Glossary

4-bit color

Refers to a way of representing bitmap or other data that can handle up to 16 (2^4) colors.

8-bit color

Refers to a way of representing bitmap or other data that can handle up to 256 (2^8) colors.

8-mm

Tape backup format and medium. Video recording format and medium, usually used for home use due to the compact size of the physical medium, and not considered of broadcast quality. See VHS and Beta.

15-bit color

Refers to a way of representing bitmap or other data that can handle up to 32,768 (2^15) colors.

16-bit color

Refers to a way of representing bitmap or other data that can handle up to 65,536 (2^16) colors.

24-bit color

Refers to a way of representing bitmap or other data that can handle up to 16,777,216 (2^24) colors.

accelerator card

A video display card with active components designed to enhance or speed up the display of image data sent to it by a rendering application. Contrast with dumb frame buffer.

active information device

An electronic device with which the user must constantly interact in order to obtain information. Video arcade games and most multimedia applications are active information devices. Contrast with passive information device.

adaptive encoding

An algorithm that has no certain prior knowledge about the format of the data it is encoding. It must adapt to the format of the data as it encodes it. LZW is an adaptive encoding algorithm.

additive system

A color system in which colors are created by adding colors to black. The more color that is added, the more the resulting color tends toward white.

aliasing

Artifact produced on a pixel-based output device where diagonal or curved edges appear jagged.

alpha channel

An additional channel of bitmap data used to store transparency data for an image, which can be on a per-pixel, per-block, or per-image basis. The degree of pixel transparency for an 8-bit alpha value ranges from 0 (the pixel is completely invisible or transparent) to 255 (the pixel is completely visible or opaque). See also overlay bit.

animation

A sequence of two or more images displayed in a rapid sequence so as to provide the illusion of continuous motion. Animations are typically played back at a rate of 12 to 15 frames per second.

anti-aliasing

The process of reducing artifacts by interpolating intermediate colors perpendicular to an aliased edge.

aperture mask

A shadow mask with vertical strips instead of round holes. See Trinitron, shadow mask, and dot pitch.

array of pixels

An ordered set of colored display elements on an output device. This term is used loosely to refer to an array of numerical values used by an application program to specify colored elements on an output device.

ART

A method of compression developed by the Johnson-Grace company.

artifact

A detectable change in an image produced by a rendering application, such as a filter, or an editing tool, such as a paint program. Such changes are said to be introduced by human intervention and are therefore artifactual influences upon natural, ecofactual data.

aspect ratio

The proportional measurement of an image or pixel based on its horizontal and vertical size. For example, an image with an aspect ratio of 4:3 has a horizontal width that is 4/3 of the vertical height.

band

See strip.

banding

Horizontal, vertical, or, more rarely, diagonal bands of discoloration inadvertently placed in an image during creation or rendering.

BBS

Bulletin Board System. A telecommunications program running on a computer that allows other computers with modems to dial in and access files. BBSs are a prime source of image files and file format information. Older names for BBSs include Computer Bulletin Board System (CBBS) and Electronic Bulletin Board System (EBBS).

Beta

Video recording format and medium, considered to be of superior quality to VHS but not widely used. Not considered broadcast quality. See VHS.

Betacam

Broadcast quality video recording format and medium. See Beta, VHS, U-Matic, M-II, and D-2.

Bezier curve

A smooth curve specified by a small set of values, including tangent and control point information.

bi-level

An image that contains only two colors: a background color and a foreground color. See monochrome and halftone.

big-endian

Refers to systems or machines that store the most significant byte (MSB) at the lowest address in a word, usually referred to as byte 0. Contrast with little-endian.

bit depth

The size of a value used to represent a pixel in bitmap graphics data. This is usually stated as the number of bits making up the individual data value, or sometimes the number of bytes. The number 2 raised to the power bit depth specifies the maximum number of values the pixel can assume. Same as pixel depth.

bit order

The order of the bits within a byte. The first bit in a byte may be either the most significant or the least significant bit. See also LSB and MSB.

bit plane

A 2D array of bits one bit deep. A bitmap containing pixels with a depth of eight bits may be said to contain eight bit planes. A monochrome image (one bit per pixel) is usually stored as a single bit plane.

bit sex

The state of a bit (0 or 1).

bitmap

A set of numerical values specifying the colors of pixels on an output device. In older usage, the term referred to data intended for display on an output device capable of displaying only two levels. It is used in this book as a synonym for raster.

bitmap data

The portion of a bitmap file containing information associated with the actual image.

bitmap image

A representation of a graphics work on a raster device or in a bitmap file. Redundant in our terminology.

bitonal

See bi-level.

bits per pixel

See bit depth.

block

See chunk.

bpp

Same as bits per pixel.

broadcast quality

Video recording medium retaining sufficient quality after multiple edit and copy operations to be broadcast on commercial television.

byte order

The order of bytes within a word of data. The first byte in a word may be either the most significant or least significant byte. See also big-endian, little-endian, LSB, and MSB.

bytes per pixel

Bit depth expressed in bytes.

