Introduction

3D game character animation is a relatively new craft. Four years ago we would not have dreamed of animating facial expressions in real time, and now the latest line of video cards and consoles brings even more complexity to what we can do. The time has come to start reflecting on 3D game animation as a craft by defining the methods we use, establish some rules, find tricks, and understand what it takes to be a game animator. It is time to go from tinkering to crafting.

This article deals with the real time, in-game character animation, however, it is also useful to many people who are involved with animation in general. I explain and discuss the very basic rules of what I call three-axis animation, and present the essence of what I have learned during the past few years. Every animator has a different approach to the same problems, but I feel my experience could be helpful to some of you, especially if you are totally new to in-game animation.

What's the big deal?

You probably remember the little sixteen by sixteen blocks of pixels creating sprites that were supposed to look and act like characters with personality in a 2D game environment. Moving pixels around in 2D animation involves creating a blurry illusion of something, while animating in 3D is close to reproducing an actual motion with all its cold mathematical strangeness: acceleration, deceleration, forces, dynamics, weights and curves. Going 3D is a huge and frightening step forward. For a 2D animator, it's a strange new world. I started, like everybody else, using the 2D method of defining a few extreme key frames from a side view, and hoping it would work. Unfortunately, it did not work, mainly because 2D is an illusion in an impossible flat world and is accepted by the player (your average Joe player) as an illusion. In contrast, 3D implies a world that obeys the same laws as ours, in a world that is truly believable. Sure Mario can hop and jump without even moving his legs in a flat universe, but in a world with dimensions and space, everybody expects him to act and react according to physical laws. The difference between 2D and 3D is not an added dimension, but a heightened expectation from the player. That's the big deal!

Now, as animators, we can ignore this fact, and still try to fix some little animations, or we can rethink our ways of animating from the ground up. That means leaving behind everything we learned from 2D, and understanding exactly what we are doing.

Gravity is the key

Let me define the technical basics of what 3D animation exactly is. As I said earlier, in a believable universe we have to consider laws of nature. The main law that directly affects any movement in a believable universe is gravity. Every motion in a 3D space either is created by gravity or is a reaction to gravity. For example, you are probably sitting while reading this article. Try to stand up and visualize what keeps you standing. Mostly, it is the muscles in your legs (especially the thighs), and possibly the muscles in your arms if you are using the table to lift you up, that produce a force strong enough to counterbalance the effect of gravity. The creation of a force is the basis for motion in a 3Dspace.

However dry this topic may sound, bear with me, because understanding the basics often makes the difference between good and bad animations. If you keep in mind that your character has to create a force to counterbalance the effects of gravity every time you want to move it, you will avoid many common mistakes.

What does it really mean? A quick example can illustrate my point. If you want to have a standing character going from point A to point B, you have two ways of solving the problem:

• You can select the mesh of the character, or his center of gravity, and move it from A to B. Here, you are creating an artificial force that allows you to move the body.
• You can create the motions, or forces, that will move this center of gravity from A to B. This is done by replicating the way a body moves in the real world.

In the first solution, you are creating the illusion of movement. The second solution illustrates a true comprehension and reproduction of the way physical laws create a movement. Some people will disagree with this because it makes inverse kinematics invalid. The truth is, inverse kinematics is a bit of a monster. Making a character walk by moving its feet is wrong; it is the total opposite of a natural walk to imply that the feet are moving because the legs are moving. Rather, it is simulating a motion, instead of reproducing a motion, and trying to create an effect without knowing what forces produce it. You should be only interested in knowing what make things move, because if you understand what you do, you can control it.

Gravity, and how it is used to create movement, is the first rule of three-axis animation. The two other rules are balance, tilt and twist. Let me explain to you how those rules are tied together and can be used to create a complex animation.