Previous chapter LLUs Home Next chapter IndexThis glossary is a brief introduction to terms used in NCAR Graphics Fundamentals, UNIX Version. Definitions here are intended to clarify issues without overwhelming new users with precise technical jargon. These definitions primarily represent usages of words in this guide, and may not be complete by industry standards.
ANSI---American National Standards Institute.
ASCII---American Standard Code for Information Interchange.
Aspect ratio---Specifies the height-to-width relationship of characters. For example, characters with an aspect ratio of 2.0 are twice as tall as they are wide.
CGM---Computer Graphics Metafile. A metafile that conforms to American National Standard ANSI X3.122-1986.
Color index---A nonnegative integer assigned a color value in a color table.
Color map--- See Color table.
Color model---See Color space.
Color palette---A table of colors that are referenced by the pixel values in an indexed color image. Color table, color map, and color palette may be used interchangeably.
Color space---A three-dimensional coordinate system, where coordinates in that system represent colors.
Color table---A list that associates RGB (red, green, blue) triplets with nonnegative integer values called color indices. Each RGB triplet defines a single color and RGB values are normalized between 0. and 1. where each value represents the intensity of the red, green, or blue hue in the color. Color table, color map, and color palette may be used interchangeably.
Color value---A value in a color model.
Dash pattern---The sequence of dashes, dots, spaces, and text used to represent a line. Dash patterns can be selected using the routines GSLN, DASHDB, and DASHDC.
Data coordinates---A coordinate system defining the location of data in an input data array. The mapping routines provided by NCAR Graphics utilities such as Conpack, Streamlines, and Vectors transform data coordinate space to user coordinate space.
Digital image---See Raster image.
Fontcap---A file that contains detailed information that can be used to plot characters. Fontcaps can have a human-readable ASCII form or a binary form that is readable by ctrans.
Fractional coordinates---The NCAR Graphics coordinate system in which each axis ranges from 0. to 1. NCAR Graphics fractional coordinates are the logical equivalent of GKS normalized device coordinates.
Function codes---Character codes that are embedded in the text string argument passed to the PLCHHQ or PLHIQU routines. These codes control text characteristics such as font, superscripting, subscripting, orientation, and position.
GKS---Graphical Kernel System. American National Standard ANSI X3.128-1985, which standardizes basic graphics functionality.
GKS segment---A collection of graphic display elements that may be manipulated as a whole.
GKS workstation---An abstraction of a physical device that allows for the display of lines, text, closed polygons, and markers on a rectangular surface of fixed resolution.
Graphcap---A file that contains detailed information that can be used to define the capabilities of a specific plotting device. Graphcaps can have a human-readable ASCII form or a binary form that is readable by ctrans.
Grid coordinates---A coordinate system coincident with the assumed dimensional boundaries of an input data array. NCAR Graphics utilities such as Conpack, Streamlines, and Vectors provide parameters for mapping the grid coordinate boundaries into a data coordinate space defining the location of the data.
GUI---Graphical User Interface. In NCAR Graphics, all GUIs run on X Window System terminals or workstations.
HDF---Hierarchical Data Format. A file format for storing raster data developed by the National Center for Supercomputing Applications (NCSA).
HLS---Hue, Lightness, and Saturation color model.
HSV---Hue, Saturation, and Value color model.
Identity transformation---A window-to-viewport mapping in which both the window and viewport range from 0. to 1. in both dimensions.
Indexed color imagery---Imagery where the pixel values reference RGB colors contained in a color table. See Raster images.
Internal parameters---Variables that control the behavior of NCAR Graphics utilities. These variables can only be accessed via FORTRAN 77 common blocks. See the specific utility's documentation for a description of how to access the utility's parameters.
Isosurface---Perspective representation of 3-D "volume" of data. The isosurface is defined by a threshold value, and it passes through all data points that equal this value. The routines EZISOS and ISOSRF draw isosurfaces. See Surface.
