Glossary


A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z

A

adjustable array
An array that is a dummy argument in a Fortran subroutine or function whose dimensionality is determined at runtime. The dimensionality of an adjustable array is supplied in the argument list in which the dummy array name appears, or by values in a COMMON block.

animation
A sequence of two or more images that, when displayed in a rapid sequence, provide the illusion of continuous motion.

annotation
Any collection of elements of a plot whose behavior can be controlled as a group.

arithmetic operator
An operator that applies to variables having a numeric data type. Examples are "+" (addition) and "*" (multiplication).

ASCII file
A file whose contents are to be interpreted as a sequence of characters as defined by the ASCII (American Standard Code for Information Interchange) standard.

attribute
A singly-dimensioned datum of any type that is assigned to a resource list using the '.' operator. An attribute contains descriptive information.

B

binary file
A file whose contents are to be interpreted as a sequence of bits, rather than characters. There are different flavors of binary files. A "flat" binary file is a sequence of bits with no ancillary information about the file contents. This type of file is created and read by C programs. Fortran creates and reads flat binary files only when in direct-access mode. All records are the same size in a flat binary file. By default, Fortran creates another type of binary file which can contain variable-length records. This is called a sequential-access binary file. In a sequential-access binary file, record length information is embedded prior to each record.

block statement
A statement that requires one or more individual statements bracketed by delimiters indicating the beginning and end of the block. Examples of block statements are: do-end do, if-then-end if, setvalues-end setvalues.

built-in function or procedure
Functions and procedures that are part of the PyNGL language and are automatically available.

C

CCM history tape format
A proprietary data format used by atmospheric climate simulation models developed at NCAR. (CCM stands for Community Climate Model.)

C function prototype
A C function declaration that declares a function's return type, how many arguments the function takes, and the types of the arguments.

class
A template for defining objects that specifies variables and procedures that operate on those variables. In the context of PyNGL, the class variables are called resources and the class procedures are called support functions.

color map (see color table)

color table (also referred to as a color map)
A table that associates integer values (called color indices) with RGB color values. In NCAR Graphics, color tables contain up to a maximum of 256 colors (including the background color).

comment line
A line in an PyNGL code beginning with a semi-colon "#". A line Comment lines contain descriptive information about the code.

contour plot
A plot of 2D data containing contour lines (lines marking points of equal elevation) to indicate surface shape. Contour plots may have color fill between contour lines and may have label bars and annotations.

D

data type
A qualifier on a variable that assigns it a specific representation. See Python and Numeric Python references for a description of data types.

decision statement
A language construct allowing for conditional program execution based on the truth or falsity of an expression.

E

Encapsulated PostScript (see EPS)

Encapsulated PostScript Interchange format (see EPSI)

EPS
EPS stands for "Encapsulated PostScript" which is a subset of regular PostScript. The restrictions placed on EPS files are for making it an appropriate format for importing into applications that import PostScript. EPS files contain information in them that allow an importing application to know the extent of the marks on the PostScript page. They are also restricted to a single page of output.

EPSI
EPSI are EPS files that have a "preview bitmap" that represents the PostScript image contained in the file. This bitmap can be used by an importing application to display quickly a picture of the imported file. This is used by applications that do not have a built-in PostScript interpreter. The "preview bitmap" output does not represent the picture contained in the EPSI file, but rather is simply a bitmap containing the message: APPLICATION CANNOT DISPLAY NCAR GRAPHIC HERE. VIEW THE POSTSCRIPT OUTPUT TO VERIFY SUCCESSFUL IMPORT.

error flag
Within the context of PyNGL, functions that are documented to return of an error flag return an integer as per:

-4 - fatal error
-3 - warning
-2 - information
-1 - no error

F

file name suffix
A suffix appended to a file name to indicate its type. PyNGL recognizes the following supported suffixes: ".nc" for netCDF, ".hdf" for HDF, ".grb" for GRIB, and ".ccm" for CCM History Tape. The obsolete suffix ".cdf" for a netCDF file is also recognized.

fill value (fill_value, _FillValue)
Another term for missing value. Often used in conjunction with masked arrays.

