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Displays : Glossary

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3-D (three dimensions or three-dimensional) - describes an image that provides the perception of depth. When 3-D images are made interactive so that users feel involved with the scene, the experience is called virtual reality. You usually need a special plug-in viewer for your Web browser to view and interact with 3-D images. Virtual reality experiences may also require additional equipment. 3-D image creation can be viewed as a three-phase process of: tessellation, geometry, and rendering. In the first phase, models are created of individual objects using linked points that are made into a number of individual polygons (tiles). In the next stage, the polygons are transformed in various ways and lighting effects are applied. In the third stage, the transformed images are rendered into objects with very fine detail.

active matrix display - a technology used in the flat panel liquid crystal displays of notebook and laptop computers. Active matrix displays provide a more responsive image at a wider range of viewing angle than dual scan (passive matrix) displays. Also known as thin film transistor display .

aliasing - In sound and image generation, the generation of a false (alias) frequency along with the correct one when doing frequency sampling. For images, this produces a jagged edge, or stair-step effect. For sound, it produces a buzz.

antialiasing - the smoothing of the image or sound roughness caused by aliasing. With images, approaches include adjusting pixel positions or setting pixel intensities so that there is a more gradual transition between the color of a line and the background color.

bit depth - the number of binary digit (bits) you have in which to describe something. Each additional bit in a binary number doubles the number of possibilities. By the time you have a 16-bit sequence, there are 65,536 possible levels. Add one more bit, and you double the possible accuracy (to 131,072 levels). When you have 24 bits to describe the color of a pixel, there are 16,777,216 available levels of color.

bit map (or bitmap) - defines a display space and the color for each pixel or "bit" in the display space. A Graphics Interchange Format and a JPEG are examples of graphic image file types that contain bit maps. A bit map does not need to contain a bit of color-coded information for each pixel on every row. It only needs to contain information indicating a new color as the display scans along a row. Thus, an image with much solid color will tend to require a small bit map. Because a bit map uses a fixed or raster graphics method of specifying an image, the image cannot be immediately rescaled by a user without losing definition. A vector graphics graphic image, however, is designed to be quickly rescaled. Typically, an image is created using vector graphics and then, when the artist is satisifed with the image, it is converted to (or saved as) a raster graphic file or bit map.

brightness - along with hue and saturation, one of the three aspects of color in the red, green, and blue (RGB) scheme. All possible colors can be specified according to hue, saturation, and brightness (also called brilliance), just as colors can be represented in terms of the R, G, and B components. Brightness is a relative expression of the intensity of the energy output of a visible light source. It can be expressed as a total energy value or as the amplitude at the wavelength where the intensity is greatest. In the RGB color model, the amplitudes of red, green, and blue for a particular color can each range from 0 to 100 percent of full brilliance. These levels are represented by the range of decimal numbers from 0 to 255, or hexadecimal numbers from 00 to FF.

browser-safe palette - a choice of 216 colors that will look the same on both the Microsoft Internet Explorer and the Netscape browsers. (These colors are shown under "browser-safe palette" on

CAD/CAM (computer-aided design/computer-aided manufacturing) - display software that is used to design products such as electronic circuit boards in computers and other devices.

CGA (Color Graphics Adapter) - a display mode introduced by IBM in 1981 that provided four colors and a maximum resolution of 320 pixels horizontally by 200 pixels vertically.

CMYK (cyan, magenta, yellow, and black) - a scheme for combining primary pigments. The C stands for cyan (aqua), M stands for magenta (pink), Y is yellow, and K stands for black. The CMYK pigment model works like an "upside-down" version of the RGB (red, green, and blue) color model. Many paint and draw programs can make use of either the RGB or the CMYK model. The RGB scheme is used mainly for computer displays, while the CMYK model is used for printed color illustrations (hard copy).

compression - minimizing the size in bytes of a graphics file without degrading the quality of the image to an unaccceptable level. The reduction in file size allows more images to be stored in a given amount of disk or memory space. It also reduces the time required for images to be sent over the Internet or downloaded from Web pages.

compression artifact - is the fuzz or distortion in a compressed image or sequence of video images. When a photo is compressed into a JPEG format, some data is lost, which is why this type of compression is called lossy compression. The data that is lost is considered to be not necessary for the viewer to perceive or perhaps not perceivable in any case. In creating a JPEG, if you wish to have a smaller file size regardless of quality, then the result may introduce perceivable compression artifacts. However, if you wish to maintain the quality of the photo and have a larger file size, then the viewer may not notice any artifacts. Artifacts are more evident on a computer image than in a printed photograph.

