The definitions below came from www.pcwebopedia.com (an on-line dictionary of computer terms).

Algorithm: A formula or set of steps for solving a particular problem. To be an algorithm, a set of rules must be unambiguous and have a clear stopping point. Algorithms can be expressed in any language, from natural languages like English or French to programming languages like FORTRAN.

We use algorithms every day. For example, a recipe for baking a cake is an algorithm. Most programs, with the exception of some artificial intelligence applications, consist of algorithms. Inventing elegant algorithms -- algorithms that are simple and require the fewest steps possible -- is one of the principal challenges in programming.

Anti-aliasing: Anti-aliasing is the process of bluring sharp edges in pictures to get rid of the jagged edges on lines. After an image is rendered, some applications automatically anti-alias images. The program looks for edges in an image, and then blurs adjacent pixels to produce a smoother edge.

In order to anti-alias an image when rendering, the computer has to take samples smaller than a pixel in order to figure out exactly where to blur and where not to. For example, if the computer finds that one pixel is on the edge of thwo objects, it then takes sub-pixel samples and checks about how many of them showed the fron obejct, and how many showed the back one. Lets say that the computer took 8 sub-samples, and 4 of them were on object one and the other 4 on object two. The computer then takes the resulting color values from the subsamples and averages them into a resulting blurred pixel, when viewed from a distance gives a smoother edge effect.

Bit Depth: Bit depth is the number of bits used to store information about each pixel. The higher the depth, the more colors are stored in an image. For example, the lowest bit-depth, 1 bit graphics are only capable of showing two colors, black & white. This is because there are only two combinations of numbers in one bit, 0 and 1. Four bit color is capable of displaying 16 colors because there are 16 different combinations of four bits:

Bit Map: A representation, consisting of rows and columns of dots, of a graphics image in computer memory. The value of each dot (whether it is filled in or not) is stored in one or more bits of data. For simple monochrome images, one bit is sufficient to represent each dot, but for colors and shades of gray, each dot requires more than one bit of data. The more bits used to represent a dot, the more colors and shades of gray that can be represented.

The density of the dots, known as the resolution, determines how sharply the image is represented. This is often expressed in dots per inch (dpi ) or simply by the number of rows and columns, such as 640 by 480.

To display a bit-mapped image on a monitor or to print it on a printer, the computer translates the bit map into pixels (for display screens) or ink dots (for printers). Optical scanners and fax machines work by transforming text or pictures on paper into bit maps.

Bit-mapped graphics are often referred to as raster graphics. The other method for representing images is known as vector graphics or object-oriented graphics. With vector graphics, images are represented as mathematical formulas that define all the shapes in the image. Vector graphics are more flexible than bit-mapped graphics because they look the same even when you scale them to different sizes. In contrast, bit-mapped graphics become ragged when you shrink or enlarge them.

Fonts represented with vector graphics are called scalable fonts , outline fonts , or vector fonts. The best-known example of a vector font system is PostScript. Bit-mapped fonts, also called raster fonts, must be designed for a specific device and a specific size and resolution.

Capture: To save a particular state of a program. The term capture often refers to saving the information currently displayed on a display screen. You can capture the screen to a printer or to a file. The act of saving a display screen is called a screen capture. Video capture refers to storing video images in a computer.

The term capture is also used to describe the recording of keystrokes during the definition of a macro.

Color Depth: The number of distinct colors that can be represented by a piece of hardware or software. Color depth is sometimes referred to as bit depth because it is directly related to the number of bits used for each pixel. A 24-bit video adapter, for example, has a color depth of 2 to the 24th power (about 16.7 million) colors. One would say that its color depth is 24 bits.

Continuous Tone: Refers to images that have a virtually unlimited range of color or shades of grays. Photographs and television images, for example, are continuous-tone images. In contrast, computer hardware and software is digital, which means that they can represent only a limited number of colors and gray levels. Converting a black-and-white continuous-tone image into a computer image is known as gray scaling.

Data Compression: Storing data in a format that requires less space than usual. Compressing data is the same as packing data.

Data compression is particularly useful in communications because it enables devices to transmit the same amount of data in fewer bits. There are a variety of data compression techniques, but only a few have been standardized. The CCITT has defined a standard data compression technique for transmitting faxes (Group 3 standard) and a compression standard for data communications through modems (CCITT V.42bis). In addition, there are file compression formats, such as ARC and ZIP.

