View Inverse Kinematics IK
Inverse Kinematics (IK) is the most natural form of keyframed anatomical limb motion in computer animation. IK is the closest thing resembling how we move as living creatures. Example: stand in front of a closed door, and reach for the door knob. As you did you, you likely just performed the action, without really thinking about it. You moved your hand to the knob. At no time did you have to plan out the rotations of your upper arm and lower arm to reach for the knob, you just did it. That, in a nutshell, is IK.
Forward Kinematics (FK) is the method of keyframed animation when IK is not used. As mentioned in the previous paragraph, FK is the device that requires the animator to rotate each joint in the limb individually, in order for the and of the limb to reach the goal, or in the case of our example, in order for the hand to reach the knob. The animator would be required to manually rotate and keyframe each joint until the goal was reached, and then refine the animation so that it looked natural. While FK is a more primitive way to animate, it does offer more control, with fewer issues thank IK, and FK is used for a majority of motion.
A joint chain can use either IK or FK for motion; the choice is really up to the riggers who created the control rig, and the animators who work with the rig (as well as specify the rig requirements). There are times where FK motion is preferred over IK, though most animators will use IK for legs, and many use both for the arms. Yes, a joint chain can be driven by both IK and FK, and the method in use can be smoothly blended between under keyframe control. So, you can have the best of both worlds, and use the best method as dictated by the action within the performance of the character.
Now there are some issues that come with the territory regarding IK. The two biggest are joint snapping or hyper extension of the joint (where the rig will suddenly snap, making the animation appear unnatural), and joint flipping (where the joint chain will literally flip upside down in orientation after it reaches a certain threshold.) Fortunately, most riggers have methods to minimize these issues, and there are tools within Cinema 4D to help with that.
As you can see, there is quite a bit of jargon in computer animation, and you may be right to question why. Suffice it to say, you don’t really need to know the innards of all of these tools and techniques if animation is your thing—after all, you don’t need to know how a car actually works in order to drive it, do you? You will need to find a friend who can build rigs for you). If you want to learn how to build rigs, then you will end up dealing with far more jargon than this, and you will need to really understand what is going on “under the hood.” If that’s your thing, great! There are never enough riggers out there…