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From Wikipedia, the free encyclopedia

An example of computer animation which is produced in the "motion capture" technique

Computer animation (or CGI animation) is the art of creating moving images with 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 for stylistic, low bandwidth, and faster real-time rendering needs. Sometimes the target of the animation is the computer itself, but sometimes the target is another medium, such as film. It is also referred to as CGI (computer-generated imagery or computer-generated imaging), especially when used in films.

To create the illusion of movement, an image is displayed on the computer screen and repeatedly replaced by a new image that is similar to the previous image, but advanced slightly in the time domain (usually at a rate of 24 or 30 frames/second). This technique is identical to how the illusion of movement is achieved with television and motion pictures.

Computer animation is essentially a digital successor to the art of stop motion animation of 3D models and frame-by-frame animation of 2D illustrations. For 3D animations, objects (models) are built on the computer monitor (modeled) and 3D figures are rigged with a virtual skeleton. For 2D figure animations, separate objects (illustrations) and separate transparent layers are used, with or without a virtual skeleton. Then the limbs, eyes, mouth, clothes, etc. of the figure are moved by the animator on key frames. The differences in appearance between key frames are automatically calculated by the computer in a process known as tweening or morphing. Finally, the animation is rendered.

For 3D animations, all frames must be rendered after modeling is complete. For 2D vector animations, the rendering process is the key frame illustration process, while tweened frames are rendered as needed. For pre-recorded presentations, the rendered frames are transferred to a different format or medium such as film or digital video. The frames may also be rendered in real time as they are presented to the end-user audience. Low bandwidth animations transmitted via the internet (e.g. 2D Flash, X3D) often use software on the end-users computer to render in real time as an alternative to streaming or pre-loaded high bandwidth animations.


A simple example

Computer animation 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 re-drawn or duplicated 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.

The moving goat is an example of shifting the location of an object. More complex transformations of object properties such as size, shape, lighting effects and color often require calculations and computer rendering instead of simple re-drawing or duplication.


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 (frame/s) 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 frame/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 one looks at for a fraction of a second, and automatically "smooth out" minor jumps. Movie film seen in theaters in the United States runs at 24 frames per second, which is sufficient to create this illusion of continuous movement.

Methods of animating virtual characters

In this .gif of a 2D Flash animation, each 'stick' of the figure is keyframed over time to create motion.

In most 3D computer animation systems, an animator creates a simplified representation of a character's anatomy, analogous to a skeleton or stick figure. The position of each segment of the skeletal model is defined by animation variables, or Avars. In human and animal characters, many parts of the skeletal model correspond to actual bones, but skeletal animation is also used to animate other things, such as facial features (though other methods for facial animation exist). The character "Woody" in Toy Story, for example, uses 700 Avars, including 100 Avars in the face. The computer does not usually render the skeletal model directly (it is invisible), but uses the skeletal model to compute the exact position and orientation of the character, which is eventually rendered into an image. Thus by changing the values of Avars over time, the animator creates motion by making the character move from frame to frame.

There are several methods for generating the Avar values to obtain realistic motion. Traditionally, animators manipulate the Avars directly. Rather than set Avars for every frame, they usually set Avars at strategic points (frames) in time and let the computer interpolate or 'tween' between them, a process called keyframing. Keyframing puts control in the hands of the animator, and has roots in hand-drawn traditional animation.

In contrast, a newer method called motion capture makes use of live action. When computer animation is driven by motion capture, a real performer acts out the scene as if they were the character to be animated. His or her motion is recorded to a computer using video cameras and markers, and that performance is then applied to the animated character.

Each method has their advantages, and as of 2007, games and films are using either or both of these methods in productions. Keyframe animation can produce motions that would be difficult or impossible to act out, while motion capture can reproduce the subtleties of a particular actor. For example, in the 2006 film Pirates of the Caribbean: Dead Man's Chest, actor Bill Nighy provided the performance for the character Davy Jones. Even though Nighy himself doesn't appear in the film, the movie benefited from his performance by recording the nuances of his body language, posture, facial expressions, etc. Thus motion capture is appropriate in situations where believable, realistic behavior and action is required, but the types of characters required exceed what can be done through conventional costuming.