CRT (cathode ray tube) - a specialized vacuum tube in which images are produced when an electron beam strikes a phosphorescent surface. Most desktop computer displays make use of CRTs. The CRT in a computer display is similar to the "picture tube" in a television receiver.

display - a computer output surface and projecting mechanism that shows text and often graphic images to the computer user, using a cathode ray tube (CRT), liquid crystal display (LCD), light-emitting diode (LED), gas plasma, or other image projection technology. The display is usually considered to include the screen or projection surface and the device that produces the information on the screen. In some computers, the display is packaged in a separate unit called a monitor. In other computers, the display is integrated into a unit with the processor and other parts of the computer.

display modes - the various display image formats and resolutions that have evolved. In addition to monochrome, color display modes include CGA, EGA, VGA, XGA, and SVGA.

dithering - the attempt by a computer program to approximate a color from a mixture of other colors when the required color is not available. For example, dithering occurs when a color is specified for a Web page that a browser on a particular operating system can't support. The browser will then attempt to replace the requested color with an approximation composed of two or more other colors it can produce. The result may or may not be acceptable to the graphic designer. It may also appear somewhat grainy since it's composed of different pixel intensities rather than a single intensity over the colored space.

dot pitch - the specification that tells how sharp the image on a display can be. The dot pitch is measured in millimeters (mm) and a smaller number means a sharper image. In desk top monitors, common dot pitches are .31mm, .28mm, .27mm, .26mm, and .25mm. Personal computer users will usually want a .28mm or finer. Some large monitors for presentation use may have a larger dot pitch (.48mm, for example). Think of the dot specified by the dot pitch as the smallest physical visual component on the display. A pixel is the smallest programmable visual element and maps to the dot if the display is set to its highest resolution. When set to lower resolutions, a pixel encompasses multiple dots.

EGA (Enhanced Graphics Adapter) - a display mode introduced by IBM in 1984 that allowed up to 16 different colors and improved the resolution to 640 pixels horizontally by 350 pixels vertically. While an improvement over CGA, it did not offer sufficient image resolution for high-level applications such as graphic design and desktop publishing.

gas plasma display - see "plasma display."

GIF (Graphics Interchange Format) - the most common type of image on the Web. A GIF file contains a rasterized image encoded in binary and uses LZW compression. There are two versions of the format, 87a and GIF89a. Version 89a (July, 1989) allows for the possibility of an animated GIF, which is a short sequence of images within a single GIF file. A GIF89a can also be specified for interlaced GIF presentation.

hologram - a three-dimensional image, created with photographic projection. Unlike 3-D or virtual reality on a two-dimensional computer display, a hologram is a truly three-dimensional and free-standing image that does not simulate spatial depth or require a special viewing device. Theoretically, holograms could someday be transmitted electronically to a special display device in your home and business. The theory of holography was developed by Dennis Gabor in 1947. The development of laser technology made holography possible.

hue - along with saturation and brightness, one of the three aspects of color in the red, green, and blue (RGB) scheme. All possible colors can be specified according to hue, saturation, and brightness (also called brilliance), just as colors can be represented in terms of the R, G, and B components. Most sources of visible light contain energy over a band of wavelengths. Hue is the wavelength within the visible-light spectrum at which the energy output from a source is greatest.

image - in computers, a picture that has been created or copied and stored in electronic form. An image can be described in terms of vector graphics or raster graphics. An image stored in raster form is sometimes called a bit map.

image map - a file containing information that associates different locations on a specified image with hypertext links (for example, a map of the world that allows a user to click on or touch any country in order to find out more about it).

imaging - the capture, storage, manipulation, and display of images. In document imaging, the emphasis is on capturing, storing, and retrieving information from the images (which are often mainly images of text). In graphical imaging, the emphasis is on the manipulation of created images in order to achieve special effects through rotating, stretching, blurring, resizing, twirling, and other changes to the original image.

