. Take an arm, for example.

Using FK, the arm would be animated from the shoulder downwards. This means that you first rotate the shoulder, then the upper arm, the lower arm, hand and finally the fingers pointing to a specific location.

The obvious disadvantage is the difficulty of trying to get the hand in a certain position in your 3D world, since any move of the shoulder will require readjustments to all the limbs that follow in the hierarchy.

Inverse kinematics offers an elegant solution to this problem. As the name suggests, the direction of the animation is inverted. This means that moving the hand will force the other joints between the hand and shoulder to reposition in order to remain in contact with the hand.

With the IK tag in Cinema 4D, it's never been easier to create stable, dynamic IK chains, where the same goal object position always gives the same result. But don't take our word for it. Test it for yourself!:

• Start a new scene.
• Choose the joint tool, set IK Solver to 2D and Ctrl-click three times in the viewport to create three joints. Make sure the joints are not all in a straight line.
• Choose the move tool (shortcut: E) and move the already selected goal object back and forth. Move the goal around in a circle around the chain's origin. Impressed?
If you're still not convinced, repeat the steps above but this time add five or six joints.

Although the IK tag and joints have detailed settings that you can adjust to fine-tune the chain's behavior, you'll find that this simple setup is perfectly adequate in most cases. There's no need here to set limits, but if you want to, you can. And there's no need to add a pole vector (although again, this is possible), because the IK tag has an internal pole vector that in many cases is actually more flexible than an external one. To test this, select the root null for your IK chain in the viewport and change its H angle. As you can see from this example, creating and starting to animate a stable rig is as easy to do as it is fast to set up.

Although the following text focuses on using the IK tag with joints, you can also use the tag with other types of object (joints do, however, have a number of additional options for solving the IK chain). In particular, you can use the tag directly on a point object's points (where a point object is an object containing points, such as a spline). If you add an IK tag to a point object, you'll see an additional option appear, Point IK, as well as two fields where you can select the first and last points you want to use. You can then animate the spline's points directly using an IK chain.

Keep in mind, when planning your IK chains, that null objects can be integrated into the joints hierarchy without causing problems. This wasn't the case previously with the old bones system. Unlike the old bones, joints are not deformers and hence their influence is not broken by nulls.