Data compression is also widely used in backup utilities, spreadsheet applications, and database management systems. Certain types of data, such as bit-mapped graphics, can be compressed to a small fraction of their normal size.

Digital: Describes any system based on discontinuous data or events. Computers are digital machines because at their most basic level they can distinguish between just two values, 0 and 1, or off and on. There is no simple way to represent all the values in between, such as 0.25. All data that a computer processes must be encoded digitally, as a series of zeroes and ones.

The opposite of digital is analog. A typical analog device is a clock in which the hands move continuously around the face. Such a clock is capable of indicating every possible time of day. In contrast, a digital clock is capable of representing only a finite number of times (every tenth of a second, for example).

In general, humans experience the world analogically. Vision, for example, is an analog experience because we perceive infinitely smooth gradations of shapes and colors. Most analog events, however, can be simulated digitally. Photographs in newspapers, for instance, consist of an array of dots that are either black or white. From afar, the viewer does not see the dots (the digital form), but only lines and shading, which appear to be continuous. Although digital representations are approximations of analog events, they are useful because they are relatively easy to store and manipulate electronically. The trick is in converting from analog to digital, and back again.

This is the principle behind compact discs (CDs). The music itself exists in an analog form, as waves in the air, but these sounds are then translated into a digital form that is encoded onto the disk. When you play a compact disc, the CD player reads the digital data, translates it back into its original analog form, and sends it to the amplifier and eventually the speakers.

Internally, computers are digital because they consist of discrete units called bits that are either on or off. But by combining many bits in complex ways, computers simulate analog events. In one sense, this is what computer science is all about.

Digitize: To translate into a digital form. For example, optical scanners digitize images by translating them into bit maps. It is also possible to digitize sound, video, and any type of movement. In all these cases, digitization is performed by sampling at discrete intervals. To digitize sound, for example, a device measures a sound wave's amplitude many times per second. These numeric values can then be recorded digitally.

Display Screen: The display part of a monitor. Most display screens work under the same principle as a television, using a cathode ray tube (CRT). Consequently, the term CRT is often used in place of display scr

Dithering: Creating the illusion of new colors and shades by varying the pattern of dots. Newspaper photographs, for example, are dithered. If you look closely, you can see that different shades of gray are produced by varying the patterns of black and white dots. There are no gray dots at all. The more dither patterns that a device or program supports, the more shades of gray it can represent. In printing, dithering is usually called halftoning, and shades of gray are called halftones.

Note that dithering differs from gray scaling. In gray scaling, each individual dot can have a different shade of gray.

Dot Pitch: A measurement that indicates the diagonal distance between like-colored phosphor dots on a display screen. Measured in millimeters, the dot pitch is one of the principal characteristics that determines the quality of display monitors. The lower the number, the crisper the image. The dot pitch of color monitors for personal computers ranges from about 0.15 mm to 0.30 mm.

Another term for dot pitch is phosphor pitch.

EPS: Abbreviation of Encapsulated PostScript. Pronounced as separate letters, EPS is the graphics file format used by the PostScript language.

EPS files can be either binary or ASCII. The term EPS usually implies that the file contains a bit-mapped representation of the graphics for display purposes. In contrast, PostScript files include only the PostScript commands for printing the graphic.

GIF: Pronounced jiff or giff (hard g) stands for graphics interchange format, a bit-mapped graphics file format used by the World Wide Web, CompuServe and many BBSs. GIF supports color and various resolutions. It also includes data compression, making it especially effective for scanned photos.

Graphics: Pertains to any computer device or program that makes a computer capable of displaying and manipulating pictures. For example, laser printers and plotters are graphics devices because they permit the computer to output pictures. A graphics monitor is a display monitor that can display pictures. A graphics board (or graphics card) is a printed circuit board that, when installed in a computer, permits the computer to display pictures.

Graphic File Formats: A file format designed specifically for representing graphical images. Graphics file formats can be broadly categorized into bit-mapped formats and vector formats.

