Computer animation: Encyclopedia - Computer animation

Computer animation

Computer animation is the art of creating moving images via the use of computers. It is a subfield of computer graphics and animation. Increasingly it is created by means of 3D computer graphics, though 2D computer graphics are still widely used. Sometimes the target of the animation is the computer itself, sometimes the target is another medium, such as film. It is also referred to as CGI (Computer generated imagery) especially when used in movies.

To create the illusion of movement, an image is displayed on the computer screen then quickly replaced by a new image that is similar to the previous image, but shifted slightly. This technique is identical to how the illusion of movement is achieved with television and motion pictures.

3D Computer animation is essentially a digital successor to the art of stop motion animation; the animated figure is built on the computer monitor and rigged with a virtual skeleton. Then the limbs, eyes, mouth, clothes, etc. of the 3D figure are moved by the animator. Finally, the animation is rendered.

Computer animation - A simple example

The screen is blanked to a background color, such as black. Then a goat is drawn on the right of the screen. Next the screen is blanked, but the goat is drawn slightly to the left of its original position. This process is repeated, each time moving the goat a bit to the left. If this process is repeated fast enough the goat will appear to move smoothly to the left. This basic procedure is used for all moving pictures in films and television.

Computer animation - Explanation

To trick the eye and brain into thinking they are seeing a smoothly moving object the pictures should be drawn at around 12 frames per second or faster (a frame is one complete image). With rates above 70 frames/s no improvement in realism or smoothness is perceivable due to the way the eye and brain process images. At rates below 12 frames/s most people can detect jerkiness associated with the drawing of new images which detracts from the illusion of realistic movement. Conventional hand-drawn cartoon animation often uses 15 frames/s in order to save on the number of drawings needed, but this is usually accepted because of the stylized nature of cartoons. Because it produces more realistic imagery computer animation demands higher frame rates to reinforce this realism.

The reason no jerkiness is seen at higher speeds is due to “persistence of vision.” From moment to moment, the eye and brain working together actually store whatever you look at for a fraction of a second, and automatically "smooth out" minor jumps. Movie film seen in a theater runs at 24 frames/s which is sufficient to create this illusion of continuous movement.

Computer animation - Creating characters and objects on a computer

Computer animation combines Vector graphics with programmed movement. The starting point is often a stick figure in which the position of each feature (limb, mouth etc) is defined by an Avars (animation variable).

The character "Woody" in Pixar's movie Toy Story, for example, uses 700 Avars. Successive sets of Avars control all movement of the character from frame to frame. Once the stick model is moving in the desired way, the avars are incorporated into a full Wire frame model or a model built of polygons. Finally surfaces are added, requiring a lengthy process of Rendering to produce the final scene.

There are several ways of generating the Avar values to obtain realistic motion. Motion tracking uses lights or markers on a real person acting out the part, tracked by a video camera. Or the Avars may be set manually using a joystick or other form input control. Toy Story uses no motion tracking, probably because manual control by a skilled animator can produce effects not easily acted out by a real person.

Computer animation - Equipment

Computer animation can be created with a computer and animation software. Some examples of animation software are: Amorphium (3D), Poser (3D), Ray Dream Studio (3D), Bryce, Maya, Blender, Truespace (3-D), 3D Studio Max (3D) and SoftImage XSI (3D) and Flash (2D). Their price will vary greatly depending on their target market. Some impressive animation can be achieved even with basic programs. However the rendering can take a lot of time on an ordinary home computer. Video game animators tend to use low resolution, low polygon count renders, such that the graphics can be rendered in real time on a home computer. Photorealistic animation would be impractical in this context.