CAD

See Computer Aided Design.

CCD

Charge coupled device. An array of electronic components which convert light into electrical signals. Used in scanners and video cameras. See scanner.

CCITT

International Telegraph and Telephone Consultative Committee. See ITU.

CD

See Compact Disc.

CD-DA

Compact Disc-Digital Audio. The standard used for encoding audio data onto a compact disc.

CD-I

Compact Disc-Interactive. The standard used for encoding audio and video information onto compact discs for use in interactive multimedia systems.

CD-R

Compact Disc-Recordable. The standard for creating write-once compact discs that may be mastered on a standard PC.

CD-ROM

Compact Disc-Read-Only Memory. A compact disc containing data encoded using the CD-XA standard. See also CD-XA and ISO-9660.

CD-XA

Compact Disc-Extended Architecture. The standard used for encoding data onto what we know as a CD-ROM. See also ISO-9660.

chroma

Term used when referring to color. Same as chrominance.

chromakeying

The process of creating an image, a portion of which is placed on a background of uniform color, usually blue, so that another image can later be added by placing it in the area of uniform color.

chrominance

The color portion of an image. It is the mixture of hue and saturation, or the combination of three primary colors, such as red, green, and blue.

chunk

A collection of data with a known format within a graphics file. Chunks are also called blocks in some graphics file format specifications. See also packet.

chunking

The breaking up of a block of data into two or more smaller pieces, usually to accommodate memory limitations or to avoid hardware dependencies.

CIE

International Commission on Illumination (Commission Internationale de l'E'clairage). The CIE established an international standard for primary colors in 1931. This standard allows all colors to be defined as a weighted sum of three primary colors.

CLUT

Color Look-Up Table. See look-up table.

CMY

Acronym for Cyan/Magenta/Yellow. A subtractive color system based on the primary colors cyan, magenta, and yellow.

CMYK

Acronym for Cyan/Magenta/Yellow/Key. A subtractive color system based on the primary colors cyan, magenta, and yellow. Key color is the color black, which is not reproducible using the CMY model alone.

codec

Shortened version of encoder/decoder (similar to modem for modulator/demodulator). A codec is any hardware device or set of software algorithms that can encode data and decode it back to it original (lossless) or reasonably original (lossy) state.

color calibration

The process of determining and adjusting the properties of a display device or the colors in an image to ensure that the rendered image is accurate to some standard, usually the human eye.

color channel

One of the numerical elements used to specify a color in a particular color model when that color is specified using an ordered n-tuple. Green is one channel in the RGB color model, which is specified using the ordered triplet (R,G,B).

color correction table

A section of a file for the storage of information designed to help a rendering application in displaying an image on a particular output device or class of output devices different from that assumed by the creator application.

color definition scheme

A system by which colors are specified, usually by numerical values or ordered sets of numbers.

color gamut

The range of colors which can be displayed using a particular color model or output device.

color map

See look-up table.

color-mapped image

Image data whose colors are not stored in the bitmap itself, but in a separate data array.

color models

The way colors are broken down and specified in a particular application or system.

color plane

A section of a file holding information about one color component of the color model currently in use.

color space

When a particular color scheme uses an ordered n-tuple to specify color, all the possible values corresponding to colors can be plotted on an n-dimensional graph. All the points plotted, which correspond to colors in the color model, constitute the color space.

color table

See look-up table.

color values

Same as pixel values.

Compact Disc

A circular plastic disc used for the storage of audio, video, textual, and other data that can be represented in a digital form, and from which data can be retrieved using an optical process. Although there are various formats, the one in most common use is 4.75 inches (12 centimeters) in diameter. See also CD-ROM.

component video

Color video information transmitted using three separate signal channels. RGB, YIQ, and YUV are examples of component video signals.

composite color

A color specified in a color model where that color is specified using an ordered n-tuple. A system where more than one color channel value exists, and where more than one channel value is needed to specify the color.

composite image

An image formed of two or more subimages stored separately in a file. Sometimes refers to a bitmap image with a lot of color variation per unit area.

composite video

Color video information transmitted using a single signal channel. NTSC, PAL, and SECAM are examples of composite video signals.

compression artifact

Spurious addition to or degradation of an image due to the process used to compress or decompress it.

compression type

The algorithm or family of algorithms used by the creator application in producing the file.

Computer Aided Design

The use of applications, usually vector-based, for the design and rendering of graphical data of architectural and mechanical drawings, electronic schematics, and 3D models. Commonly referred to as CAD.

contiguous data

Image data stored as a continuous block of pixel values without scan-line or block delimiters.

continuous tone

An image consisting of smooth gradations of color between adjacent elements, requiring an output device capable of displaying thousands or millions of colors at high resolution in order to prevent image artifacts.

convenience revision

A file format version created by an application vendor to accommodate a bug or quirk in a program. This is sometimes caused by ignorance or honest error, but in many cases is intentional. There is ample evidence that at least one vendor, the custodian (but not the originator) of a file format specification, knowingly released format revisions so as to avoid shipping delays caused by bugs introduced by junior programmers.

convolution

The process of transforming the value of a pixel, or a field of pixels, based on a mathematical formula. Convolution is used to alter the color of an image (filtering), or to re-encode the data (compression).

copyright

The exclusive rights to the production, publication, and sale of a work of authorship, such as a photograph or a captured image.

