Since large drawings cannot fit in their entirety on display screens, they can either be compressed to fit, thereby obscuring details and creating clutter, or only a portion of the total drawing can be displayed. The portion of a 2D or 3D object to be displayed is chosen through specification of a rectangular window that limits what part of the drawing can be seen.

A 2D window is usually defined by choosing a maximum and minimum value for its x- and y-coordinates, or by specifying the center of the window and giving its maximum relative height and width. Simple subtractions or comparisons suffice to determine whether a point is in view. For lines and polygons, a clipping operation is performed that discards those parts that fall outside of the window. Only those parts that remain inside the window are drawn or otherwise rendered to make them visible.

Built into your computer is a mapping system, or grid, complete with the ability to pinpoint any location or coordinate in the application window. This grid is laid out in the common x,y format-x being the horizontal units of measure starting from the left side of the screen, and y being the units starting from the top of the screen. It’s easy to see that 0,0 would be the upper left corner of the screen. Now, if you’re only using your computer for word processing, then you have no real use for knowing exactly where your cursor is. But in the painting and drawing world, knowing these coordinates is very helpful-to say the least-and it’s essential in a lot of instances. Nearly all graphic and page layout applications give you a separate window which shows the coordinates of where your cursor is located at any given moment. By watching your coordinates you can move, create, shape, or select objects or portions thereof with great precision.

The processing of a set of data in order to reduce its size. Compression may be performed both to reduce the amount of storage space occupied (say, to fit the data onto a single CD) and to reduce the time it takes to transmit (say, over a slow telephone line). Compressed data must be decompressed by reversing the process before it can be read or modified.

When you compress computerized information, you make it smaller (taking up less space on the disk), meaning that less data is needed to represent exactly the same information. Using a compression utility, you can compress files stored on disk so that they take up less disk space and leave more space for other files. Some programs have the ability to compress data that’s being held in memory, allowing the computer to keep more data in memory and thus spend less time retrieving data from the disk. And some modems and communications software can compress the data they send back and forth to one another. Since there’s less data, it takes less time to transfer them.

Keep in mind that even when you are archiving files you never want to compress your original and only file. All it takes is to lose one bit, one electronic signal, from a compressed file and that file is destroyed. Bits get lost all the time. In an uncompressed file, it’s a minor problem, but in a compressed file, it can be a catastrophe of considerable dimension.

There are many known compression/ decompression ALGORITHMaSn,d the search for better ones has become commercially important since most new communications technologies (such as digital television) can only work with effective data compression.

All compression algorithms work by uncovering and eliminating redundancy in the data, and there is an important distinction between those that preserve all the information in the data (loss/ess methods) and those that sacrifice some information for greater compression (lossy methods). Lossy algorithms such as JPEG and MPEG are suitable only for final delivery of data to end users, as the information losses accumulate each time the material is recompressed and decompressed.

CODEC is a shorthand way of saying “compressor/decompressor.” It refers to a variety of software products that determine how a movie file, such as QuickTime, should be condensed, or compressed, to save space on the hard disk and to make the movie run faster. You might choose a different CODEC for video images than you would for still photography images. The different choices strike a different balance between picture quality and the size of the file (how many megabytes it requires to store it on the hard disk).

The acronym CMYK (pronounced as the individual letters: CM Y K) stands for the process colors cyan, magenta, yellow, and black. These four process colors are the transparent ink colors that a commercial press uses to recreate the illusion of a full-color photograph or illustration on the printed page. If you look at any printed color image in a magazine, especially if you look at it through a magnifying glass (a “loupe”), you will see separate dots of ink in each of the four colors. These four colors, in varying intensities determined by the dot size and space around the dot, combine together to create the wide range of colors you appear to see.

To get these four colors from the full-color image, the image must be separated into the varying percentages of each of the colors. There are several very sophisticated methods of doing this, and the result is a four color separation.

Desktop color systems and the powerful page layout and art programs are now capable of making four-color separations for us. I can also create a color in my publication to match a color in the photograph. For instance, if I want to print my headlines in the same slate blue as in the model’s tie, the computer could separate the headline color into the four different layers (sometimes called “plates”) as follows: 91%Cyan, 69%Magenta, 9% Yellow, and 2% Black. The photograph itself would be separated into its variations of CMYK. When these four percentages of transparent ink are printed on top of each other, the colors combine to make the full-color photograph and the slate blue of the headlines, all at the same time.

This is different from spot color, where each spot of color is a separate, opaque ink color out of a can, such as red or blue or peach.

