Secrets of 3D computer graphics

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You're probably reading this on the screen of a computer monitor -- a display that has two real dimensions, height and width. But when you look at a movie like "Toy Story II" or play a game like Tomb Raider, you see a window into a three-dimensional world. One of the truly amazing things about this window is that the world you see can be the world we live in,

the world we will live in tomorrow, or a world that lives only in the minds of a movie’s or game's creators. And all of these worlds can appear on the same screen you use for writing a report or keeping track of a stock portfolio.

How does your computer trick your eyes into thinking that the flat screen extends deep into a series of rooms? How do game programmers convince you that you're seeing real characters move around in a real landscape? In this edition of How Stuff Works, we will tell you about some of the visual tricks 3D graphic designers use, and how hardware designers make the tricks happen so fast that they seem like a movie that reacts to your every move.

What Makes a Picture 3D

A picture that has or appears to have height, width and depth is three-dimensional (or 3D). A picture that has height and width but no depth is two-dimensional (or 2-D). Some pictures are 2-D on purpose. Think about the international symbols that indicate which door leads to a restroom, for example.

The symbols are designed so that you can recognize them at a glance. That’s why they use only the most basic shapes. Additional information on the symbols might try to tell you what sort of clothes the little man or woman is wearing, the color of their hair, whether they get to the gym on a regular basis, and so on, but all of that extra information would tend to make it take longer for you to get the basic information out of the symbol: which restroom is which. That's one of the basic differences between how 2-D and 3D graphics are used: 2-D graphics are good at communicating something simple, very quickly. 3D graphics tell a more complicated story, but have to carry much more information to do it.

Take a look at the triangles above. Each of the triangles on the left has three lines and three angles -- all that's needed to tell the story of a triangle. We see the image on the right as a pyramid -- a 3D structure with four triangular sides. Note that it takes five lines and six angles to tell the story of a pyramid -- nearly twice the information required to tell the story of a triangle.

For hundreds of years, artists have known some of the tricks that can make a flat, 2-D painting look like a window into the real, 3D world. You can see some of these on a photograph that you might scan and view on your computer monitor: Objects appear smaller when they're farther away; when objects close to the camera are in focus, objects farther away are fuzzy; colors tend to be less vibrant as they move farther away. When we talk about 3D graphics on computers today, though, we're not talking about still photographs -- we're talking about pictures that move.

If making a 2-D picture into a 3D image requires adding a lot of information, then the step from a 3D still picture to images that move realistically requires far more. Part of the problem is that we’ve gotten spoiled. We expect a high degree of realism in everything we see. In the mid-1970s, a game like "Pong" could impress people with its on-screen graphics. Today, we compare game screens to DVD movies, and want the games to be as smooth and detailed as what we see in the movie theater. That poses a challenge for 3D graphics on PCs, Macintoshes, and, increasingly, game consoles like the Dreamcast and the Playstation II.

What Are 3D Graphics

For many of us, games on a computer or advanced game system are the most common ways we see 3D graphics. These games, or movies made with computer-generated images, have to go through three major steps to create and present a realistic 3D scene:

1. Creating a virtual 3D world.

2. Determining what part of the world will be shown on the screen.

3. Determining how every pixel on the screen will look so that the whole image appears as realistic as possible.

Creating a Virtual 3D World

A virtual 3D world isn't the same thing as one picture of that world. This is true of our real world also. Take a very small part of the real world -- your hand and a desktop under it. Your hand has qualities that determine how it can move and how it can look. The finger joints bend toward the palm, not away from it. If you slap your hand on the desktop, the desktop doesn't splash -- it's always solid and it's always hard. Your hand can't go through the desktop. You can't prove that these things are true by looking at any single picture. But no matter how many pictures you take, you will always see that the finger joints bend only toward the palm, and the desktop is always solid, not liquid, and hard, not soft. That's because in the real world, this is the way hands are and the way they will always behave. The objects in a virtual 3D world, though, don’t exist in nature, like your hand. They are totally synthetic. The only properties they have are given to them by software. Programmers must use special tools and define a virtual 3D world with great care so that everything in it always behaves in a certain way.

What Part of the Virtual World Shows on the Screen?

At any given moment, the screen shows only a tiny part of the virtual 3D world created for a computer game. What is shown on the screen is determined by a combination of the way the world is defined, where you choose to go and which way you choose to look. No matter where you go -- forward or backward, up or down, left or right -- the virtual 3D world around you determines what you will see from that position looking in that direction. And what you see has to make sense from one scene to the next. If you're looking at an object from the same distance, regardless of direction, it should look the same height. Every object should look and move in such a way as to convince you that it always has the same mass, that it's just as hard or soft, as rigid or pliable, and so on.

Programmers who write computer games put enormous effort into defining 3D worlds so that you can wander in them without encountering anything that makes you think, “That couldn't happen in this world!" The last thing you want to see is two solid objects that can go right through each other. That’s a harsh reminder that everything you’re seeing is make-believe.

The third step involves at least as much computing as the other two steps and has to happen in real time for games and videos. We'll take a longer look at it next.

How to Make It Look Like the Real Thing

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