CRC

Cyclic Redundancy Check. An algorithm that recursively generates a 16- or 32-bit numerical value based on a stream of data. The value can be used to verify whether the data has changed. See digital signature.

D-2

Broadcast quality video recording format and medium. See Beta, VHS, U-Matic, M-II, and Betacam.

data compression

The process of converting data from one format to another format that is physically smaller in size. The same logical information is stored using less physical information.

data element

Typically the smallest units of readable data with a collection of data. Bits, bytes, WORDs, and DWORDs are all data elements.

data encoding

A generic term for the process of converting data from one format to another. Data compression and data encryption are both forms of data encoding.

data encryption

The process of converting data from an intelligible format to an unintelligible, but decryptable, format.

DCT

Discrete Cosine Transform. A mathematical transform used to convert data from a 3D to a 2D form. Used by lossy compression methods such as JPEG and MPEG.

DDB

Device Dependent Bitmap. A bitmap format designed to support the capabilities of a specific type of display hardware. A bitmap format not designed with portability in mind.

decimation

The process of throwing away portions of a bitmap image when reducing it in size.

decoder

An algorithm that converts encoded data to a raw format.

DIB

Device Independent Bitmap. A bitmap format that is designed not to be limited by the capabilities of a specific type of display hardware. A portable or interchange data format.

dichroic filter

A light producing mechanism which concentrates and directs visible light but not infrared radiation (heat). Used in movie and multimedia production, and in some scanners. See scanner.

digital camera

An input device in the form of a camera, capable of delivering bitmap image data of real-world scenes in digital form to a creator application.

digital signature

An electronic method of verifying the authenticity of a message or file. A block of data is attached to the message that can be used to validate who sent the message and when it was sent. If a change is made to the file, the signature will not verify.

digitizing

The process of converting an analog signal to a digital signal. See sampling.

digitizing device

A device that creates a version of a physical graphical representation by creating a digital version. Common digitizing devices are scanners, image capture boards that work with video cameras, and digital cameras.

digitizing tablet

An input device incorporating a pen-like component and a flat surface, and meant to provide a simulation of the interaction between the hand, a pen or pencil, and paper. Can be pressure-sensitive.

display adapter

See video display card.

display surface

The portion of an output device where an image appears. The screen of a monitor, or printed paper.

dithering

The process of displaying colors not available on an output device. Patterns of other colors are created by intermixing monochrome pixel values with color pixels to produce shading and highlighting that appears to the eye as differing colors. See monochrome and contrast with halftone.

dot pitch

The measure of the spacing between the centers of physical pixels on an output device, usually a monitor. On monitors, dot pitch is expressed in millimeters (mm) between the centers of like-colored pixels. See shadow mask.

down-bit ordering

Where the most significant bit (MSB) is the first bit read in a byte. See MSB.

drawing surface

The area on an output device where a rendered image appears.

drum scanner

A high-resolution scanning device used in the most demanding professional applications. They can often be the source of extremely large files. See scanner and film scanner.

dumb frame buffer

A video display card consisting mainly of a frame buffer with few enhancements. Contrast with accelerator card.

EDIP

See Electronic Document Image Processing.

Electronic Document Image Processing

A subfield of image processing specializing in the creation, storage, and manipulation of black-and-white images derived from printed documents. At least 75 percent of the image-processing market today is based on EDIP systems and applications.

element attributes

Information, such as color, line width, pen style, and fill color, stored in a file for use by a rendering application in reconstructing an image element.

encoder

An algorithm that converts raw data to an encoded form, usually to physically compress the data.

entropy encoder

See lossless encoding.

FAX files

Graphics files produced by a program that manages FAX-modem hardware. These are generally bitmap files and may be compressed. They are often in a proprietary format, although versions of TIFF and PCX are popular.

field

A fixed-size data structure in a file.

fields

Images stored in a video or animation file designed to support various display technologies. NTSC video consists of two sets of images meant to be displayed alternately, each of which is a field.

file element

The smallest unit of logical information within a file. Examples include fields within graphics file headers and color triples used to store RGB pixel data.

file header

A data structure containing information on the data stored within a file. Graphics file headers contain information such as the height and width of an image and the number of bits per pixel.

file ID

See file identifier value.

file identifier value

A specific value, or set of values, used to positively identify a file as being of a particular file format. File ID values may be an integer, such as 59A66A95h, or a string of ASCII characters, such as BITMAP, and they usually appear in the first field of a file header. Also called magic number.

fill attributes

Fill color and/or other information associated with an image element, and used by the rendering application to reconstruct an image.

fill color

A color meant to be used by a rendering application when filling a closed area, usually polygonal, when reconstructing an image element.

filler

See padding.

