Computer-generated imagery (CGI) is a specific application of computer graphics for creating or improving images in art, printed media, simulators, videos, and video games. These images are either static (i.e. still images) or dynamic (i.e. moving images). CGI both refers to 2D computer graphics and (more frequently) 3D computer graphics with the purpose of designing characters, virtual worlds, or scenes and special effects (in films, television programs, commercials, etc.). The application of CGI for creating or improving animations is called computer animation (or CGI animation).
History
The first feature film to use CGI as well as the composition of live-action film with CGI was Vertigo, which used abstract computer graphics by John Whitney in the opening credits of the film. The first feature film to make use of CGI with live action in the storyline of the film was the 1973 film Westworld. The first feature film to present a fully CGI character was the 1985 film Young Sherlock Holmes, showcasing a fully animated stained glass knight character. Other early films that incorporated CGI include Demon Seed (1977), Star Wars (1977), Tron (1982), Star Trek II: The Wrath of Khan (1982), Golgo 13: The Professional (1983), The Last Starfighter (1984), The Return of Godzilla (1984), The Abyss (1989), Terminator 2: Judgement Day (1991), and Jurassic Park (1993). The first music video to use CGI was Will Powers' "Adventures in Success" (1983). In 1995, Pixar's Toy Story became the first fully CGI feature film, marking a historic milestone for both animation and film-making. Prior to CGI being prevalent in film, virtual reality, personal computing, and gaming, one of the early practical applications of CGI was for aviation and military training, namely the flight simulator. Visual systems developed in flight simulators were also an important precursor to three-dimensional computer graphics and Computer Generated Imagery (CGI) systems today. Namely because the object of flight simulation was to reproduce on the ground the behavior of an aircraft in flight. Much of this reproduction had to do with believable visual synthesis that mimicked reality. The Link Digital Image Generator (DIG) by the Singer Company (Singer-Link), was considered one of the world's first generation CGI systems. It was a real-time, 3D capable, day/dusk/night system that was used by the NASA Space Shuttles, for F-111s, Black Hawk, and the B-52. Link's Digital Image Generator had architecture to provide a visual system that realistically corresponded with the view of the pilot. The basic architecture of the DIG and subsequent improvements contained a scene manager followed by geometric processor, video processor, and into the display with the end goal of a visual system that processed realistic texture, shading, translucency capabilities, and free of aliasing.
Combined with the need to pair virtual synthesis with military-level training requirements, CGI technologies applied in flight simulation were often years ahead of what would have been available in commercial computing or even in high-budget film. Early CGI systems could depict only objects consisting of planar polygons. Advances in algorithms and electronics in flight simulator visual systems and CGI in the 1970s and 1980s influenced many technologies still used in modern CGI adding the ability to superimpose texture over the surfaces as well as transition imagery from one level of detail to the next one in a smooth manner.
The evolution of CGI led to the emergence of virtual cinematography in the 1990s, where the vision of the simulated camera is not constrained by the laws of physics. Availability of CGI software and increased computer speeds have allowed individual artists and small companies to produce professional-grade films, games, and fine art from their home computers.
Static images and landscapes
Not only do animated images form part of computer-generated imagery; natural looking landscapes (such as fractal landscapes) are also generated via computer algorithms. A simple way to generate fractal surfaces is to use an extension of the triangular mesh method, relying on the construction of some special case of a de Rham curve, e.g., midpoint displacement. For instance, the algorithm may start with a large triangle, then recursively zoom in by dividing it into four smaller Sierpinski triangles, then interpolate the height of each point from its nearest neighbors. The creation of a Brownian surface may be achieved not only by adding noise as new nodes are created but by adding additional noise at multiple levels of the mesh. Thus a topographical map with varying levels of height can be created using relatively straightforward fractal algorithms. Some typical, easy-to-program fractals used in CGI are the plasma fractal and the more dramatic fault fractal.Game programming gems 2 by Mark A. DeLoura 2001 page 240
Many specific techniques have been researched and developed to produce highly focused computer-generated effects — e.g., the use of specific models to represent the chemical weathering of stones to model erosion and produce an "aged appearance" for a given stone-based surface.Digital modeling of material appearance by Julie Dorsey, Holly Rushmeier, François X. Sillion 2007 page 217
Architectural scenes
Modern architects use services from computer graphic firms to create 3-dimensional models for both customers and builders. These computer generated models can be more accurate than traditional drawings. Architectural animation (which provides animated movies of buildings, rather than interactive images) can also be used to see the possible relationship a building will have in relation to the environment and its surrounding buildings. The processing of architectural spaces without the use of paper and pencil tools is now a widely accepted practice with a number of computer-assisted architectural design systems.