LLU---Low Level Utility. Refers to any of the Fortran-callable utilities in NCAR Graphics.
Log scaling---Describes a Cartesian axis in which values range logarithmically from one end to the other.
Markers---Symbols (dots, plus signs, asterisks, circles, crosses, and FORTRAN 77 characters) drawn using routines such as GPM, POINT, and POINTS.
Metafile---Generally synonymous with NCGM in this document. A metafile can be any mechanism for retaining and transporting graphical data and control information. This information contains a device-independent description of one or more pictures.
Mirror imaging---Describes a Cartesian X or Y axis in which values decrease from left to right or top to bottom.
NCGM---NCAR Computer Graphics Metafile. A CGM created by NCAR Graphics, using the NCAR Graphics GKS library. This is an ANSI standard CGM with an NCAR-specific wrapper on it.
NDC---Normalized Device Coordinates. A device-independent GKS coordinate system in which the horizontal and vertical coordinate ranges are both 0. to 1.
Normalization transformation---The transformation that occurs between the world coordinate window and the normalized device coordinates.
Parameters---Variables that control the behavior of NCAR Graphics utilities and are typically accessed via parameter-access routines that can retrieve or set the parameter value.
PAU---Plotter Address Unit. A measure of length. By default, 1 PAU is equal to 1/1024th of the maximum screen size. NCAR Graphics utilities are migrating away from the use of the PAU terminology.
Pixel---Picture element.
Polyline---A curve made up of line segments. Polylines are drawn using the routines GPL, CURVE, and CURVED.
Polymarkers---A series of markers drawn using the GKS routine GPM.
PWRITX database---One of the character databases used by the Plotchar utility. The database contains the complex and duplex character sets.
Raster image---A digital image is a 2-D array of pixels (picture elements), where each pixel either directly or indirectly defines the color of that (X, Y) location. In the case of "indexed-color" imagery, the pixel values are mapped into a color by using the pixel values as indices into a separate color palette. In "true-color" imagery, each pixel value maps directly into the actual pixel color (RGB, HSV, and so on).
RGB---Red, Green, and Blue color model.
Special value---The number that represents missing or special data in a dataset. Typically, this value is outside the range of the "normal" dataset values.
SPPS---An NCAR Graphics utility that draws points, lines, and text, and supplies a few low-level shortcuts, such as opening and closing GKS.
SPPS polyline buffer---An area of memory that contains the commands for polyline segments so that NCGM files can be kept small.
Stereo pairs---Two identical images that are drawn at some offset on a single or separate frame.
Subsampling---A method for reducing the size of a dataset by extracting a subset of the data.
Surface---A perspective representation of 2-D data, where each value of the data represents height in the surface. The routines EZSRFC and SRFACE draw surfaces. See the "Drawing Surfaces and Isosurfaces" chapter.
True color imagery---Imagery where each pixel has a red, green, and blue value. See Raster images.
User coordinates---The NCAR Graphics Cartesian coordinate system in which you perform your calculations. This is identical to the GKS world coordinate system when mirror imaging and/or log scaling of an axis is not used. In user coordinates, windows can be defined in which the coordinates decrease from left to right or top to bottom (mirror imaging) or range in a logarithmic scale (log scaling). GKS world coordinates, however, always increase from left to right and bottom to top in a linear fashion. See Chapter 8 "Coordinate systems in NCAR Graphics" and Appendix A "The Use of X/Y Coordinates in NCAR Graphics" for more information.
WC---World Coordinates. The GKS Cartesian coordinate system in which you perform your calculations. See Chapter 8 "Coordinate systems in NCAR Graphics" and Appendix A "The Use of X/Y Coordinates in NCAR Graphics" for more information.
X Window System---A network-based graphics window system that was developed at the Massachusetts Institute of Technology (MIT) in 1984.
YIQ---Broadcast television color model.
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