film scanner

See slide scanner.

filter

A section of code or program which operates on an entire block of data. Contrast with scanner.

fixed

Refers to an element in a file that has a known position, usually identified by an offset from a landmark in a file.

fixed frequency

A monitor or other output device with fixed frame rate and horizontal frequency, only capable of displaying a small set of resolutions. See horizontal frequency and frame rate.

footer

A data structure similar to a header but appended to the end of a file.

format creator

The person or organization responsible for the definition of the physical structure of, and conventions associated with, a file format. Often this person is a programmer called on to produce a file format in association with an application. In some cases, the format creator is a standards committee.

fractal

Repetitious patterns that naturally occur in the texture of all surfaces. Mathematics is used to described the properties of fractals.

fractal compression

The use of fractal encoding to reduce the amount of physical data required to store an image. See fractal encoding.

fractal encoding

The process of describing a bitmapped image as a sequence of fractals and by its fractal properties. See fractal.

frame

A single image. Multiple frames of slightly differing images displayed in rapid sequence are used to create animations.

frame buffer

Older term for video display card. Technically, the portion of a video display adapter containing memory in which digital image data is assembled prior to sending it to a monitor. See video display card.

frame rate

Number of full images which can be displayed by an output device, usually a monitor. Number of fields a monitor can display in a given time. Usually expressed in hertz (Hz). See fields, interlaced, non-interlaced, and vertical retrace.

frames

Series of single images stored in a video, animation, or multimedia format file, meant to give the illusion of motion when rendered in rapid succession.

frequency

Factor that determines the quality of the image a monitor or other output device can display. Higher frequency monitors update the screen faster and theoretically can display more information in a given amount of time. See vertical retrace, interlaced, and non-interlaced.

FTP

File Transfer Protocol. A low-level protocol used to transfer files between computers over computer networks. FTP is the primary means by which binary files are transferred between machines on the Internet.

fullcolor display

A term sometimes used to imply that a device is capable of displaying 2^15 (32,768) or 2^16 (65,536) colors; however, this actually describes hicolor; fullcolor tends to be a marketing term, rather than a technical one.

full-motion video

Video image frames displayed at a rate of 30 frames per second for NTSC and 25 frames per second for PAL.

fuzzy logic

A sub-discipline of mathematics used to quantify subjective linguistic concepts, such as bright, dark, very far, quite close, most usually, almost impossible, etc.

G3

Abbreviation for CCITT Group 3 encoding.

G4

Abbreviation for CCITT Group 4 encoding.

gamma

A numerical value used to indicate the non-linear response curve of an output device to light intensity. Used to correct the intensity of an image on a display device (gamma correction). Also called gamma value.

gradient fill

An elaboration on a fill color consisting of two colors placed at opposite ends of a closed area of an image element. The area is filled with a continuous blend of color intermediate to the two colors and between the two ends. See fill color.

graphic work

The end result of effort by a graphic artist. A drawing or other artifact.

graphics adapter

See video display card.

graphics card

See video display card.

graphics data

Data which may or may not have a physical representation, intended for display on an output device.

graphics file

A file containing graphics data.

graphics file format

The definition of, and conventions associated with, a file structure used for the storage of graphics data.

graymap

In older terminology, raster data composed of values with more than two levels, intended for an output device capable of displaying only shades of gray.

gray-scale

A term used when referring to an image. A gray shade is any color whose three primary colors are the same value. Gray shades only have intensity (luminance) and no color (chrominance).

halftone

The use of bi-level pixels or dots to create the appearance of shades of gray by grouping the pixels in patterns that produce the desired shades. Used in printing and liquid crystal displays. See bi-level and contrast with dithering.

HBL

Acronym for Hue/Brightness/Luminosity. See HSI.

heuristics

A set of rules derived from experimentation or experience.

Hi8

Video recording format offering resolution of over 400 lines. See VHS, S-VHS, 8-mm, and Beta.

hicolor display

A term used to imply that a device is capable of displaying 2^15 (32,768) or 2^16 (65,536) colors.

HLS

Acronym for Hue/Lightness/Saturation. See HSI.

horizontal frequency

Measure of the speed at which the electron beam in a monitor sweeps across the width of the active area of the screen. Usually expressed in kilohertz (kHz).

HSB

Acronym for Hue/Saturation/Brightness. See HSI.

HSI

Acronym for Hue/Saturation/Intensity. An additive color system based on the attributes of color (hue), percentage of white (saturation), and brightness (intensity). Similar or identical color systems include HBL, HLS, HSB, HSL, and HSV.

HSL

Acronym for Hue/Saturation/Luminosity. See HSI.

HSV

Acronym for Hue/Saturation/Value. See HSI.

hue

Any color, such as red, violet, orange, and so on.

hybrid text

The storage and display of bitmap and textual data using a single graphics file format. GIF89A is an example of a format with a hybrid text capability.

hybrid database

The ability to store complex and highly organized database information in conjunction with graphical data. See also hybrid text.

hypertext

A collection of graphical and textual data organized in such a way as to facilitate easy access to all of the information it contains. Hypertext may be thought of as a precursor to multimedia, or simply as an extension of it. Certain extensions of hypertext are becoming known as hypermedia.

icon

A small bitmap image used as a placeholder to represent an object. An icon may also be though of as a graphical file name. Contrast with thumbnail image.