Fortran 90 interface block
A sequence of Fortran 90 statements (bracketed by special delimiting statements) used to describe a procedure interface. The statements in the interface block contain a declaration for the procedure and declarations for the dummy arguments and no executable statements.

G

GRIB
GRids In Binary - a read-only network-transparent data format. GRIB is a format used by many of the world's foremost operational weather centers. It is a very space-efficient format.

GSUN
Acronym for "Getting Started Using NCL."

H

HDF
Hierarchical Data Format - a network-transparent and self-describing data format developed at NCSA.

High Level Utilities
Objects, like XyPlot objects, Contour objects, TextItem objects, and so forth, that can be created and manipulated by a set of library functions, callable from either a C program, a Fortran program, the NCAR Command Language, or a GUI. High Level Utilities are also called HLUs.

HSV
Acronym for Hue/Saturation/Value. An additive color system based on the attributes of color (hue), percentage of white (saturation), and value (brightness or intensity).

HLUs (see High Level Utilities)

I

interpreter
A program that transforms statements into machine code a statement at a time. The the Python executable is an interpreter of the Python language.

irregular rectangular coordinate space
A 2-dimensional rectangular grid that has unequal spacing along the X and/or Y axes.

J

K

L

label bar
A specialized label consisting of a bar of filled rectangular areas that are labeled to correspond with areas from an adjoining plot. Label bars can be filled with black-and-white patterns, with color, or with both. Label bars are commonly used with contour plots and with other types of plots where area pattern fills or color are used to differentiate values in the plot.

landscape
See portrait

legend
A specialized annotation that formats a series of lines or markers of varying styles along with adjoining explanatory labels. Legends are designed to serve as "keys" for an associated plot.

loop statement
A language construct that allows for code repetition with incremental values set for a variable or variables.

logical operator
A operator that returns a true value or a false value depending on the truth or falsity of its operands.

M

machine-independent data format
Same as Network-transparent data format.

masked arrays
Masked arrays are arrays that may have missing or invalid entries. Module numpy.core.ma provides a nearly work-alike replacement for NumPy that supports data arrays with masks. As of version 1.3.0b1, support is available for numpy masked arrays.

metafile
A file containing encoded graphical elements. Metafiles are used for storing and transporting graphics images. In the context of PyNGL, "metafile" is generally synonymous with "NCGM".

missing value
A special value for a variable or array element indicating that no legal data has been specified for that quantity.

N

named colors
A string containing the name of a color, like "HotPink". Named colors can be used in place of color index values when setting color resources. See the table of named colors for details.

named dimension
A dimension of a variable or file variable that has been assigned a name using the '!' operator.

NCL
The NCAR Command Language, a programming language designed specifically for the analysis and visualization of data.

ncl
Refers to the interpreter that interprets NCL statements.

NCGM
Stands for "The NCAR encoding of the Computer Graphics Metafile ANSI standard," (see metafile)

NDC (Normalized Device Coordinates)
A coordinate system that describes positions on a virtual plotting device. The lower left corner corresponds to (0,0), and the upper right corner corresponds to (1,1). NDC space will be mapped onto the largest square that will fit on an actual plotting device.

netCDF
The Network Common Data Format - a network-transparent and self-describing data format developed at Unidata.

network-transparent data format
A format for encoding data that removes any machine dependencies that might be involved in encoding the data. Typical examples of such data formats are netCDF and HDF.

numeric data type
Any of the Python numeric types (integers, long integers, floating-point numbers, or complex numbers), or any of the many Numerical Python numeric types.

O

object
An instance of a class, formed by assigning specific values to the variables in the class.

output primitive
Procedures and functions for producing graphics output at the lowest level. PyNGL procedures/functions exist for drawing lines (Ngl.polyline, Ngl.polyline_ndc, Ngl.add_polyline), text (Ngl.text, Ngl.text_ndc, Ngl.add_text), filled areas (Ngl.polygon, Ngl.polygon_ndc, Ngl.add_polygon), and markers (Ngl.polymarker, Ngl.polymarker_ndc, Ngl.add_polymarker).

