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What a Designers Should Know About Physics

January 21, 2003 Article Start Page 1 of 2 Next

During my years as game design director at Angel Studios, there wasn’t a day where I didn’t have to deal with physics in one or several of our products. Impressive physic demos have always been one of Angel Studios ticket to get contracts, so we had to have some in our games. And actually, we had a lot of physics in our games: from driving physics to ragdolls, to collisions with stacking and more, the entire spectrum was covered. They all helped make our games look and feel different from the rest of the crowd. That, and our ability to tame physics so our games stayed playable and fun.

In this article, I will try to explain how to tune physics from a designer’s point of view with some fundamentals you have to know and end with some classic traps to avoid. But first, do you approach physics as a game designer?

A Hands-On Approach

Physics can make a game look and feel different. They can bring shocking realism to impacts, to vehicle controls and really enhance the critical reactions a player’s action generate.
Physics can help pull a game into reality. The ultimate goal, however, is to enhance that reality. If you get it right, you will give unique sensations to players and show them things they have never experienced before. Unfortunately, there is a price to pay for that. Physics can take a lot of CPU time and it requires a lot of tuning to keep a game fun to play.

Badly tuned physics can be a disaster: A vehicle can become impossible to control, objects can get in the way of the player, things just don’t feel right and what should have made the game fun just generates frustration. As a game designer, you are responsible for the fun of the game, so it is your role to make sure physics are tuned properly and not left solely to a programmer’s whim.

If you don't play a role designing and tuning the physics, your game might ship with you unhappy with the tuning, wishing you had learned more sooner (also called regrets) and you will miss a lot of opportunities to discover fun things you might have done with the physics. You will have to get your hands dirty and tweak many variables many times before you get some good results. There are no other ways around that. Besides, if you are a game designer, you already have to tune a lot of variables for your controls and your cameras.

But before you get there though, you need to learn the theory behind physics.

What Are They Talking About?

As a game designer, it is also your role to sit down and talk to your team and create a communication flow between all team members. Usually, you do the talking. You listen, yes, but you also have to explain a lot of things to a lot of people.

Well, this time, it is your turn to listen and to listen well! It is essential for the designer to understand the language of physics, to understand its limitations in real case scenarios, to make sure you get a grasp of what is possible or not. Only then, after hours of questioning and drawings (Paper and pencil are your best friends when sitting with a physics programmer) you will start to grasp how you can control this beast and make it behave the way you want it to.

It is ok to ask questions, again and again. It’s time to take your game designer’s ego and throw it out the door. If you don’t understand something, just ask and ask again, until you get it right -- Everything has to be crystal clear. You might look dumb, that’s for sure, but you cannot leave anything unexplained.

Understanding what the physics guy means is crucial if you plan to start to tweak things around and understand what is happening in your game. It will also allow both of you to work on solutions that solve these “special cases” occurrences. You know, it’s a game and sometimes you don’t want physics to do this or that. If you don’t know what you are talking about, physics wise, you will not be able to convince your programmer to tweak the code to make things work. After all, he wants it to be real but you want it to be really FUN.

Before you start talking to your programmer (Yes, you have to!), here is a very simple to understand primer on game physics as well as some basic techniques to help you tune your game for fun.

Center of Gravity

The center of gravity is a crucial parameter in physics. It will determine how your object will react to forces applied to it. The center of gravity is at the center of mass of an object (center of mass is another name for center of gravity), where the object will likely be balanced. If you move the position of the center of gravity of an object, you will change it’s behavior dramatically: If a tall object has its center of gravity very low, it will be very stable and hard to roll. If it is placed way up there though, the object will roll very easily. If you want a car to make nice drifts, or roll a bit into turns, move your center of gravity around. You will get very different driving feelings when you do so.

Control, It’s All About Control

Physics in games are about forces and impulsions (a force is a continuous impulse). You can’t just move a physically based object by giving it a position in 3D space. You have to push it or apply a torque to rotate it. Also, the heavier the object, the stronger the force or the impulsion you need to apply to move it (except if your object and the ones you are colliding with have a low friction…). Some physics engine allow you to specify x,y and z coordinates and will try to calculate the forces needed to bring your object there, but it will still be an approximation.

To control all these forces and to prevent things to get too crazy (like an object spinning all over the place, a car that falls on it’s side too easily in turns) you have a friend that can help you. And this friend is called dampening. This is your Tylenol, your savior -- the magic word. If in your game physics make things fly out of control, go to your programmer and say the magic word. He will know what you are talking about. You should also ask him about dampening when things feel too slow, sluggish, smooth, controlled. Then, he probably already has dampening coded in and he did the tuning himself. Get these tuning parameters exposed and play with them.

Article Start Page 1 of 2 Next

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