interlaced display - a cathode-ray tube (CRT) display in which the lines are scanned alternately in two interwoven raster scans. In a CRT display, there are several hundred horizontal lines in a frame (full screen). These lines are scanned from left to right, and from top to bottom. The refresh rate (number of frames scanned per second) varies, but it is normally between 60 and 100 hertz. Refresh rates slower than 60 Hz produce distracting screen flicker, which can cause headaches and eye fatigue. Most CRT computer monitors scan each line in turn from top to bottom at the lowest resolution levels (640 x 480 and 800 x 600 pixel). However, at the higher resolutions, such as 1024 x 768 or 1200 x 800, the frame is sometimes scanned in interlaced fashion: first the odd-numbered lines, and then the even-numbered lines. This allows for a lower refresh rate without producing flicker. With text and fixed graphics displays, this scheme can work well. However, with animated graphics -- especially images that move or change form rapidly -- interlacing can produce a fluttering effect at least as irritating as screen flicker. For serious animated-graphics work and video editing, a non-interlaced CRT display is recommended. The refresh rate should be as high as the system will allow, ideally 70 Hz or more.

interlaced GIF - a GIF image that seems to arrive on your display like an image coming through a slowly-opening Venetian blind. A fuzzy outline of an image is gradually replaced by seven successive waves of bit streams that fill in the missing lines until the image arrives at its full resolution. The faster the connection, the less advantage an interlaced GIF has over a non-interlaced GIF.

JPEG (Joint Photographic Experts Group) - a graphic image created by choosing from a range of compression qualities (actually, from one of a suite of compression algorithm). When you create a JPEG or convert an image from another format to a JPEG, you are asked to specify the quality of image you want. Since the highest quality results in the largest file, you can make a trade-off between image quality and file size. Formally, the JPEG file format is ISO standard 10918. The JPEG scheme includes 29 distinct coding processes although a JPEG implementor may not use them all. JPEG is an acronym for Joint Photographic Experts Group, the committee that established the baseline algorithms.

LCD (liquid crystal display) - the predominant technology used for displays in notebook and other smaller computers. Like light-emitting diode (LED) and gas-plasma technologies, LCDs allow displays to be much thinner than cathode ray tube (CRT) technology. LCDs consume much less power than LED and gas-display displays because they work on the principle of blocking light rather than emitting it. An LCD is made with either a passive matrix or an active matrix display display grid. The active matrix LCD is also known as a thin film transistor (TFT) display.

LED (light-emitting diode) - a semiconductor device that emits visible light when an electric current passes through it. The light is not particularly bright, but in most LEDs it is monochromatic, occurring at a single wavelength. The output from an LED can range from red (at a wavelength of approximately 700 nanometers) to blue-violet (about 400 nanometers).

lossless and lossy compression - With lossless compression, every single bit of data that was originally in the file remains after the file is uncompressed. All of the information is completely restored. This is generally the technique of choice for text or spreadsheet files, where losing words or financial data could pose a problem. The Graphics Interchange File (GIF) is an image format used on the Web that provides lossless compression. On the other hand, lossy compression reduces a file by permanently eliminating certain information, especially redundant information. When the file is uncompressed, only a part of the original information is still there (although the user may not notice it).

monitor - a computer display and related parts packaged in a physical unit that is separate from other parts of the computer. Notebook computers don't have monitors because all the display and related parts are integrated into the same physical unit with the rest of the computer. In practice, the terms monitor and display are used interchangably.

palette - in art, the board on which an artist puts selected colors and also the set of colors themselves. On the Web, choosing the colors you use not only involves understanding which colors work well together but also understanding the capabilities of display screens and browser for displaying the colors you choose. In computer display technology, a color is set for each individual pixel or addressable illumination element on the screen. Each pixel has a red, a green, and a blue (RGB) component. By specifying the amount of intensity for each of these components, a distinct color is given to that pixel.

pixel (short for "picture element") - the basic unit of programmable color on a computer display or in a computer image. Think of it as a logical - rather than a physical - unit. The physical size of a pixel depends on how you've set the resolution for the display screen. If you've set the display to its maximum resolution, the physical size of a pixel will equal the physical size of the dot pitch (let's just call it the dot size) of the display. If, however, you've set the resolution to something less than the maximum resolution, a pixel will be larger than the physical size of the screen's dot (that is, a pixel will use more than one dot). The specific color that a pixel describes is some blend of three components of the color spectrum - RGB. Up to three bytes of data are allocated for specifying a pixel's color, one byte for each color. A true color or 24-bit color system uses all three bytes. However, some color display systems use only eight-bits (which provides up to 256 different colors).

pixels per inch (ppi) - a measure of the sharpness (that is, the density of illuminated points) on a display screen. The dot pitch determines the absolute limit of the possible pixels per inch. However, the displayed resolution of pixel (picture elements) that is set up for the display is usually not as fine as the dot pitch. The pixels per inch for a given picture resolution will differ based on the overall screen size since the same number of pixels are being spread out over a different space. The term "dots per inch (dpi)," extended from the print medium, is sometimes used instead of pixels per inch.