Gray Scale: The use of many shades of gray to represent an image. Continuous-tone images, such as black-and-white photographs, use an almost unlimited number of shades of gray. Conventional computer hardware and software, however, can only represent a limited number of shades of gray (typically 16 or 256). Gray-scaling is the process of converting a continuous-tone image to an image that a computer can manipulate.

While gray scaling is an improvement over monochrome, it requires larger amounts of memory because each dot is represented by from 4 to 8 bits. At a resolution of 300 dpi, you would need more than 8 megabytes to represent a single 8½ by 11-inch page using 256 shades of gray. This can be reduced considerably through data compression techniques, but gray scaling still requires a great deal of memory.

Many optical scanners are capable of gray scaling, using from 16 to 256 different shades of gray. However, gray scaling is only useful if you have an output device -- monitor or printer -- that is capable of displaying all the shades. Most color monitors are capable of gray scaling, but the images are generally not as good as on dedicated gray-scaling monitors.

Note that gray scaling is different from dithering. Dithering simulates shades of gray by altering the density and pattern of black and white dots. In gray scaling, each individual dot can have a different shade of gray.

Halftone: In printing, a continuous tone image, such as a photograph, that has been converted into a black-and-white image. Halftones are created through a process called dithering, in which the density and pattern of black and white dots are varied to simulate different shades of gray.

In conventional printing, halftones are created by photographing an image through a screen. The screen frequency, measured in lines per inch, determines how many dots are used to make each spot of gray. In theory, the higher the screen frequency (the more lines per inch), the more accurate the halftone will be. However, actual screen frequencies are limited by the technology because higher screen frequencies create smaller, more tightly packed dots. If you are printing on a low resolution device, therefore, you may get better results with a lower screen frequency.

Modern desktop publishing systems can create halftones by simulating the conventional photographic process. This is why some programs allow you to specify a screen frequency even when no actual screen is used.

Interlacing: (1) A display technique that enables a monitor to provide more resolution inexpensively. With interlacing monitors, the electron guns draw only half the horizontal lines with each pass (for example, all odd lines on one pass and all even lines on the next pass). Because an interlacing monitor refreshes only half the lines at one time, it can display twice as many lines per refresh cycle, giving it greater resolution. Another way of looking at it is that interlacing provides the same resolution as noninterlacing, but less expensively.

A shortcoming of interlacing is that the reaction time is slower, so programs that depend on quick refresh rates (animation and video, for example), may experience flickering or streaking. Given two display systems that offer the same resolution, the noninterlacing one will generally be better.

(2) Preparing a graphic image so that alternating rows are displayed in separate passes. Interlaced images give a nice effect because the entire image is displayed quickly and then details are filled in gradually. They are especially prevalent on the World Wide Web because of the slow transmission speed. Web pages with interlaced GIFs appear in a browser more quickly than pages with normal GIF images. The interlaced GIFs look blurry at first, but then become sharp as the rows are filled in.

JPEG: Short for Joint Photographic Experts Group, and pronounced jay-peg. JPEG is a lossy compression technique for color images. Although it can reduce files sizes to about 5% of their normal size, some detail is lost in the compression.

Lossless Compression: Refers to data compression techniques in which no data is lost. The PKZIP compression technology is an example of lossless compression. For most types of data, lossless compression techniques can reduce the space needed by only about 50%. For greater compression, one must use a lossy compression technique. Note, however, that only certain types of data -- graphics, audio, and video -- can tolerate lossy compression. You must use a lossless compression technique when compressing data and programs.

Lossy Compression: Refers to data compression techniques in which some amount of data is lost. Lossy compression technologies attempt to eliminate redundant or unnecessary information. Most video compression technologies, such as MPEG, use a lossy technique.

LZW: Short for Lempel-Zif-Welch, a popular data compression technique developed in 1977 by J. Ziv and A Lempel, and later refined by Terry Welch. It is the compression algorithm used in the GIF graphics file format, which is one of the standard graphic formats used by CompuServe and the World Wide Web.

The patent for LZW is owned by Unisys, which for many years allowed anyone to use the algorithm for free. Then in 1995, Unisys suddenly decided to charge a license fee. There was an uproar from the Compuserve and Web communities, and Unisys backed down somewhat, though they still enforce the licensing requirement for commercial applications.