P

PDF
Portable Document Format - a device independent page description format introduced by Adobe Systems Incorporated in the early 1990s.

portrait
(Definition taken from Wikipedia) Portrait mode and landscape mode refer to the orientation of text (and pictures) on a printed page. (The paper must be a rectangle, however in practice square sheets are hardly ever used.) In portrait mode the text is printed on the paper such that the reader will turn the long side of the paper vertical and the short side horizontal. In landscape mode on the other hand, the long side is horizontal, and the short side vertical (like most landscape paintings).

primitive
(see output primitive)

Q

R

relational operator
A operator that returns a true value or false value depending on a relation between its operands.

resource
A name that designates a particular attribute that will affect plotting with PyNGL. Resources can be assigned values. A couple examples of resources are xyLineColors, used for defining line colors in an XY plot, and cnLineLabelsOn, used for controlling whether line labels appear on contour plots. There are hundreds of legal resources.

resource file
A file that can be used to set values for resources.

RGB values
Stands for the red, green, blue color space where colors are specified as triples of floating point numbers between 0.0 and 1.0 inclusive. The number triple gives the intensities for the red, green, and blue components of a color. The RGB value (1,.0.,0.) would indicate red, for example.

S

script
A file containing a sequence of program statements that can be submitted to an interpreter for execution.

self-describing data format
A format for encoding data that can contain information that describes the data being encoded. Typical examples of such data formats are netCDF and HDF.

scalar variable
A singly-dimensioned variable of size one.

shape
The number of dimensions of an array. The statement a = Numeric.zeros([2,3,5]) would create an array a of shape 3 (i.e. a has three dimensions).

size
The number of elements in array dimensions. The statement a = Numeric.zeros([2,3,5]) would create an array a that has a first dimension of size 2, a second dimension of size 3, and a third dimension of size 5.

stipple
To cover an area with small dots.

streamline
The path an idealized particle would follow if introduced into a wind or fluid flow. For example (as an approximation to the ideal), the path a speck of dust would take in a wind.

Streamline plot
A plot representing a vector field using streamlines, based upon 2-dimensional data. It may also contain tick marks and titles.

stride
The increment indicator in a subscript specifier. Using m:n:i as a subscript means to take the individual subscript values starting with m and ending with n in strides of i. The stride must always be an integer. A stride of 2 means to take every second value after the first, a stride of 3 means take every third value, and so forth.

statement
A non-comment line of code (or a line of code prior to any commenting). A line of PyNGL code contains all continuations resulting from use of the "\" symbol.

supported data format
Any of the formats that can be read by a PyNGL module.

T

tick marks
Marks along an axis of a plot that are perpendicular to the axis and serve to divide the axis (or parts of the axis) into equal or logarithmically spaced parts.

text function codes
Special characters embedded in a text string, usually starting and ending with the colon (":") character. Text function codes provide the capability for selecting various fonts, doing superscripts and subscripts, taking complete control of the positioning of characters relative to one another, and last but not least, "zooming" characters in either width or height, or both.

U

V

variable
A symbol that denotes an array, a dimension name, an attribute, and so forth.

Vector plot
A plot representing a vector field by drawing glyphs that represent magnitude and direction at grid points based on 2-dimensional data. It may also contain tick marks, titles, and/or a label bar. Three glyph styles are available: a basic line-drawn arrow, a filled arrow with an option edge, and a standard wind barb.

viewport
The viewport is a rectangular subregion of NDC space that specifies where the graphics will be placed when drawn. The precise meaning of the viewport depends on the type of graphic being drawn. For example, for functions drawing XY plots (like Ngl.xy), the viewport specifies where the grid containing the curves will be placed, and the labeling (if any) will be drawn outside of the viewport.

W

workstation
Used to mean a valid output device such as an X11 Window, a PostScript file, a PDF file, or an NCGM.

wrapper function
A C function that provides an interface between PyNGL and an existing Fortran or C subroutine or function. The wrapper intercepts an PyNGL function or procedure, does the appropriate argument checks and conversions, then calls the existing code.

X

XY plot
A plot containing curves made up of X/Y coordinate pairs. It may also contain tick marks, titles, and/or a legend.

Y

Z