plasma display - a display in which each pixel on the screen is illuminated by a tiny bit of plasma or charged gas, somewhat like a tiny neon light. Plasma displays are thinner than cathode ray tube (CRT) displays and brighter than liquid crystal displays (LCDs). Plasma displays are sometimes marketed as "thin-panel" displays and can be used to display either analog video signals or display modes digital computer input. In addition to the advantage of slimness, a plasma display is flat rather than slightly curved as a CRT display is and therefore free of distortion on the edges of the screen. Unlike many LCD displays, a plasma display offers a very wide viewing angle. Plasma displays come in conventional PC displays sizes and also in sizes up to 60 inches for home theater and high definition television.

polymer LED (sometimes called light-emitting polymer or polyLED) - a technology based on the use of polymer as the semiconductor material in LEDs. Polymers are chemical substances that consist of large molecules that are, themselves, made from many smaller and simpler molecules: proteins and DNA are examples of naturally occuring polymers; many others, such as nylon, are artificially created. Because of their flexibility and strength, polymers are used for products such as car bumpers and bullet-proof vests. Because polymers are lightweight and flexible, they can simply and inexpensively be used for the creation of thin film displays for future technologies such as the electronic newspaper.

progressive JPEG - the JPEG equivalent of the interlaced GIF file, an image created using the JPEG suite of compression algorithms that will "fade in" in successive waves of lines until the entire image has completely arrived. Like the interlaced GIF, a progressive JPEG is a more appealing way to deliver an image at modem connection speeds.

raster - the region of a cathode ray tube (CRT) or liquid crystal display (LCD) monitor that is capable of rendering images.

raster graphics - digital images created or captured (for example, by scanner in a photo) as a set of samples of a given space. A raster is a grid of x and y coordinates on a display space. (And for three-dimensional images, a z coordinate.) A raster image file identifies which of these coordinates to illuminate in monochrome or color values. The raster file is sometimes referred to as a bit map because it contains information that is directly mapped to the display grid.

resolution - the number of pixels (individual points of color) contained on a display monitor, expressed in terms of the number of pixels on the horizontal axis and the number on the vertical axis. The sharpness of the image on a display depends on the resolution and the size of the monitor. The same pixel resolution will be sharper on a smaller monitor and gradually lose sharpness on larger monitors because the same number of pixels are being spread out over a larger number of inches. A given computer display system will have a maximum resolution that depends on its physical ability to focus light (in which case the physical dot size - the dot pitch - matches the pixel size) and usually several lesser resolutions. For example, a display system that supports a maximum resolution of 1280 by 1023 pixels may also support 1024 by 768, 800 by 600, and 640 by 480 resolutions. Note that on a given size monitor, the maximum resolution may offer a sharper image but be spread across a space too small to read well.

RGB (red, green, blue) - a system for representing the colors to be used on a computer display. Red, green, and blue can be combined in various proportions to obtain any color in the visible spectrum. Levels of R, G, and B can each range from 0 to 100 percent of full intensity. Each level is represented by the range of decimal numbers from 0 to 255 (256 levels for each color), equivalent to the range of binary numbers from 00000000 to 11111111, or hexadecimal 00 to FF. The total number of available colors is 256 x 256 x 256, or 16,777,216 possible colors.

saturation - along with brightness and hue, one of the three aspects of color in the red, green, and blue (RGB) scheme. All possible colors can be specified according to hue, saturation, and brightness (also called brilliance), just as colors can be represented in terms of the R, G, and B components. Saturation is an expression for the relative bandwidth of the visible output from a light source. As saturation increases, colors appear more "pure." As saturation decreases, colors appear more "washed-out."

screen - the physical surface on a computer display on which visual information is presented. This surface is usually made of glass. The screen size is measured from one corner to the opposite corner diagonally. Common screen sizes for desktop display screens are 12, 14, 17, 19, and 21 inches. This term is not to be confused with panel , which is a representation of what information will appear on the screen in given circumstances.

SVGA (originally Super Video Graphics Array) - an informal name for the Video Electronics Standards Association's standard, the VESA BIOS Extension. Typically, an SVGA display can support a palette of up to 16,000,000 colors, although the amount of video memory in a particular computer may limit the actual number of displayed colors to something less than that. Image-resolution specifications vary. In general, the larger the diagonal screen measure of an SVGA monitor, the more pixels it can display horizontally and vertically. Small SVGA monitors (14-inch diagonal) usually display 800 pixels horizontally by 600 pixels vertically. The largest monitors (20 inches or more diagonal measure) can display 1280 x 1024, or even 1600 x 1200, pixels.