Monitor: (1) Another term for display screen. The term monitor, however, usually refers to the entire box, whereas display screen can mean just the screen. In addition, the term monitor often implies graphics capabilities.

Optical Scanner: A device that can read text or illustrations printed on paper and translate the information into a form the computer can use. A scanner works by digitizing an image -- dividing it into a grid of boxes and representing each box with either a zero or a one, depending on whether the box is filled in. (For color and gray scaling, the same principle applies, but each box is then represented by up to 24 bits.) The resulting matrix of bits, called a bit map, can then be stored in a file, displayed on a screen, and manipulated by programs.

Pixel: Short for Picture Element, a pixel is a single point in a graphic image. Graphics monitors display pictures by dividing the display screen into thousands (or millions) of pixels, arranged in rows and columns. The pixels are so close together that they appear connected.

The number of bits used to represent each pixel determines how many colors or shades of gray can be displayed. For example, in 8-bit color mode, the color monitor uses 8 bits for each pixel, making it possible to display 2 to the 8th power (256) different colors or shades of gray.

On color monitors, each pixel is actually composed of three dots -- a red, a blue, and a green one. Ideally, the three dots should all converge at the same point, but all monitors have some convergence error that can make color pixels appear fuzzy.

The quality of a display system largely depends on its resolution, how many pixels it can display, and how many bits are used to represent each pixel. VGA systems display 640 by 480, or about 300,000 pixels. In contrast, SVGA systems display 1,024 by 768, or nearly 800,000 pixels. True Color systems use 24 bits per pixel, allowing them to display more than 16 million different colors.

PNG: Short for Portable Network Graphics, and pronounced ping, a new bit-mapped graphics format similar to GIF. In fact, PNG was approved as a standard by the World Wide Web consortium to replace GIF because GIF uses a patented data compression algorithm called LZW. In contrast, PNG is completely patent- and license-free. The most recent versions of Netscape Navigator and Microsoft Internet Explorer now support PNG.

Post Script: A page description language (PDL) developed by Adobe Systems. PostScript is primarily a language for printing documents on laser printers, but it can be adapted to produce images on other types of devices. PostScript is the standard for desktop publishing because it is supported by imagesetters, the very high-resolution printers used by service bureaus to produce camera-ready copy.

PostScript is an object-oriented language, meaning that it treats images, including fonts, as collections of geometrical objects rather than as bit maps. PostScript fonts are called outline fonts because the outline of each character is defined. They are also called scalable fonts because their size can be changed with PostScript commands. Given a single typeface definition, a PostScript printer can thus produce a multitude of fonts. In contrast, many non-PostScript printers represent fonts with bit maps. To print a bit-mapped typeface with different sizes, these printers require a complete set of bit maps for each size.

The principal advantage of object-oriented (vector) graphics over bit-mapped graphics is that object-oriented images take advantage of high-resolution output devices whereas bit-mapped images do not. A PostScript drawing looks much better when printed on a 600-dpi printer than on a 300-dpi printer. A bit-mapped image looks the same on both printers.

Every PostScript printer contains a built-in interpreter that executes PostScript instructions. If your laser printer does not come with PostScript support, you may be able to purchase a cartridge that contains PostScript.

There are three basic versions of PostScript: Level 1, Level 2 and PostScript 3. Level 2 PostScript, which was released in 1992, has better support for color printing. PostScript 3, release in 1997, supports more fonts, better graphics handling, and includes several features to speed up PostScript printing.

Refresh: (1) Generally, to update something with new data. For example, some Web browsers include a refresh button that updates the currently display Web pages. This feature is also called reload.

(2) To recharge a device with power or information. For example, dynamic RAM needs to be refreshed thousands of times per second or it will lose the data stored in it.

Similarly, display monitors must be refreshed many times per second. The refresh rate for a monitor is measured in hertz (Hz) and is also called the vertical frequency, vertical scan rate, frame rate or vertical refresh rate. The old standard for monitor refresh rates was 60Hz, but a new standard developed by VESA sets the refresh rate at 75Hz for monitors displaying resolutions of 640x480 or greater. This means that the monitor redraws the display 75 times per second. The faster the refresh rate, the less the monitor flickers.

