Computer graphics
Overview
Computer graphics is a subfield of computer science and is concerned with digitally synthesizing and manipulating visual content. Although the term often refers to three-dimensional computer graphics, it also encompasses two-dimensional graphics and image processing. Graphics is often differentiated from the field of visualization, although the two have many similarities. Entertainment (in the form of animated movies and video games) is perhaps the most well-known application of graphics.
Some major subproblems in computer graphics include:
- describing the shape of an object (modeling)
- describing the motion of an object (animation)
- creating an image of an object (rendering)
Branches of Computer Graphics
Modeling
Modeling describes the shape of an object. The two most common sources of 3D models are those created by an artist using some kind of 3D modeling tool, and those scanned into a computer from real-world objects. Models can also be produced procedurally or via physical simulation.
Because the appearance of an object depends largely on the exterior of the object, boundary representations are most common in computer graphics. Two dimensional surfaces are a good analogy for the objects used in graphics, though quite often these objects are non-manifold. Since surfaces are not finite, a discrete digital approximation is required: polygonal meshes (and to a lesser extent subdivision surfaces) are by far the most common representation, although point-based representations have been gaining some popularity in recent years. Level sets are a useful representation for deforming surfaces which undergo many topological changes such as fluids.
Subfields
- Subdivision surfaces
- Digital geometry processing - surface reconstruction, mesh simplification, mesh repair, parameterization, remeshing, mesh generation, mesh compression, and mesh editing all fall under this heading.
- Discrete differential geometry - DDG is a recent topic which defines geometric quantities for the discrete surfaces used in computer graphics.
- Point-based graphics - a recent field which focuses on points as the fundamental representation of surfaces.
Shading
Texturing, or more generally, shading is the process of describing surface appearance. This description can be as simple as the specification of a color in some colorspace or as elaborate as a shader program which describes numerous appearance attributes across the surface. The term is often used to mean texture mapping, which maps a raster image to a surface to give it detail. A more generic description of surface appearance is given by the bidirectional scattering distribution function, which describes the relationship between incoming and outgoing illumination at a given point.
Animation
Animation refers to the temporal description of an object, i.e., how it moves and deforms over time. There are numerous ways to describe these motion, many of which are used in conjunction with each-other. Popular methods include keyframing, inverse kinematics, and motion capture. As with modeling, physical simulation is another way of specifying motion.
Rendering
Rendering converts a model into an image either by simulating light transport to get physically-based photorealistic images, or by applying some kind of style as in non-photorealistic rendering. See Rendering for more information.
Subfields
- physically-based rendering - concerned with generating images according to the laws of geometric optics
- real time rendering - focuses on rendering for interactive applications, typically using specialized hardware like GPUs
- non-photorealistic rendering
- relighting - recent area concerned with quickly re-rendering scenes
History
William Fetter was credited with coining the term Computer Graphics in 1960, to describe his work at Boeing. One of the first displays of computer animation was Futureworld (1976), which included an animation of a human face and hand — produced by Ed Catmull and Fred Parke at the University of Utah.
The most significant results in computer graphics are published annually in a special edition of the ACM (Association for Computing Machinery) Transactions on Graphics and presented at SIGGRAPH.
An extensive history of computer graphics can be found at [1].
History of the Utah teapot
The Utah teapot or Newell teapot is a 3D model that has become a standard reference object (and something of an in-joke) in the computer graphics community. The model was created in 1975 by early computer graphics researcher Martin Newell, a member of the pioneering graphics program at the University of Utah.
Newell needed a moderately simple mathematical model of a familiar object for his work. At the suggestion of his wife Sandra, he sketched their entire tea service by eye. Then he went back to the lab and edited Bezier control points on a Tektronix storage tube, again by hand. While a cup, saucer, and teaspoon were digitized along with the famous teapot, only the teapot itself attained widespread usage.
The teapot shape contains a number of elements that made it ideal for the graphics experiments of the time. Newell made the mathematical data that describe the teapot's geometry publicly available, and soon other researchers began to use the same data for their computer graphics experiments. They needed something with roughly the same characteristics that Newell had, and using the teapot data meant they did not have to laboriously enter geometric data for some other object. Although technical progress has meant that the act of rendering the teapot is no longer the challenge it was in 1975, the teapot continued to be used as a reference object for increasingly advanced graphics techniques. Over the following decades, editions of computer graphics journals regularly featured versions of the teapot: faceted or smooth-shaded, wireframe, bumpy, translucent, refractive, even leopard-skin and furry teapots were created.
