Every Frame a Computer

Hi! I'm Colleen Schmit. I'm 20, I'm a junior in computer science here in Salt Lake City, and I spend most of my nights writing ray tracing scenes in POV-Ray when I should be doing my compilers homework. So when the first feature film in history made entirely on computers came out, I was in line on opening night, Wednesday, November 22. I've been back twice since, once with my roommate Johanna Danley, who wanted to see what I kept talking about, and once by myself, so I could watch the backgrounds instead of the story.

This page is about how they did it. No spoilers, I promise. Go see it.

On this page: The facts | How you make a movie out of numbers | What Pixar's computers actually did | Pixar before Toy Story | Why the people look stiffer than the toys | Buzz is sold out | My tribute (coming soon)

The headings are in Arial if your browser does fonts. Microsoft's new Internet Explorer 2.0 does. Netscape shows them in plain old Times, which is fine too.

The facts

One more fact for the computer people: Pixar went public on the stock market the week after the movie opened. My professor says that's the first time he's ever seen the financial page and the graphics journals excited about the same thing.

How you make a movie out of numbers

People keep asking me if "computer animated" means the computer does it by itself. It does not! Every motion in Toy Story was decided by a human animator. The computer is more like a camera, a set, a lighting crew and a box of paint that will do exactly what you tell it, very fast, forever, without complaining. Here's the pipeline, in plain words.

1. Modeling. First somebody builds every object as a shape the computer understands: a mesh of polygons or a set of curved surfaces called patches, all described by numbers (coordinates in 3-D space). Woody's head, a Tinkertoy, the bedpost, the doorknob, the slats in the blinds: every one is a model. Imagine sculpting out of chicken wire, except the chicken wire can be perfectly smooth.

2. Rigging the controls. Then the models get controls the animator can move, like strings on a puppet: bend this elbow, raise this eyebrow, open the mouth this much. Woody has hundreds of them, a lot of those just in his face. The animators set the controls at key moments and the computer fills in the motion in between.

3. Texture maps. A bare model looks like gray plastic. (Which, for a few of the toys, is perfect.) To make the rest look like something, you wrap a picture around the shape, the way a label goes around a soup can. That picture is the texture map. The wood grain, the scuffs on the floor, the pattern on the bedspread, the wallpaper: texture maps. You can also map bumpiness, shininess and dirt. Look at the edges of things in the movie. Nothing is perfectly clean. That's on purpose, and it's a lot of work.

4. Lighting and shading. Next you put in lights, just like on a real set, and write little programs called "shaders" that say how each surface reacts to light. Plastic gets a hard, bright highlight. Cloth soaks up light and stays soft. Every material is a shader.

5. Rendering. Finally the computer takes all of it (models, positions, textures, lights, shaders, camera) and works out the color of every single dot in the picture. That's one frame. Film runs at 24 frames a second. An 81-minute movie is more than a hundred thousand frames, and I've read that some of the hard ones took the better part of a day each, even on Pixar's machines. That's why they need a whole room of computers working at once.

What Pixar's computers actually did (a note for my fellow ray tracers)

Here's something that surprised me. From everything I've read, RenderMan mostly does not ray trace. In a ray tracer like POV-Ray, the program fires a ray from the camera through every pixel and follows it as it bounces off mirrors and through glass, and it fires more rays at every light to see what's in shadow. That gives you perfect reflections and sharp shadows, and it is SLOW. Pixar's renderer cuts the scene into tiny pieces smaller than a pixel and shades those instead. Shadows come from "shadow maps" (basically a picture taken from each light's point of view) and reflections usually come from pictures wrapped around the scene. It's faking it, very cleverly, and it lets them do motion blur and enormous scenes without waiting a year for every shot.

My conclusion: ray tracing is more correct, but Pixar is more done. I'm thinking about that a lot this week, for reasons you'll understand if you click on my tribute.

And a bit of local pride: a lot of computer graphics was invented right here in Utah. Ed Catmull, one of the founders of Pixar, did his Ph.D. at the University of Utah in the seventies, and texture mapping came out of that work. Gouraud shading and Phong shading were both developed there too, and so was the famous teapot that every graphics student has rendered at least fifty times. (Me: I quit counting.)

Pixar before Toy Story

Toy Story didn't come out of nowhere. John Lasseter and the Pixar people spent ten years making short films to prove computers could tell a story, and you can see the movie growing up in them:

• The Adventures of André and Wally B. (1984). Made before Pixar was even its own company, back when the group was part of Lucasfilm. Short, simple, and one of the first times anybody saw computer characters squash and stretch like cartoons.
• Luxo Jr. (1986). Two desk lamps, a parent and a kid, and a ball. No faces, no words, and you know exactly what each lamp is feeling. It was nominated for an Academy Award, the first computer film ever to be.
• Red's Dream (1987). A unicycle in a bike shop on a rainy night. Sad and sweet.
• Tin Toy (1988). A wind-up one-man band trying to get away from a baby. It won the Oscar for animated short film, the first computer-animated film to win one. Toy Story's whole idea, toys with feelings, is right there.
• Knick Knack (1989). A snow globe snowman who wants out. Pure cartoon slapstick.

Our department's graphics lab has a tape of the shorts that gets passed around like a library book. I've had it twice.

Why the people look stiffer than the toys

Johanna noticed this before I said a word: the toys look amazing and the humans look a little... off. Like dolls of themselves. She's right, and there's a good reason.

Plastic is the easiest material in the world to draw with a computer. It's smooth, it's shiny, it's one color, and it's supposed to bend in a few places and not in others. Woody is cloth and vinyl, Buzz is hard plastic, the army men are molded plastic with that little base on their feet. A computer is good at that out of the box.

People are the hardest. Skin isn't a solid color: light goes a little way into it and comes back out. Hair is thousands of strands, every one moving. Clothes wrinkle and fold. And our faces move in a hundred tiny ways we don't even notice until they're missing, and then we notice right away. Anyone who's seen the baby in Tin Toy knows how strange an almost-real person can look. I've read that one reason they picked toys for the first feature is that the computer could make toys look exactly right. I believe it. Watch the dog next time, too. Fur is right up there with hair.

Give it ten years. Somebody will crack hair.

Buzz is sold out

If you are trying to get a talking Buzz Lightyear for a kid this Christmas: good luck. Nobody expected the movie to do this well, so there weren't nearly enough of the toys made. My mom has been to every toy store in Boise for my little brother and says the shelf where Buzz should be is always empty, and the Woody dolls are going fast too. She asked me to look in Salt Lake. I have. Same story. I think there's something wonderful about a computer-made toy becoming the toy everybody can't find.

Coming soon: my tribute

I'm ray-tracing the rocket from the Pizza Planet sign in POV-Ray, my own model from scratch.
Status: rendering, 38 hours so far.
Read about the scene and watch it crawl.

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Comments, corrections, POV-Ray tips: [email protected]
Last updated Sunday, December 17, 1995. Home to Boise on Tuesday for break; back in January.