Resolution: Refers to the sharpness and clarity of an image. The term is most often used to describe monitors, printers, and bit-mapped graphic images. In the case of dot-matrix and laser printers, the resolution indicates the number of dots per inch. For example, a 300-dpi (dots per inch) printer is one that is capable of printing 300 distinct dots in a line 1 inch long. This means it can print 90,000 dots per square inch.

For graphics monitors, the screen resolution signifies the number of dots (pixels) on the entire screen. For example, a 640-by-480 pixel screen is capable of displaying 640 distinct dots on each of 480 lines, or about 300,000 pixels. This translates into different dpi measurements depending on the size of the screen. For example, a 15-inch VGA monitor (640x480) displays about 50 dots per inch.

Printers, monitors, scanners, and other I/O devices are often classified as high resolution, medium resolution, or low resolution. The actual resolution ranges for each of these grades is constantly shifting as the technology improves.

Sampling: A technique used to capture continuous phenomena, whereby periodic snapshots are taken. If the sampling rate is fast enough, the human sensory organs cannot discern the gaps between each snapshot when they are played back. This is the principle behind motion pictures.

Sampling is the key technique used to digitize analog information. For example, music CDs are produced by sampling live sound at frequent intervals and then digitizing each sample. The term sampling is also used to describe a similar process in digital photography

Screen Flicker: The phenomenon whereby a display screen appears to flicker. Screen flicker results from a variety of factors, the most important of which is the monitor's refresh rate, the speed with which the screen is redrawn. If the refresh rate is too slow, the screen will appear to glimmer. Another factor that affects screen flicker is the persistence of the screen phosphors. Low-persistence phosphors fade more quickly than high-persistence monitors, making screen flicker more likely. Screen flicker can also be affected by lighting. Finally, screen flicker is a subjective perception that affects people differently. Some people perceive screen flicker where others do not. Most people perceive no screen flicker if the refresh rate is 72 Hz or higher.

TIFF: Acronym for tagged image file format, one of the most widely supported file formats for storing bit-mapped images on personal computers (both PCs and Macintosh computers). Other popular formats are BMP and PCX.

TIFF graphics can be any resolution, and they can be black and white, gray-scaled, or color. Files in TIFF format often end with a .tif extension.

True Color: Refers to any graphics device or software that uses at least 24 bits to represent each dot or pixel. Using 24 bits means that more than 16 million unique colors can be represented. Since humans can only distinguish a few million colors, this is more than enough to accurately represent any color image

Vector graphics: Vector graphics are really just a list of graphical objects such as lines, rectangles, ellipses, arcs, or curves—called primitives. Draw programs, also called vector graphics programs, are used to create and edit these vector graphics. These programs store the primitives as a set of numerical coordinates and mathematical formulas that specify their shape and position in the image. This format is widely used by computer-aided design programs to create detailed engineering and design drawings. It has also become popular in multimedia when 3D animation is desired. 

Video Adapter: A board that plugs into a personal computer to give it display capabilities. The display capabilities of a computer, however, depend on both the logical circuitry (provided in the video adapter) and the display monitor. A monochrome monitor, for example, cannot display colors no matter how powerful the video adapter.

Many different types of video adapters are available for PCs. Most conform to one of the video standards defined by IBM or VESA.

Each adapter offers several different video modes. The two basic categories of video modes are text and graphics. In text mode, a monitor can display only ASCII characters. In graphics mode, a monitor can display any bit-mapped image. Within the text and graphics modes, some monitors also offer a choice of resolutions. At lower resolutions a monitor can display more colors.

Modern video adapters contain memory, so that the computer's RAM is not used for storing displays. In addition, most adapters have their own graphics coprocessor for performing graphics calculations. These adapters are often called graphics accelerators.

Video adapters are also called video cards, video boards, video display boards, graphics cards and graphics adapters.

VRAM: Short for video RAM, and pronounced vee-ram. VRAM is special-purpose memory used by video adapters. Unlike conventional RAM, VRAM can be accessed by two different devices simultaneously. This enables the RAMDAC to access the VRAM for screen updates at the same time that the video processor provides new data. VRAM yields better graphics performance but is more expensive than normal RAM.

A special type of VRAM, called Windows RAM (WRAM), yields even better performance than conventional VRAM.