CLUT stands for color look-up table. A CWT is a software palette or set of 256 colors (it’s actually a resource) that resides within the system software and most color-capable applications. On a computer with 8-bit color (those that are only capable of displaying a total of 256 colors), a CWT is a necessary reference to let the computer know which 256 colors out of the available 16.7 million colors (24-bit color) it can use at one time. If you think of all those 16.7 million colors as being a big (ok, very big) box of crayons, you can visualize a CWT as being a small box of handpicked colors that someone has handed you to work with. Many applications give you the option of choosing which 256 colors you want to work with. You often can set up your own palette for each particular file. For instance, if you were painting a picture of a man’s face, a palette of 256 different flesh tones would be more useful than a palette containing 256 colors found in the range between black and burgundy. Take the time to explore your particular application and its documentation for a variable palette feature.

CGM stands for computer graphics metafile, which is an international standard file format for graphic images. Most CGM files are vector graphics, although it is possible to store raster graphics in the CGM format. The purpose of creating a standard is to enable users of different systems and different programs to exchange the same graphic file. It is extremely difficult, though, to create a standard so strict that it can work seamlessly everywhere. A CGM file created in one program may not necessarily be read by every other program.

The Windows metafile format (WPM) developed by Microsoft Corporation may eventually supplant CGM as the vector graphics standard.

A technique called double buffering permits one set of data to be used while another is collected. It is used with graphics displays, where one frame buffer holds the current screen image while another acquires the bits that will make up the next image. When it is ready, the buffers are switched, the new screen is displayed, and the process continues. 

This reduces the minimum time between successive frames to the time required to switch buffers, rather than the time required to RENDER a whole frame, so avoiding a lengthy dark space between frames.

In a typical situation, a processor will be capable of producing data several orders of magnitude faster than a peripheral can accept it. In order to make most efficient use of the processor, the data will be placed in a buffer and its location made known to the peripheral. The peripheral then proceeds to empty the buffer while the processor is freed for other work.

Bump Mapping: An extension of the technique Of TEXTURE MAPPING to create more realistic 3D images, in which an additional BITMAP (the bump map) applied to a surface contains not colour data but small displacements to be applied to the surface normal at each point. After the image is rendered, these displacements alter the angles of reflected rays in such a way as to convey the illusion of surface relief, even though the surface actually remains completely smooth.

Bitmap: A table of digital BITS used to represent, for example, a picture or a text character, each bit in the table being interpreted as the presence or absence of a screen PIXEL or a printed dot. The principle can be illustrated by the following table, which represents the letter Z as a 6 x 6 table of bits:

1  1  1 1 1 1

0 0 0 0 1 0

0 0 0 1 0 0

0 0 1 0 0 0

0 1 0 0 0 0

1 1 1  1  1 1

Bitmap Editor: The generic name for programs, such as Adobe’s PHOTOSHOP, whose function is to create and manipulate BITMAPPED images: that is files in which pictures are stored as a collection of individual PIXELS rather than as geometric descriptions. bitmap editor enables its user to change the colour of individual pixels or whole areas of pixels at once, using a variety of tools that mimic the effect of paintbrushes, pencils, spray cans and more. Typical bitmap editors also support a variety of graphics FILTER operations, such as EDGE ENHANCEMENT that can be applied automatically to the whole image.

Bitmapped Display: Strictly, a display in which each PIXEL on the screen is represented by a BIT stored in VIDEO MEMORY which would limit its applicability to black-and-white images only. More frequently used, however, to describe any display in which each pixel corresponds to a byte or word in video memory, which covers all contemporary computer colour displays. The term was coined in distinction to the now-obsolete VECTOR DISPLAY, which drew lines instead of pixels.

Bit Block Transfer (bitblt, bitblit): An operation used in computer graphics programming that moves a block of bits en masse from one location in memory to another. If these bits represent display pixels, the effect is to move part of an image from one place to another, and so bitblt is much used in graphical user interface code to display WINDOWS, ICONS and FONT characters quickly. Because this operation is used so extensively, many modern microprocessors provide special instructions to speed it up and a hardware GRAPHICS ACCELERATOR usually contains a dedicated unit called a BLITTER that performs the operation as quickly as possible.

Bitmapped Font, Bitmap Font: A character FONT in which each individual letter form is stored as a table of PIXELS (a picture), in contrast to an OUTLINE FONT where each character is stored as a set of lines or strokes (a description of how to draw the character). Bitmapped fonts are fast and easy to RENDER onto a screen or printer – by simply copying the bits for the character – and for this reason were preferred on older computer systems (up to and including MS-DOS PCs) that used CHARACTER-BASED displays.

