Sun 18 Feb 2007
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 for low bandwidth and faster real-time rendering needs. Sometimes the target of the animation is the computer itself, but it 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 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.
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.
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. Many people are tricked into seeing the movement without any stopping because the frames are shot at a quick rate.
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 as Avars (animation variable).
The character “Woody” in Toy Story, for example, uses 700 Avars with 100 Avars in his face alone. 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 only manual control by a skilled animator can produce effects not easily acted out by a real person.
Computer animation development equipment
Computer animation can be created with a computer and animation software. Some examples of animation software are: Amorphium, Art of Illusion, Poser, Ray Dream Studio, Bryce, Maya, Blender, TrueSpace, Lightwave, 3D Studio Max, SoftImage XSI, Alice, and Adobe Flash (2D). There are many more. Prices will vary greatly depending on 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. 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 $5,000 to $8,000. 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.
Hardware animation display technology
An example of computer rendered animation
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. John MacArthur is the one that thought up this process. 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.
The future
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 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. 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. However, living actors will be needed for voice-over acting and motion capture body movements. Complete human realism is not likely to happen very soon, however such concepts obviously bear certain philosophical implications for the future of the film industry.
Then we have the animation studios who are not interested in photorealistic CGI features, or to be more precise, they want some alternatives to choose from and may prefer one style over another, depending on the movie.
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 Corpse Bride). None of them are as mentioned perfected yet, but the progress continues. The non-photorealistic/cartoonish direction is more like an extension and improvement 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 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 “texturesâ€, 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.
Movies
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 first completely computer-generated animated movie was Toy Story. See List of computer animated films for more.
Amateur animation
The popularity of sites such as YouTube, 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, anyone with the tools can have their animation viewed by thousands.
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article “Computer animation“.
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