ID value

See file identifier value.

IEC

International Electrotechnical Commission. See ISO.

image

A visual representation of graphics data displayed on the display surface of an output device. Output of a rendering application. One end of the graphics production pipeline. A single frame from an animation or video sequence.

image bitmap

See bitmap image.

image data

A term used loosely to refer to bitmap data, or the portion of a bitmap file containing bitmap data.

image elements

Portions of an image, often repeated, from which the image is composed by duplication, rotation, scaling, and translation.

image file index

Image offset table. An area of a file designed to hold descriptions of subimages, image components, or individual images in a multi-image file.

image height

The vertical size of an image, usually expressed in pixels or scan-lines or other non-device-dependent units.

image length

The horizontal size of an image, usually expressed in pixels or other non-device-dependent units.

image offset table

A portion of a file designed to hold offsets, usually measured in bytes, where subimages, image components, or individual images in a multi-image file can be found.

index map

See look-up table.

index values

Pairs of numbers arranged in a table so that an application can match numbers it knows about to numbers representing colors that an output device knows about.

indexed-color image

See color-mapped image.

indirect color

The specification of colors through the use of a palette or look-up table.

input

Generic term in computer technology referring to any data which is processed or transformed.

interframe encoding

The creation of encoded data from two or more image frames. MPEG encoding is an interframe encoding method.

interlaced

Refers to a strategy used by televisions and some older monitors, where the electron gun draws every other line during a single sweep across the active area of the screen. The alternate lines are filled in during a second pass. See vertical retrace, non-interlaced, and fields.

interlaced encoding

See interleaved encoding.

interleaved encoding

The storage of bitmap scan-lines, or pixels within scan lines, in a non-sequential pattern. Contrast with sequential encoding.

interleaving

In reference to a single image, the storage of two or more subimages which are combined to create a final image by displaying alternate scan-lines from each subimage. In reference to video, multimedia, or animation formats, the process of storing or displaying information other than that used to reconstruct the video portion between video frames.

interpolation

The addition of pixels between pairs of others. Usually made necessary when enlarging a bitmap.

intraframe encoding

The creation of encoded data from a single image frame. JPEG encoding is an intraframe encoding method.

ISO

International Standards Orgainzation. The primary organization for creating world-wide technical standards. Along with the IEC and the ITU, the ISO authors and maintains standards for everything from nuts and bolts to computer languages.

ISO-9660

A file system standard developed for CD-ROMs using the CD-XA encoding standard. An ISO-9660 file system is readable by many operating systems, including MS-DOS, Macintosh OS, and UNIX.

ITU

International Telecommunications Union. An agency of the United Nations responsible for telecommunications. The ITU replaced the CCITT on Februrary 28, 1993.

ITU-T

ITU Telecommunications Standardization Sector. The body within the ITU responsible for setting world telecommunications standards (Recommendations).

jaggies

Term denoting the presence of aliasing in an image.

JBIG

Joint Bi-level Image Experts Group. The ISO/IEC JTC1/SC29/WG9 and CCITT SGVIII group formed in 1988 to establish a standard for the progressive encoding of bi-level image data. JBIG is also the name given to the codec created by this group.

JPEG

Joint Photographic Experts Group. The group of the ISO responsible for the creation and maintenance of the JPEG still-image compression standard.

JTC1

Joint Technical Committee 1. A group within the ISO/IEC that handles information technology.

key points

Points necessary for the reconstruction of a graphics object from vector data. These are usually the minimum needed to specify the object. Two points at the corners of a rectangle are the key points.

landmark

Refers to an element in a file from which other positions and offsets are calculated. The canonical landmarks are the beginning, end, and current position. Other features, such as prominent data structures, may at times act as landmarks.

laser disc

Recording medium used for video, similar to a large CD or CD-ROM, offering the advantage of random access and high quality.

leak

Image artifact, usually generated from vector data, produced by a rendering application from an image element incorrectly designated as closed. Usually consists of horizontal lines of the fill color in an inappropriate area of the image.

little-endian

Refers to systems or machines which store the least significant byte (LSB) at the lowest address in a word, usually referred to as byte 0. Contrast with big-endian.

logical pixels

Idealized pixels having perfectly defined characteristics and occupying no physical extent. The graphics equivalent of a mathematical point. Contrast with physical pixels.

look-up table

A series of pairs of numerical values whereby a program can match a meaningful value to one which specifies a color on an output device.

lossless encoding

A data compression or encoding algorithm that does not lose or discard any input data during the encoding process.

lossy encoding

A data compression or encoding algorithm that loses, or purposely throws away, input data during the encoding process to gain a better compression ratio. JPEG is an example of a lossy encoding method.

