Definitions

Definitions:


Address Bus: A transport and identification system whereby circuits route (bus) information from a memory location where informations is stored (an address) to another location.

Aliasing: In computer graphics, the process by which smooth curves and other lines become jagged because the resolution of the graphics device or file is not high enough to represent a smooth curve. Smoothing and antialiasing techniques can reduce the effect of aliasing.

Animated GIF: A type of GIF image that can be animated by combining several images into a single GIF file. Applications that support the animated GIF standard, GIF89A, cycle through each image. GIF animation doesn't give the same level of control and flexibility as other animation formats but it has become extremely popular because it is supported by nearly all Web browsers. In addition, animated GIF files tend to be quite a bit smaller that other animation files, such as Java applets.

Anti-Aliasing: The removal of the "jaggies" to recreate smoother diagonal lines through a filtering technique. The technique involves use of intermediate intensities between neighboring pixels to soften the "jaggies".

Analog Signal: A signal such as that produced by a TV camera which varies in voltage over time in a continuous manner. A digital signal has two or more discrete levels.

B»zier curve: Pronounced bez-ee-ay, curved lines (splines) defined by mathematical formulas. Nearly all draw programs support B»zier curves. Named after the French mathematician Pierre B»zier, B»zier curves employ at least three points to define a curve. The two endpoints of the curve are called anchor points. The other points, which define the shape of the curve, are called handles, tangent points, or nodes. Attached to each handle are two control points. By moving the handles themselves, or the control points, you can modify the shape of the curve.



Binary: Having two states, often complementary to one another. on/off, black/white.

Bitmap: 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.



Byte: A group of 8 bits. The byte is a common unit of measure for memory and file sizes. A single byte can represent any of 256 different characters or 256 discrete levels of brightness.

Central Processing Unit: Contains the circuits that control the execution of instructions. In a personal computer, the CPU is often on a single chip called the microprocessor. It performs arithmetic operations, controls instruction processing and provides timing signals.

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. Continuous-tone printers can print each dot at many different shades of lightness and darkness. Though this isn't true continuous-tone because the level of shades is limited, there are enough shades (256 or more) so that the difference between one shade and the next is imperceptible to the human eye.

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.)



GrayScale: 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 8? 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.

Histogram: A representation of the distribution of samples over a range of values, often in the form of a bar graph. A histogram of an image can indicate where values of brightness and darkness are concentrated.

Image Map: A single graphic image containing more than one hot spot. For example, imagine a graphic of a bowl of fruit. When you click on a banana, the system displays the number of calories in a banana and when you click on an apple, it displays the number of calories in an apple. Image maps are used extensively on the World Wide Web. Each hot spot in a Web image map takes you to a different Web page.

Line Art: A type of graphic consisting entirely of lines, without any shading. Most art produced on computers is not line art because the computer makes it so easy to add subtle shadings.

Mask: A filter that selectively includes or excludes certain values. For example, when defining a database field, it is possible to assign a mask that indicates what sort of value the field should hold. Values that do not conform to the mask cannot be entered.

Moir»: An undesirable pattern that appears when a graphic image is displayed or printed with an inappropriate resolution. Moir» patterns are difficult to predict because they result from a complex combination of parameters: the size of the image, resolution of the image, resolution of the output device, halftone screen angle, etc.

A radiating curved pattern seen when two geometrically regular patterns (as two halftone screens) are superimposed.A moire pattern can result from scanning or rescreening a halftone, which puts a screen over another screen.Usually moire patterns are not wanted because they interfere with viewing the image.Sometimes this effect can be controlled by adjusting the angle of the halftone, or refining the image with a photo editing program.

Object-Oriented Graphics: The representation of graphical objects, such as lines, arcs, circles, and rectangles, with mathematical formulas. This method of describing objects enables the system to manipulate the objects more freely. In an object-oriented system, for example, you can overlap objects but still access them individually, which is difficult in a bit-mapped system. Also, object-oriented images profit from high-quality output devices. The higher the resolution of a monitor or printer, the sharper an object-oriented image will look. In contrast, bit-mapped images always appear the same regardless of a device's resolution. One of the most widely used formats for object-oriented graphics is PostScript. PostScript is a page description language (PDL) that makes it possible to describe objects and manipulate them in various ways. For example, you can make objects smaller or larger, turn them at various angles, and change their shading and color. A font described in PostScript, therefore, can easily be transformed into another font by changing its size or weight. Object-oriented fonts are called outline fonts, scalable fonts, or vector fonts. Object-oriented graphics is also called vector graphics, whereas bit-mapped graphics is sometimes called raster graphics.

Operating System: A set of programs which monitors and controls the execution of user programs. The operating system must be present in the computer before the application programs can be loaded or run.

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.

RAM: (random-access memory) The memory avaliable to the user for storing information or instruction. Ram is read/write memory; it lets you retrieve information you have put un the computer or add new information. Data stored in RAM will be lost when the computer is turned off unless it is saved on a magnetic medium such as a disk.

Resampling: Changing the resolution of a bitmap without changing its size. When you reduce the resolution of a bitmap, it's called Downsampling. When you increase the resolution , it's called Interpolation. Interpolation can occur in variety of ways (nearest neighbor, bilinear, bicubic), by adding a sample point to the bitmap. Unlike downsampling, which simply removes existing information, interpolation is more problematic, because it's attempting to add information that doesn't exist in the orginal image.

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.

ROM: (read-only memory) The memory the computer uses to store its operating instructions. The information in ROM is unalterable and remains in the computer even when you turn it off.