Computer Animation.

Computer Animation. I INTRODUCTION Computer Animation, creation of the illusion of motion by viewing a succession of computer-generated still images. Prior to the advent of computers, animation was accomplished by filming hand-drawn or painted sequences on plastic or paper, called cels, one frame at a time. Computers were first used to control the movements of the artwork and the camera. Now computers create the artwork and simulate the camera. Computer animation can be used to create special effects and to simulate images that would be impossible to show with nonanimation techniques, such as a spacecraft flying by the planet Saturn. Computer animation can also produce images from scientific data, and it has been used to visualize large quantities of data in the study of interactions in complex systems, such as fluid dynamics, particle collisions, and the development of severe storms. These mathematically based models use animation to help researchers see relationships that might otherwise be overlooked. Computer animation has also been used in legal cases to reconstruct accidents. II HOW COMPUTER ANIMATION WORKS In traditional frame-by-frame animation, the illusion of motion is created by filming a sequence of hand-painted cels and then playing the images back at high speeds, typically 14 to 30 frames per second. In computer animation, the art is created using computer programs, frame by frame, and then recorded, edited, and played back. Another computer animation technique is real-time animation, in which the frames are created using a computer and then immediately displayed on a computer monitor. This technique eliminates the interim step of digitally recording the images; however, real-time animation currently does not produce high quality or richly detailed results. It is best suited for creating simple animations for video games. III COMPUTER-ASSISTED ANIMATION In the traditional process of animation, a storyboard (a scene-by-scene illustration of the plot) is drawn first, the soundtrack is completed, and a senior animator creates key animation frames. Other animators then draw the frames in between the key scenes, color is added, and each frame is then filmed. Computers can be used to assist or replace every phase of this animation process. A In-Betweening The process of creating the intermediate frames to fill in the action from key scene to key scene is called in-betweening. Techniques have been developed that allow the computer to create the in-between frames by estimating common points from key frame to key frame. In the simplest case, the computer draws the in-between movement of two corresponding points by calculating the mid-point distance. Repeated calculation of mid-points can provide the illusion of smooth and continuous motion. B Painting Systems The hand painting of animated cels is a painstaking process, with an average output of 25 cels per day per painter. Sometimes cels are stacked together to create different images--for example, the cels may interact, overlap, or provide backgrounds for one another. When a large number of cels are stacked, the transparent layers become slightly opaque. The cel painter must then compensate for this effect by varying the image colors; this process often introduces errors. Computers can eliminate these errors and increase production by consistently coloring the most complex areas of frames. Computer painting uses a coloring, or filling, process in which the artist specifies a color and then selects a pixel, the smallest individual picture element on the computer screen. The computer then changes all adjacent pixels that have the same color (or nearly the same color) to the newly specified color. C Camera Stands and Editing Once the frames are painted they must be filmed. Traditionally, an animation stand positions both the cels and the camera to allow the layers of cels and the camera to move independently. Computers simulate the animation stand and the camera. The computer controls this virtual camera in three-dimensional space, while focusing on the series of two-dimensional images or cels in the computer's memory. In effect, both the cels and the camera reside within the computer. Special characteristics of real cameras, such as fish-eye lenses and lens flare, can be simulated by the virtual camera. This ability to control a virtual camera, combined with powerful digital video editing, enables the animator to complete the film entirely in a computer-generated environment. IV COMPUTER-MODELED ANIMATION Computer-modeled animation is the process of creating three-dimensional models of animated objects. Typically, this is achieved by representing the objects using the following methods: wire-frame, surface, and solid. Wire-frame representations are specified by a set of line segments, typically the object's edges and a set of points on the surface called vertices. While a wire-frame representation often does not produce very realistic images, it is good for quick studies, such as how the object will move and fit in a particular scene. Surface representations are specified by a set of primitive features, such as a collection of polygons to produce smooth curves and surfaces. While it is possible to perfectly model an object's surface as a collection of primitive features, it may not be practical to measure and store these features because complex objects may require an infinite number of features to create a perfectly smooth surface. Solid representations are specified by a set of primitive shapes or portions of primitive shapes. For example, a human might be represented by a sphere for the head and cubes that compose the torso and limbs. Solid representations can specify both inner and outer surfaces of an object. A Image Rendering The process of creating a realistic three-dimensional scene is called rendering. The computer is given a detailed description of the objects that comprise the scene, along with the specifications of the camera. To create photographiclike images, the computer must calculate the viewers' perspective of the image, the visible objects and surfaces; add shading, by determining the available light on each surface; add reflections and shadows; provide surfaces with textures, patterns, and roughness to make objects appear more realistic; add transparency of objects; and remove surfaces hidden by other objects (see Computer Graphics). Once the objects and lights in a three-dimensional scene are rendered, the animator specifies their movement within the scene as well as the motions of the camera. Key frames synchronize the movement of the objects just as in the computer-assisted model, and the in-between frames must be created. One technique, called parametric key-frame animation, interpolates, or blends, the in-between images. Another technique, algorithmic animation, controls motion by applying rules that govern how the objects move. When the objects and their behaviors have been specified, each scene is rendered frame-by-frame by the virtual camera and stored; then the final animated feature is played back. Despite the power of today's computers, and the innovations used to accelerate traditional animation processes, modern computer animations require still faster and more powerful computers to exploit new techniques and potentially photorealistic effects. In the fully animated Disney feature, Toy Story (1995), it took PIXAR Animation Studios an average of 3 hours to render a single frame, and some frames took as long as 24 hours. For this 77-minute movie, 110,880 frames were rendered, requiring approximately 38 years of computing time distributed among many computers. Contributed By: J. Donald Hubbard Kenneth R. O'Connell Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.