2D computer graphics
2D computer graphics is the computer-based generation of digital images—mostly from two-dimensional models (such as 2D geometric models, text, and digital images) and by techniques specific to them. The term may stand for the branch of computer science that comprises such techniques, or for the models themselves.
2D computer graphics are mainly used in applications that were originally developed upon traditional printing and drawing technologies, such as typography, cartography, technical drawing, advertising, etc.. In those applications, the two-dimensional image is not just a representation of a real-world object, but an independent artifact with added semantic value; two-dimensional models are therefore preferred, because they give more direct control of the image than 3D computer graphics (whose approach is more akin to photography than to typography).
In many domains, such as desktop publishing, engineering, and business, a description of a document based on 2D computer graphics techniques can be much smaller than the correspoding digital image—often by a factor of 1/1000 or more. This representation is also more flexible since it can be rendered at different resolutions to suit different output devices. For these reasons, documents and illustrations are often stored or transmitted as 2D graphic files.
2D computer graphics started in the 1950s, based on vector graphics devices. These were largely supplanted by raster-based devices in the following decades. The PostScript language and the X Window System protocol were landmark developments in the field.
2D graphics techniques
2D graphics models may combine geometric models (also called vector graphics), digital images (also called raster graphics), text to be typeset (defined by content, font style and size, color, position, and orientation), mathematical functions and equations, and more. These components can be modified and manipulated by two-dimensional geometric transformations such as translation, rotation, scaling.
In object oriented graphics, the image is described indirectly by an object endowed with a self-rendering method—a procedure which assigns colors to the image pixels by an arbitrary algorithm. Complex models can be built by combining simpler objects, in the paradigms of object-oriented programming.
Direct painting
A convenient way to create a complex image is to start with a blank "canvas" raster map (an array of pixels, also known as a bitmap) filled with some uniform background color and then "draw", "paint" or "paste" simple patches of color onto it, in an appropriate order. In particular, the canvas may be the frame buffer for a computer display.
Some programs will set the pixel colors directly, but most will rely on some 2D graphics library and/or the machine's graphics card, which usually implement the following operations:
- paste a given image at a specified offset onto the canvas;
- write a string of characters with a specified font, at a given position and angle;
- paint a simple geometric shape, such as a triangle defined by three corners, or a circle with given center and radius;
- draw a line segment, arc of circle, or simple curve with a virtual pen of given width.
Extended color models
Text, shapes and lines are rendered with a client-specified color. Many libraries and cards provide color gradients, which are handy for the generation of smoothly-varying backgrounds, shadow effects, etc.. (See also Gouraud shading). The pixel colors can also be taken from a texture, e.g. a digital image (thus emulating rub-on screentones and the fabled "checker paint" which used to be available only in cartoons).
Painting a pixel with a given color usually replaces its previous color. However, many systems support painting with transparent and translucent colors, which only modify the previous pixel values. The two colors may also be combined in fancier ways, e.g. by computing their bitwise exclusive or. This technique is known as inverting color or color inversion, and is often used in graphical user interfaces for highlighting, rubber-band drawing, and other volatile painting—since re-painting the same shapes with the same color will restore the original pixel values.
Layers
The models used in 2D computer graphics usually do not provide for three-dimensional shapes, or three-dimensional optical phenomena such as lighting, shadows, reflection, refraction, etc.. However, they usually can model multiple layers (conceptually of ink, paper, or film; opaque, translucent, or transparent—stacked in a specific order. The ordering is usually defined by a single number (the layer's depth, or distance from the viewer).
Layered models are sometimes called 2 1/2-D computer graphics. They make it possible to mimic traditional drafting and printing techniques based on film and paper, such as cutting and pasting; and allow the user to edit any layer without affecting the others. For these reasons, they are used in most graphics editors. Layered models also allow better anti-aliasing of complex drawings and provide a sound model for certain techniques such as mitered joints and the even-odd rule.
Layered models are also used to allow the user to suppress unwanted information when viewing or printing a document, e.g. roads and/or railways from a map, certain process layers from an integrated circuit diagram, or hand annotations from a business letter.