Bitmapped fonts render correctly only at the size they were created: to enlarge or reduce their characters involves duplicating or removing pixels, which gives the letters an unattractive jaggy appearance. In contrast, outline fonts can be scaled to any size (above a minimum) with little loss of quality and hence they have almost entirely displaced bitmapped fonts, except for applications such as instruments and hand-held computers with small fixed-size displays. Examples of bitmapped fonts include the fixed-pitch Courier and MS Serif fonts supplied with Windows.

All fonts (typefaces) that you see on the screen are bitmapped. That’s the only way the computer can display the typeface on the screen, since the screen is composed of dots (pixels). Some fonts have no other information to them than the bitmapped display you see on the screen, while other fonts have additional data that is used by the printer to print the typeface smoothly on a page (outline, or scalable fonts).

A bitmap is an image or shape of any kind-a picture, a text character, a photo-that’s composed of a collection of tiny individual dots. A wild landscape on your screen is a bitmapped graphic, or simply a bitmap. Remember that whatever you see on the screen is composed of tiny dots called pixels. When you make a big swipe across the screen in a paint program with your computerized “brush,” all that really happens is that you turn some of those pixels on and some off. You can then edit that bitmapped swipe dot by dot; that is, you can change any of the pixels in the image. Bitmaps can be created by a scanner, which converts drawings and photographs into electronic form, or by a human artist (like you) working with a paint program.

A computer screen is made up of thousands of dots of light, called pixels (short for picture elements). A single pixel is composed of up to three rays of light, red, blue, and green, blended into a single dot on-screen. By combining these rays and changing their intensity, virtually any color can be displayed on-screen. The number of bits required to display a single pixel onscreen varies by the total number of colors a particular monitor can display. The larger the number of possible colors, the larger the number of bits’ required to describe the exact color needed. Regardless of the actual number of bits required, a bit map is a series of these bits stored in memory, which form a pattern when read left to right, top to bottom. When decoded by the computer and displayed as pixels on-screen, this pattern forms the image of a picture.

The simplest bitmaps are monochrome, which have only one color against a background. For these, the computer needs just a single bit of information for each pixel (remember, a bit is the smallest unit of data the computer recognizes). One bit is all it takes to turn the dot off (black) or on (white). To produce the image you see, the bits get “mapped” to the pixels on the screen in a pattern that displays the image.

In images containing more than black and white, you need more than one bit to specify the colors or shades of gray of each dot in the image. Multicolor images are bitmaps also. An image that can have many different colors or shades of gray is called a “deep bitmap,” while a monochrome bitmap is known as a “bilevel bitmap.” The “depth” of a bitmap-how many colors or shades it can contain – has a huge impact on how much memory and/or disk space the image consumes. A 256-color bitmap needs 8 times as much information, and thus disk space and memory, as a monochrome bitmap.

The resolution of a bitmapped image depends on the application or scanner you use to create the image, and the resolution setting you choose at the time. It’s common to find bitmapped images with resolutions of 72 dots per inch (dpi), 144 dpi, 300 dpi, or even 600 dpi. A bitmap’s resolution is permanently fixed-a bitmapped graphic created at 72 dpi will print at 72 dpi even on a 300 dpi printer such as the LaserWriter. On the other hand, you can never exceed the resolution of your output device (the screen, printer, or what have you); even though you scanned an image at 600 dpi, it still only prints at 300 dpi on a LaserWriter, since that’s the LaserWriter’s top resolution.

You can contrast bitmapped images with vector or object-oriented images, in which the image is represented by a mathematical description of the shapes involved. You can edit the shapes of an object graphic, but not the individual dots. On the other hand, object-oriented graphics are always displayed or printed at the maximum resolution of the output device. But keep in mind that an object-oriented graphic is still displayed as a bitmap on the screen.

Bit-mapped fonts and bit-mapped graphics use pixels to form pictures or letters. However, because of the number of bits required to encode a single pixel, bit-mapped fonts and graphics consume a great deal of memory. In comparison, trying to create a perfect circle by coloring the squares on a piece of graph paper demonstrates the problems inherent with this method of displaying text and graphics. Because a computer screen is layed out in a grid of dots (pixels) like graph paper, a distortion will show up along the angled and curved lines in an image. This distortion is called “jaggies” or “aliasing.”

The positively charged ELECTRODE that attracts ELECTRONS within a current-consuming device such as an electrolytic cell, discharge tube or valve. In a current-producing BATTERY, the anode is the electrode that receives electrons internally and hence is connected to the external negative terminal.