Computer animation development equipment

Computer animation can be created with a computer and animation software. Some impressive animation can be achieved even with basic programs; however the rendering can take a lot of time on an ordinary home computer. Because of this, 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, television, and video sequences on computer games make photorealistic animation with high detail. This level of quality for movie animation would take tens to hundreds 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 is not comprised solely of rendering, however). A workstation typically costs $2,000 to $16,000, with the more expensive stations being able to render much faster, due to the more technologically advanced hardware that they contain. Pixar's Renderman is rendering software which is widely used as the movie animation industry standard, in competition with Mental Ray. It can be bought at the official Pixar website for about $3,500. It will work on Linux, Mac OS X, and Microsoft Windows based graphics workstations 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.

The future

One open challenge in computer animation is a photorealistic animation of humans. Currently, most computer-animated movies show animal characters (A Bug's Life, Finding Nemo, Ratatouille, Newt, Ice Age, Over the Hedge), fantasy characters (Monsters Inc., Shrek, Monsters vs. Aliens), anthropomorphic machines (Cars, WALL-E, Robots) or cartoon-like humans (The Incredibles, Despicable Me, Up, Jimmy Neutron: Boy Genius, Meet the Robinsons, Cloudy with a Chance of Meatballs). The movie Final Fantasy: The Spirits Within is often cited as the first computer-generated movie to attempt to show realistic-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. Another problem is the distasteful psychological response to viewing nearly perfect animation of humans, known as "the uncanny valley." 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 could 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. Complete human realism is not likely to happen very soon, and when it does it may have major repercussions for the film industry.

For the moment it looks like three dimensional computer animation can be divided into two main directions; photorealistic and non-photorealistic rendering. Photorealistic computer animation can itself be divided into two subcategories; real photorealism (where performance capture is used in the creation of the virtual human characters) and stylized photorealism. Real photorealism is what Final Fantasy tried to achieve and will in the future most likely have the ability to give us live action fantasy features as The Dark Crystal without having to use advanced puppetry and animatronics, while Antz is an example on stylistic photorealism (in the future stylized photorealism will be able to replace traditional stop motion animation as in Corpse Bride). None of them mentioned are perfected as of yet, but the progress continues.

The non-photorealistic/cartoonish direction is more like an extension of traditional animation, an attempt to make the animation look like a three dimensional version of a cartoon, still using and perfecting the main principles of animation articulated by the Nine Old Men, such as squash and stretch.

While a single frame from a photorealistic computer-animated feature will look like a photo if done right, a single frame vector from a cartoonish computer-animated feature will look like a painting (not to be confused with cel shading, which produces an ever simpler look).

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
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 representation 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 ‘furthest’ 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 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.


CGI short films have been produced as independent animation since 1976, though the popularity of computer animation (especially in the field of special effects) skyrocketed during the modern era of U.S. animation. The first completely computer-generated television series was ReBoot, and the first completely computer-generated animated movie was Toy Story (1995). See List of computer-animated films for more.

Amateur animation

The popularity of sites such as Newgrounds, which allows members to upload their own movies for others to view, has created a growing number of what is often considered amateur computer animators. With many free utilities available and programs such as Windows Movie Maker or iMovie, which are included in the operating system, anyone with the tools and a creative mind can have their animation viewed by thousands. Many high end animation software options are also available on a trial basis, allowing for educational and non-commercial development with certain restrictions. Several free and open source animation software applications exist as well, Blender as an example. One way to create amateur animation is using the GIF format, which can be uploaded and seen on the web easily.

Architectural animation

Architects use services from animation companies to create a 3-dimensional models for both the customers and builders. It can be more accurate than traditional drawings. Architectural animation can also be used to see the possible relationship the building will have in relation to the environment and its surrounding buildings.

See also

Animated images in Wikipedia

  • Computer animation example
  • An animated pentakisdodecahedron
  • Animation of an MRI brain scan, starting at the top of the head and moving towards the base


External links

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