TFT (thin film transistor) - a liquid crystal display (LCD), common in notebook and laptop computers, that has a transistor for each pixel (that is, for each of the tiny elements that control the illumination of your display). Having a transistor at each pixel means that the current that triggers pixel illumination can be smaller and therefore can be switched on and off more quickly. TFT is also known as active matrix display technology (and contrasted with "passive matrix" which does not have a transistor at each pixel).

touch screen - a computer display screen that is sensitive to human touch, allowing a user to interact with the computer by touching pictures or words on the screen. Touch screens are used with information kiosks, computer-based training devices, and systems designed to help individuals who have difficulty manipulating a mouse or keyboard. There are three types of touch screen technologies: resistive, surface wave, and capacitive.

transparentized GIF - a GIF image with a color that has been made to disappear into or blend in with whatever color the background of your Web page is. Typically, images are created in some rectangular shape against a background color and you want only the main image, not the entire rectangle with background, to be presented. For example, if you've created a ball, you may want only the ball to show. Transparentizing the image allows you to specify a color that will be transparent or disappear against the background of the browser page.

true color - the specification of the color of a pixel on a display screen using a 24-bit value, which allows the possibility of up to 16,777,216 possible colors. Many displays today support only an 8-bit color value, allowing up to 256 possible colors. The number of bits used to define a pixel's color shade is its bit-depth. True color is sometimes known as 24-bit color. Some new color display systems offer a 32-bit color mode. The extra byte, called the alpha channel, is used for control and special effects information.

twip (twentieth of a point) - a measure used in laying out space or defining objects on a page or other area that is to be printed or displayed on a computer screen. A twip is 1/1440th of an inch or 1/567th of a centimeter. That is, there are 1440 twips to an inch or 567 twips to a centimeter. The twip is 1/20th of a point, a traditional measure in printing. A point is approximately 1/72nd of an inch. A number of computer software programs, notably Microsoft's Visual Basic, as well as its rich text file format (RTF), require programmers to specify screen positions and image and icon sizes in twips rather than in another common measure, the pixel. Like the pixel, the twip can be adjusted in size as screen resolution is changed, but, unlike the pixel, expresses an absolute value for printing.

vector - a quantity or phenomenon that has two independent properties: magnitude and direction. The term also denotes the mathematical or geometrical representation of such a quantity.

vector graphics - the creation of digital images through a sequence of commands or mathematical statements that place lines and shapes in a given two-dimensional or three-dimensional space. In physics, a vector is a representation of both a quantity and a direction at the same time. In vector graphics, the file that results from a graphic artist's work is created and saved as a sequence of vector statements. For example, instead of containing a bit in the file for each bit of a line drawing, a vector graphic file describes a series of points to be connected. One result is a much smaller file.

VGA (Video Graphics Array) - a display mode introduced by IBM in 1987 that a;llowed a choice between 16 colors at 640 x 480 pixels or 256 colors at 320 x 200 pixels. All IBM-compatible computers support the VGA standard.

voxel - a unit of graphic information that defines a point in three-dimensional space. Since a pixel (picture element) defines a point in two dimensional space with its x and y coordinates, a third z coordinate is needed. In 3-D space, each of the coordinates is defined in terms of its position, color, and density. Think of a cube where any point on an outer side is expressed with an x, y coordinate and the third, z coordinate defines a location into the cube from that side, its density, and its color.

x and y coordinates - the horizontal and vertical addresses of any pixel or addressable point on a computer display screen. The x coordinate is a given number of pixels along the horizontal axis of a display starting from the pixel (pixel 0) on the extreme left of the screen. The y coordinate is a given number of pixels along the vertical axis of a display starting from the pixel (pixel 0) at the top of the screen. Together, the x and y coordinates locate any specific pixel location on the screen. x and y coordinates can also be specified as values relative to any starting point on the screen or any subset of the screen such as an image.

XGA (Extended Graphics Array) - a display mode introduced by IBM in 1990 as a successor to its 8514/A display. A later version, XGA-2 offers 800 by 600 pixel resolution in true color (16 million colors) and 1,024 by 768 resolution in 65,536 colors.

z coordinate - the third-dimensional coordinate in a volume pixel, or voxel. Together with x and y coordinates, the z coordinate defines a location in a three-dimensional space.

This was last updated in June 2006

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