Analogue video: A video signal that is captured transmitted and stored as a continuously varying voltage, rather than as a stream of bits as in digital video. Up until the advent of digital TV in the late 1990S, television worked by transmitted analogue video signals, and older video tape recorders such as VHS, PAL, Betamax and Umatic all store analogue signals.

The disadvantage of analogue video is that it is prone to noise interference, while its advantage is its great density: a domestic 3- hour VHS cassette holds the equivalent of 16 gigabytes of digital data.

Aspect ratiois a fancy term for “proportion,” or the ratio of width to height. for example 4:3 for a computer screen. For instance, if a direction in a software manual tells you to “hold down the Shift key while you resize a graphic in order to maintain the aspect ratio,” it simply means that if youdon’thold down the Shift key you will stretch the image out of proportion.

Some combinations of computers and printers have trouble maintaining the correctaspect ratio when the image goes from the screen to the printer, or when the image is transferred from one system to another, so the aspect ratio can be an important specification to consider when choosing hardware.

The aspect ratio of the screen determines the most efficient screen RESOLUTIONS and the most desirable shape for individual PIXELS, all of which may have to change upon the introduction of HIGH DEFINITION TELEVISION.

Aliasing has two definitions, depending on whether you’re talking about pictures or sounds.

When a diagonal line or a curved arc drawn on the screen looks as if it was made out of bricks, when it looks like stair steps instead of a slide, the effect is technically called aliasing. Most of us would say it had the jaggies. It can be ameliorated by the technique of ANTIALIASING.

The phenomenon by which a digitized sound sample may pick up unwanted spurious frequencies. It affects digitally reproduced sound, which is the kind of sound your computer probably makes. (The beeps you hear from a standard PC speaker aren’t digital, but a sound board you plug into your PC creates sound digitally; the Mac has built-in digital sound.)

Digital sound is based on a sequence of numbers (digits) that are converted into sound waves by electronic circuits. The computer has to guess at what sound to make between each number in the sequence. If the time between each value is too long (if the “sampling rate” is low), you hear the mistaken guesses as a metallic, static distortion called aliasing. To squelch aliasing, you need a soundcard with a sampling rate of around 40 kilohertz (40,000 times a second) or higher.

Both these phenomena result from sampling the data at a frequency below its NYQUIST FREQUENCY: that is, displaying a line on a screen of too Iowa resolution or sampling the sound at too Iowa frequency. In fact aliasing is a potential source of distortion when sampling any form of data, another example being the optical illusion that wheels are rotating backwards often seen when old movies are shown on television.

On a color monitor, each pixel has three dots arranged in a triad-red, green, and one blue dot. Each dot can deal with a maximum of 8 bits, which makes a total of 24 bits per pixel. With the possibility of combining the 256 levels of color in each of the three color dots, 24-bit color gives you the awesome potential of 16.7 million colors on your screen (256 times 3). Many of these colors differ so slightly that even the most acute observer couldn’t tell the difference between them. Simply stated: 16 million colors is more than enough. (How do you get black and white if there are three colored dots? If all dots are on, the pixel is white; if all dots are off, the pixel is black.)

Now, you will often hear of 32-bit color, which there isn’t, really. Those other 8 bits don’t offer any extra color, but they do offer the capacity for masking and channeling.

On a grayscale monitor, each pixel can accept from 1 to 8 bits of data, which will show from 1 to 256 shades of gray.

If there are 2 bits per pixel, there are four possible combinations of on and off: on/on, off/off, on/off, and off/on. Each of these combinations displays a different shade of gray (including black and white).

If there are 4 bits per pixel (24), you will have 16 levels of gray

If there are 8 bits per pixel, there are 256 possible combinations (28). This is the maximum number of grays possible on any grayscale monitor, which is plenty because our eyes can’t distinguish more than that number of grays anyway.

If an item is monochrome, that means it uses only one color on a differently colored background. In a monochrome monitor, these pixels have only one color phosphor. The picture is created with, say, black dots (or lines) against a white background. A monochrome image pixel can have two values, on (white) or off (black), and this can be represented by 1-bit as either 0 or 1. Most printers are monochrome, meaning they only print black toner on white paper.

If an image is One-bit that means 1-bit of information is sent to the pixel on the screen. That bit can turn the pixel on (white) or off (black). All 1-bit images are black-and-white. On amonochrome monitor, the pixels can’t deal with more than just that one bit of data so that’s all you can ever get is black and white.