In a layer-based model, the target image is produced by "painting" or "pasting" each layer, in order of decreasing depth, on the virtual canvas. Conceptually, each layer is first rendered on its own, yielding a digital image with the desired resolution which is then painted over the canvas, pixel by pixel. Fully transparent parts of a layer need not be rendered, of course. The rendering and painting may be done in parallel, i.e. each layer pixel may be painted on the canvas as soon as it is produced by the rendering procedure.
Layers that consist of complex geometric objects (such as text or polylines) may be broken down into simpler elements (characters or line segments, respectively), which are then painted as separate layers, in some order. However, this solution may create undesirable aliasing artifacts wherever two elements overlap the same pixel.
See also Portable Document Format#Layers.
2D graphics hardware
Modern computer graphics card displays almost overwhelmingly use raster techniques, dividing the screen into a rectangular grid of pixels, due to the relatively low cost of raster-based video hardware as compared with vector graphic hardware. Most graphic hardware has internal support for blitting operations and sprite drawing. A co-processor dedicated to blitting is known as a Blitter chip.
Classic 2D graphics chips of the late 1970s and early 80s, used in the 8-bit video game consoles and home computers, include:
- Atari's ANTIC (actually a 2D GPU), TIA, CTIA, and GTIA
- Commodore/MOS Technology's VIC and VIC-II
2D graphics software
Many graphical user interfaces (GUIs), including Mac OS, Microsoft Windows, or the X Window System, are primarily based on 2D graphical concepts. Such software provides a visual environment for interacting with the computer, and commonly includes some form of window manager to aid the user in conceptually distinguishing between different applications. The user interface within individual software applications is typically 2D in nature as well, due in part to the fact that most common input devices, such as the mouse, are constrained to two dimensions of movement.
2D graphics are very important in the control peripherals such as printers, plotters, sheet cutting machines, etc.. They were also used in most early video and computer games; and are still used for card and board games such as solitaire, chess, mahjongg, etc..
2D graphics editors or drawing programs are application-level software for the creation of images, diagrams and illustrations by direct manipulation (through the mouse, graphics tablet, or similar device) of 2D computer graphics primitives. These editors generally provide geometric primitives as well as digital images; and some even support procedural models. The illustration is usually represented internally as a layered model, often with a hierarchical structure to make editing more convenient. These editors generally output graphics files where the layers and primitives are separately preserved in their original form. MacDraw, introduced in 1984 with the Macintosh line of computers, was an early example of this class; recent examples are the commercial products Adobe Illustrator and CorelDRAW, and the free editors such as xfig or Inkscape. There are also many 2D graphics editors specialized for certain types of drawings such as electrical, electronic and VLSI diagrams, topographic maps, computer fonts, etc.
Image editors are specialized for the manipulation of digital images, mainly by means of free-hand drawing/painting and signal processing operations. They typically use a direct-painting paradigm, where the user controls virtual pens, brushes, and other free-hand artistic instruments to apply paint to a virtual canvas. Some image editors support a multiple-layer model; however, in order to support signal-processing operations like blurring each layer is normally represented as a digital image. Therefore, any geometric primitives that are provided by the editor are immediately converted to pixels and painted onto the canvas. The name raster graphics editor is sometimes used to contrast this approach to that of general editors which also handle vector graphics. One of the first popular image editors was Apple's MacPaint, companion to MacDraw. Modern examples are the free GIMP editor, and the commercial products Photoshop and Paint Shop Pro. This class too includes many specialized editors — for medicine, remote sensing, digital photography, etc.
See also
- Graphics
- Digital image editing
- Bit blit
- Computer painting
- Image scaling
- Logo programming language
- Macromedia Flash
- PostScript
- Transparency in graphics
- SVG
Applications
- Special effects
- Video games
Current Challenges
Connected Studies
- Computer vision
- Image processing
See Also
Numerous sub-areas of computer graphics can be found in.
Miscellaneous
- Digital geometry
- Digital image editing
- Graphics processing unit (GPU)
- Graphical output devices
- Utah Teapot
- Stanford Bunny
- SIGGRAPH
- ASCII art
External Links
- A Critical History of Computer Graphics and Animation
- History of Computer Graphics series of articles
- The ARTS: Episode 5 An in depth interview with Legalize on the subject of the History of Computer Graphics. (Available in MP3 audio format)
- CGSociety The Computer Graphics Society
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