Architectural modeling tools allow an architect to visualize a space and perform "walk-throughs" in an interactive manner, thus providing "interactive environments" both at the urban and building levels.Interactive environments with open-source software: 3D walkthroughs by Wolfgang Höhl, Wolfgang Höhl, 2008, , pp. 24–29. Specific applications in architecture not only include the specification of building structures (such as walls and windows) and walk-throughs but the effects of light and how sunlight will affect a specific design at different times of the day.Advances in Computer and Information Sciences and Engineering by Tarek Sobh 2008 pages 136-139
Architectural modeling tools have now become increasingly internet-based. However, the quality of internet-based systems still lags behind sophisticated in-house modeling systems.Encyclopedia of Multimedia Technology and Networking, Volume 1 by Margherita Pagani 2005 page 1027
In some applications, computer-generated images are used to "reverse engineer" historical buildings. For instance, a computer-generated reconstruction of the monastery at Georgenthal in Germany was derived from the ruins of the monastery, yet provides the viewer with a "look and feel" of what the building would have looked like in its day.Interac storytelling: First Joint International Conference by Ulrike Spierling, Nicolas Szilas 2008 pages 114-118
Anatomical models
Computer generated models used in skeletal animation are not always anatomically correct. However, organizations such as the Scientific Computing and Imaging Institute have developed anatomically correct computer-based models. Computer generated anatomical models can be used both for instructional and operational purposes. To date, a large body of artist-produced medical images continue to be used by medical students, such as images by Frank H. Netter, e.g. Cardiac images. However, a number of online anatomical models are becoming available.
A single patient X-ray is not a computer generated image, even if digitized. However, in applications which involve CT scans a three-dimensional model is automatically produced from many single-slice x-rays, producing "computer generated image". Applications involving magnetic resonance imaging also bring together a number of "snapshots" (in this case via magnetic pulses) to produce a composite, internal image.
In modern medical applications, patient-specific models are constructed in 'computer assisted surgery'. For instance, in total knee replacement, the construction of a detailed patient-specific model can be used to carefully plan the surgery.Total Knee Arthroplasty by Johan Bellemans, Michael D. Ries, Jan M.K. Victor 2005 pages 241-245 These three-dimensional models are usually extracted from multiple CT scans of the appropriate parts of the patient's own anatomy. Such models can also be used for planning aortic valve implantations, one of the common procedures for treating heart disease. Given that the shape, diameter, and position of the coronary openings can vary greatly from patient to patient, the extraction (from CT scans) of a model that closely resembles a patient's valve anatomy can be highly beneficial in planning the procedure.I. Waechter et al. Patient Specific Models for Minimally Invasive Aortic Valve Implantation in Medical Image Computing and Computer-Assisted Intervention -- MICCAI 2010 edited by Tianzi Jiang, 2010 pages 526-560
Cloth and skin images
Models of cloth generally fall into three groups: The geometric-mechanical structure at yarn crossing The mechanics of continuous elastic sheets The geometric macroscopic features of cloth.Cloth modeling and animation by Donald House, David E. Breen 2000 page 20 To date, making the clothing of a digital character automatically fold in a natural way remains a challenge for many animators.Film and photography by Ian Graham 2003 page 21
In addition to their use in film, advertising, and other modes of public display, computer generated images of clothing are now routinely used by top fashion design firms.Designing clothes: culture and organization of the fashion industry by Veronica Manlow 2007 page 213
The challenge in rendering human skin images involves three levels of realism: Photo realism in resembling real skin at the static level Physical realism in resembling its movements Function realism in resembling its response to actions.Handbook of Virtual Humans by Nadia Magnenat-Thalmann and Daniel Thalmann, 2004 pages 353-370 The finest visible features such as fine wrinkles and skin pores are the size of about 100 μm or 0.1 millimetres. Skin can be modeled as a 7-dimensional bidirectional texture function (BTF) or a collection of bidirectional scattering distribution function (BSDF) over the target's surfaces.
When animating a texture like hair or fur for a computer generated model, individual base hairs are first created and later duplicated to demonstrate volume. The initial hairs are often different lengths and colors, to each cover several different sections of a model. This technique was notably used in Pixar's Monsters Inc (2001) for the character Sulley, who had approximately 1,000 initial hairs generated that were later duplicated 2,800 times. The quantity of duplications can range from thousands to millions, depending on the level of detail sought after.
Interactive simulation and visualization