LPI

Lines per inch, usually used to refer to screen size or the resolution of an output device. See screen.

luminance

The brightness or intensity of a color. The pixels in a monochrome image have a luminance of either 100 percent or 0 percent.

LSB

Depending on context, either the least significant byte (of more than one juxtaposed bytes) or the least significant bit (of the bits in a byte or word of data). Contrast with MSB.

LUT

See look-up table.

M-II

Broadcast quality video recording format and medium. See Beta, VHS, U-Matic, Betacam, and D-2.

magic number

See file identifier value.

magic values

Arbitrary numbers or text strings, often picked "out of the air" by a format creator for the purpose of identifying the format.

message digest function

A family of algorithms used to create digital signatures.

metadata

Data comprised of attributes, parameters, notebooks, and other types of miscellaneous complex data aggregates associated with primary scientific data.

metafile

A file format capable of storing two or more types of image data, usually vector and bitmap, in the same file.

MIDI

Acronym for Musical Instrument Digital Interface. A standard for digital signals used to control electronic musical instruments. MIDI information may be stored as a data file and is found in many multimedia file formats.

MMR

Modified Modified READ, The compression algorithm used in CCITT Group 4 facsimile compression.

monochrome

An image composed of a single color and black. Most monochrome images are black and white, although any color might be substituted for white. Also called 1-bit images. Although the term monochrome, of course, means single-colored, in computer graphics it is used to denote a system where two colors can be specified: the foreground color and the background.

MPEG

Motion Picture Experts Group. The group of the ISO responsible for the creation and maintenance of the MPEG video compression standard.

MR

Modified READ, The compression algorithm used in CCITT Group 3 facsimile compression.

MSB

Depending on context, either the most significant byte (of more than one juxtaposed bytes) or the most significant bit (of the bits in a byte or word of data). Contrast with LSB.

multi-channel palette

A palette with two or more individual color values per color element. Contrast with single-color palette.

multi-sync

A monitor or other output device with frame rate and horizontal frequency adaptable to demand. See horizontal frequency, fixed frequency, and frame rate.

multimedia

The concept of creating, storing, and playing back two or more forms of electronic information simultaneously. Such information includes still-images, motion-video, animations, digitized sound, and control information such as MIDI codes.

non-interlaced

Refers to a strategy used by higher-frequency monitors where the electron gun draws all lines of the active area of the screen during a single sweep. See interlaced, vertical retrace, and fields.

NTSC

Acronym for National Television Standards Committee. The standards committee responsible for, among other things, the creation of the color television signal used in the United States (NTSC video).

objects

Image elements, particularly in vector files. Descriptions of complex image elements. Image element information stored along with code for use by the rendering application.

output

Generic term in computer technology meaning the result of any process or transformation of data.

output device

Physical mechanism used to create a display.

output device language

A computer language or set of commands created by a vendor to communicate with a particular output device, such as a printer. Hewlett Packard's PCL is one well-known output device language and is understood by Hewlett Packard printers and HP-compatible printers. It may or may not be easily human-readable. See page description language.

overflow

A condition which results when data of a certain size is placed in a storage cell, such as location in memory or a register, which is too small to hold it. This is usually a problem resulting in loss or corruption of data.

overlay bit

An additional bit found in a pixel or pixel plane that indicates whether the pixel is displayed as visible (opaque) or transparent (overlayed). See alpha channel.

packet

A block of data with a known structure, usually used to denote elements of a stream.

padding

Portion of a file usually included to accommodate machine dependencies or to increase reading or writing speed.

page description language

A computer language created by a vendor to communicate with output devices. It may be a fully functional language and is always human-readable. It is generally more sophisticated than an output device language and is not tailored to any particular output device. The most popular page description language in use today is Adobe's PostScript.

page table

An array of offset values used to index the location of multiple bitmaps within a single graphics file. Each offset value indicates the starting position of each bitmap.

PAL

Acronym for Phase Alternation Line. PAL is a standard of color television and video signals developed in West Germany and used throughout Europe (PAL video).

palette

The gamut of colors which a device can display; a software data structure used to match numbers that are meaningful to a software program to numbers that cause colors to appear on an output device.

passive information device

An electronic device with which the user need not interact in order to obtain data. Television and newspapers are examples of passive information devices. Contrast with active information device.

PDL

Page Description Language. A computer language used for describing the layout, font information, and graphics of printed and displayed pages.

pel

See pixel.

pen

A logical device used by creator applications to draw lines or curves or objects composed of them, having the properties of width, color, and possibly line style. An area in a file holding information used by a rendering application in reconstructing lines.

persistence

A term often used in object-oriented technology to describe data that is stored in a static medium, such as a disk file or database. The data is said to "persist" even after the application that created it is no longer in memory. Spreadsheet, word processing, and graphics files are examples of persistent data.