Professional animators of movies, TV, and video sequences on computer games make photorealistic animation with high detail. This kind of movie animation would take 10s to 100s of years to create on a home computer. Many powerful workstation computers are used instead. Graphics workstation computers use two to four processors and thus are a lot more powerful than a home computer, and are specialized for rendering. A large number of workstations (known as a render farm) are networked together to effectively act as a giant computer. The result is a computer animated movie that can be completed in about one to five years (this process will not just comprise rendering, however). A workstation typically costs $2000 to $16000. Pixar's Renderman is rendering software and is widely used as the movie animation industry standard competing with Mental Ray. It can be bought at the official Pixar website for about $5000 to $8000. It will work on Linux, Mac OS X, and Microsoft Windows based graphics workstation along with an animation program such as Maya and Softimage XSI. Professionals also use digital movie cameras, motion capture or performance capture, bluescreens, film editing software, props, and other tools for movie animation.

Computer animation - Technical details

When an image is rendered to the screen, it is normally rendered to something called a back buffer. There the computer can draw the image, making any necessary changes to it before it is done. While the computer is rendering, the screen is showing the contents of what is called the primary or active buffer.

When the image is completed, the computer tells the screen to draw from the back buffer. This can be done in one of two ways: the contents of the back buffer can be copied to the primary buffer (or active buffer—the buffer which is currently being shown) or the computer can switch where it is drawing from and make the back buffer the new primary buffer. In this case, the primary buffer becomes the back buffer. This process is usually called double buffering or, informally, "flipping," because the computer is flipping its use of primary and back buffers.

This switching should be carried out when it is imperceptible to the user. Therefore it needs to take place during what is called the "v-sync" or vertical retrace. The v-sync, in CRTs, takes place when the electron guns reach the bottom right of the screen and need to reposition the beam to the top left of the screen. This happens very quickly and the image the guns had just projected remain on the screen as they are moving back to their starting position. While the guns are repositioning themselves, the computer has enough time to flip buffers and the new image will be rendered on the screen on the next pass of the guns. The new image will continued to be displayed until the buffers are flipped once more.

When the computer fails to wait for the v-sync, a condition called sprite breakup or image breakup is perceptible. This is highly undesirable and should always be avoided when possible to maintain the illusion of movement.

Computer animation - The future

The future of animation is unimaginable, there is no way we can know what will be released next. Every day a new program, a new rendering technique enters the market, flipping everything around. Some are so good that the prices for the new capability can be in the tens of thousands of dollars.

In the future, some people expect the current level of animation (2D, 3D) will be replaced with virtual reality; meaning the person watching a movie will see himself in the movie as a person watching from the side.

One open challenge in computer animation is a photorealistic animation of humans. Currently, most computer-animated movies show animal characters (Finding Nemo), fantasy characters (Shrek, Monsters Inc.), or cartoon-like humans (The Incredibles). The movie Final Fantasy: The Spirits Within is often cited as the first computer-generated movie to attempt to show realistically-looking humans. However, due to the enormous complexity of the human body, human motion, and human biomechanics, realistic simulation of humans remains largely an open problem. It is one of the "holy grails" of computer animation. Eventually, the goal is to create software where the animator can generate a movie sequence showing a photorealistic human character, undergoing physically-plausible motion, together with clothes, photorealistic hair, a complicated natural background, and possibly interacting with other simulated human characters. This should be done in a way that the viewer is no longer able to tell if a particular movie sequence is computer-generated, or created using real actors in front of movie cameras. Achieving such a goal would mean that conventional flesh-and-bone human actors are no longer necessary for this kind of movie creation, and computer animation would become the standard way of making every kind of a movie, not just animated movies. This is not likely to happen very soon, however such concepts obviously bear certain philosophical implications for the future of the film industry.

There's no way of knowing how far computer animation can go, every day new effects are created which make it more realistic and more immersive.