PGP

Pretty Good Privacy. A powerful public-key encryption system authored by Phil Zimmermann that is freely available on the Internet.

physical pixels

The actual pixels which appear on the display surface of a raster output device. Contrast with logical pixels.

picture element

See pixel.

pixel

In traditional usage, short for "picture elements." These are irreducible elements of color created by an output device on its display surface. The term is sometimes used loosely to refer to the values of bitmap data elements used by an application to order the display of color elements on an output device.

pixel depth

See bit depth.

pixelmap

In older terminology, bitmap data composed of values with more than two levels, intended for an output device capable of displaying color.

pixel values

Numerical data items in a graphics file indicating the color or other information associated with an individual pixel.

pixmap

See pixelmap.

planar data

Image data stored as separate color planes, and meant to be assembled into the final image by the rendering application. Contrast with scan-line data.

planar files

Graphics files with image data stored as bit planes or color planes rather than as pixels.

plotter

A bitmap rendering of vector or 3D graphical data used to display an approximation of the graphical data. See thumbnail.

predictive encoding

An algorithm that has certain prior knowledge about the format of the data it is encoding. Huffman is a predictive encoding algorithm.

preview

A bitmap rendering of vector or 3D graphical data used to display an approximation of the graphical data. See thumbnail.

primary colors

Colors in a particular color model from which other colors can be constructed. In the RGB color model, red, green, and blue are the primary colors because other colors can be produced by mixing them.

production pipeline

The series of operations involved in defining, creating, and displaying an image, from conception to its realization or recording on an output device.

progressive encoding

The storage of a single bitmap as several different images, each at a different level of resolution.

pseudo-color

A color specified through the use of a palette or look-up table.

quantization

The process of reducing the number of colors defined in the source data to match the number available on an output device.

quantization artifacts

Generally refers to features introduced in an image when the data used to render that image is converted to a data format capable of displaying fewer colors than the original. Banding and false color are two examples of possible quantization artifacts. Usually considered undesirable.

raster

Refers to graphics data represented by color values at points, which taken together describe the display on an output device. Bitmap is used in preference to raster in this book.

raw

Image data without header information. Sometimes refers to image data, especially bitmap data, which is uncompressed or otherwise unencoded.

READ

Relative Element Address Differentiation code. A compression method used by CCITT Group 3 and 4 facsimile transmission.

realization

The representation of an image on an output device. Sometimes meant to signify the current rendered version of some particular graphics data.

render

To produce a visual representation of graphics data on an output device.

rendering

The actual representation of an image on an output device.

representation

The actual artifact produced as the end result of the computer graphics production process, which may be an image on a monitor or on paper.

reserved fields

Fields in a file designated in the format specification as reserved space.

reserved space

Portion of a file designated in the format specification as space for additional information should it become necessary in the future.

resolution

The measure of detail within an image. The resolution of an image is its physical size (number of pixels wide by number of scan lines long). The resolution of a display is the number of scan lines it may display (800x600 is a higher resolution than 320x200).

RGB

Acronym for Red/Green/Blue. An additive color system based on the primary colors red, green, and blue. The RGB model is loosely patterned after human eyes, which have a peak sensitivity to the colors red, green, and blue light.

RLE

Run-Length Encoding. A simple method of compressing runs of identical byte sequence values into a code only a few bytes in length.

S-VHS

Video recording format and medium offering horizontal resolution of over 400 lines. See VHS and Beta.

sample rate

The number of digital samples recorded per second. The sample rate increases with the number of samples recorded per second. Same as sample resolution.

sample resolution

See sample rate.

sampling

The process of reading an analog signal at specific increments in time (sample rate) and storing the data as digital values. Sampling is the basic process used to create digital audio and video.

saturation

The percentage of white in a color. Zero percent saturation is full white (no color). 100 percent saturation is no white (pure color).

scalable

An image, such as that stored as vector data, which can be scaled without introducing artifacts.

scaling

The process of enlarging an image in one or more directions.

scan line

A row of pixels. The term comes from the scanning action of raster CRT output devices, which produces successive lines of output on the display surface.

scan-line data

Image data stored as scan lines, and meant to be displayed a line at a time by the rendering application. Contrast with planar data.

scan-line table

An array of offset values used to index the location of each scan line or tile within a collection of bitmap data, which may or may not be compressed. Each offset value indicates the starting position of each scan line or tile.

scanner

An input device generating a bitmap image of a surface. A section of code or program allowing random access to a block of data and which treats different portions of the block differently in accord with its informational content. Contrast with filter.

scene description language

A computer language used to describe the position and attributes of objects within a 2D or 3D image. A file produced by such a language is called a scene format or scene description file.

screen

Term borrowed from traditional graphics denoting a device or process designed to turn a continuous tone image into an array of dots, usually for display on a low-resolution output device. See LPI.

SECAM

Acronym for Sequential Coleur Avec Memoire (sequential color with memory). SECAM is a standard of color television and video signals used in France and several other European countries (SECAM video).

segment

An independent section of a data stream. For example, a JPEG data stream is composed of many different types of information, each stored in a separate segment.

sequential encoding

The storage of a bitmap using the natural order of its scan lines from the top-to-bottom or bottom-to-top of image. Contrast with interleaved encoding.

shadow mask

A plate with tiny holes, usually in a monitor, which helps locate and focus the electron beam. The spacing of the holes in monitors with a shadow mask determines the dot pitch. See dot pitch.

single-channel palette

A palette with one color value per element. Contrast with multi-channel palette.

single pass scanner

A scanner which images an object in a single pass of its mechanism. See scanner, three-light method, and three-filter method.

slide scanner

A device allowing the digitization of slides taken with conventional film cameras. Usually much higher resolution devices than scanners designed with paper in mind. See scanner.

stream

Data with no fixed position in a file, composed of sub-elements with a known structure.

strip

A collection of one or more contiguous scan lines in a bitmap. Scan lines are often grouped in strips to buffer them in memory more efficiently. Also called bands in some file format specifications.

stripe mask

See aperture mask.

stroke font

A file of character information meant to be rendered by drawing single lines, usually by a plotter or other device responding only to pen up, pen down, and move to commands.

subsampling

See decimation.

subtractive system

A color system in which colors are created by subtracting colors from white. The more color that is added, the more the resulting color tends toward black.

tag

A data structure in a file which can vary in both size and position.

three-filter method

Process whereby color images are produced, usually in scanners, by illuminating the object to be imaged with white light which is made to pass through three successive colored filters. Contrast with three-light method. See color model and scanner.

three-light method

Process whereby color images are produced, usually in scanners, by successively illuminating the object to be imaged with three colors, usually red, green, and blue. Contrast with three-filter method. See color model and scanner.

thumbnail

A small image derived from a larger image used to quickly display an approximation of the contents of an image. See preview.

tile

A 2D sub-section of a bitmap. For example, a bitmap 100x100 pixels in size may be divided into four 25x25 pixel tiles. Pixels are often grouped as tiles rather than scan lines to achieve a more efficient use of memory.

trailer

See footer.

transcode

Convert from one encoded data format to another encoded data format--for example, converting CCITT Group 3-encoded data to RLE-encoded data.

transform

See convolution.

transparency

The degree of visibility of a pixel against a fixed background. A totally transparent pixel is invisible. See also alpha channel.

trichromatic colorimetric

Color models that use three color channels to specify a color. The RGB color model is a trichromatic colorimetric system.

Trinitron

Trademark of Sony Corporation, referring to picture tubes used in monitors and televisions using aperture mask technology, and where dot pitch is an expression of the horizontal distance between strips in the aperture mask. See aperture mask, shadow mask, and dot pitch.

triple sensor scanner

A scanner which images an object using three recording devices at once, usually CCDs. See scanner and CCD.

truecolor display

A term used to imply that a device is capable of displaying 2^24 (16,777,216) colors or more (said to match or exceed the color-resolving power of the human eye). Truecolor formerly referred to any device capable of displaying 2^15 (32,768) colors or more, but hicolor more accurately describes the display of 2^15 (32,768) or 2^16 (65,536) colors.

U-Matic

Broadcast quality video recording format and medium. See Beta, VHS, Betacam, M-II, and D-2.

up-bit ordering

Where the least significant bit (LSB) is the first bit read in a byte. See LSB.

vector

Refers to graphics data composed mainly of representations of lines and outlines of objects, which can be compactly represented by specifying sets of key points. A program displaying vector data must know how to draw lines by interpolating points between the key points.

vertical refresh rate

See frame rate.

vertical retrace

The interval between when an electron beam in a monitor reaches the end of its sweep across the active area of the screen and when it returns to the start.

vertical scanning frequency

See frame rate.

VHS

Video recording format and medium in wide use in conjunction with television technology, offering horizontal resolution of 240 lines. Not considered broadcast quality. See Beta.

video adapter

See video display card. Also, a device allowing output in NTSC, PAL, or other video format.

video adapter card

See video display card.

video controller

See video display card. Also, a video adapter which may have circuitry for controlling a video recording or playback deck.

video display card

Device which takes digital information from a rendering application and converts it to an analog format suitable for output on a monitor.

virtual output

Data or an image that is produced, but that can't be seen--in other words, for which no physical representation yet exists. Data in a file.

virus

A computer program that mimics a biological virus in its characteristics and actions, including hiding, replicating, and possibly causing the death of its host.

voxel

A 3D pixel. Voxels contain all of the components of a pixel (such as color values) and include an extra component that specifies the distance of the voxel from the point of observation.

VRML

Virtual Reality Modeling Language. An interpreted language used to render both still and animated 3D objects.

VTR

Video tape recorder used as an output device for video, animation, and multimedia creator applications.

x origin of image

The point in an image from which pixels are numbered in the horizontal direction. Usually at a corner or at the center of the image.

y origin of image

The point in an image from which to start counting scan-lines. Usually at the top, bottom, or center of the rendered image.

YIQ

The color model used by NTSC video signals. See NTSC.

YUV

Acronym for Y-signal, U-signal, and V-signal, which is based on early color television terminology. A luminance/chrominance-base color model (Y specifies gray-scale or luminance, U and V chrominance) used by many video compression algorithms, such as MPEG.



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