Computer animation - Detailed examples and pseudocode

In 2D computer animation, moving objects are often referred to as “sprites.” A sprite is an image that has a location associated with it. The location of the sprite is changed slightly, between each displayed frame, to make the sprite appear to move. The following pseudocode makes a sprite move from left to right:

 var int x := 0, y := screenHeight ÷ 2;
 while x < screenWidth
     drawBackground()
     drawSpriteAtXY(x, y)  // draw on top of the background
     x := x + 5  // move to the right

Modern (2001) computer animation uses different techniques to produce animations. Most frequently, sophisticated mathematics is used to manipulate complex three dimensional polygons, apply “textures”, lighting and other effects to the polygons and finally rendering the complete image. A sophisticated graphical user interface may be used to create the animation and arrange its choreography. Another technique called constructive solid geometry defines objects by conducting boolean operations on regular shapes, and has the advantage that animations may be accurately produced at any resolution.

Let's step through the rendering of a simple image of a room with flat wood walls with a grey pyramid in the center of the room. The pyramid will have a spotlight shining on it. Each wall, the floor and the ceiling is a simple polygon, in this case, a rectangle. Each corner of the rectangles is defined by three values referred to as X, Y and Z. X is how far left and right the point is. Y is how far up and down the point is, and Z is far in and out of the screen the point is. The wall nearest us would be defined by four points: (in the order x, y, z). Below is a representaion of how the wall is defined.

 (0, 10, 0)                        (10, 10, 0)





 (0,0,0)                           (10, 0, 0)

The far wall would be:

(0, 10, 20)                        (10, 10, 20)





(0, 0, 20)                         (10, 0, 20)

The pyramid is made up of five polygons: the rectangular base, and four triangular sides. To draw this image the computer uses math to calculate how to project this image, defined by three dimensional data, onto a two dimensional computer screen.

First we must also define where our view point is, that is, from what vantage point will the scene be drawn. Our view point is inside the room a bit above the floor, directly in front of the pyramid. First the computer will calculate which polygons are visible. The near wall will not be displayed at all, as it is behind our view point. The far side of the pyramid will also not be drawn as it is hidden by the front of the pyramid.

Next each point is perspective projected onto the screen. The portions of the walls ‘farthest’ from the view point will appear to be shorter than the nearer areas due to perspective. To make the walls look like wood, a wood pattern, called a texture, will be drawn on them. To accomplish this, a technique called “texture mapping” is often used. A small drawing of wood that can be repeatedly drawn in a matching tiled pattern (like wallpaper) is stretched and drawn onto the walls' final shape. The pyramid is solid grey so sp its surfaces can just be rendered as grey. But we also have a spotlight. Where its light falls we lighten colors, where objects blocks the light we darken colors.

Next we render the complete scene on the computer screen. If the numbers describing the position of the pyramid were changed and this process repeated, the pyramid would appear to move.

Computer animation - Movies and TV shows

CGI short films have been produced as independent animation since the 1970s, though the popularity of computer animation (especially in the field of special effects) skyrocketed during the modern era of U.S. animation. The very first totally computer-generated animated series was ReBoot.

Below is a selected list of films and television shows that employ computer animation.

• The Ant Bully
• Animusic
• Antz
• Beast Wars
• A Bug's Life
• Barnyard
• Chicken Little
• Dragon Booster
• Father of the Pride
• Futurama
• The Fairly OddParents
• Final Fantasy: The Spirits Within
• Finding Nemo
• Foster's Home for Imaginary Friends
• Game Over (TV series)
• Ice Age
• The Incredibles
• Adventures of Jimmy Neutron: Boy Genius
• Madagascar
• Monsters Inc.
• The Polar Express
• ReBoot
• Robots
• Rolie Polie Olie
• Shadow Raiders
• Shark Tale
• Shrek
• Thomas & Friends
• Toy Story
• Veggie Tales
• Waking Life

See also

• Animation - Introductory Root page
• Computer-generated imagery (CGI)
• Computer Graphics Lab and the project The Works
• Wire frame model
• Computer representation of surfaces
• Motion capture
• Avar (animation variable)
• Pixar Animation Studios
• Rhythm and Hues Studios
• Skeletal animation
• Timeline of CGI in movies

Adapted from the Wikipedia article "Computer animation", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki