*Provided by Gitta Domik, University of Paderborn, Germany*

**animation**

A movie. A sequence of related images viewed in rapid succession to see and experience the
apparent movement of objects. [KEL93]

**back-to-front**

A volume viewing algorithm in which the traversal for viewing is performed from the
farthest voxel backward to the closest one. Voxels nearer to the viewer overwrite voxels
that are farther to the back. [KAU91]

**brightness**

The apparent intensity of light. Often a synonym for intensity [KEL93]

**contour plots**

A technique for plotting scalar data of the form f(x,y) by constructing closed (level)
curves of equal values of f. [WOL93]

**effectiveness**

An effective graph presents all information clearly in view of visualization aims. [MAC86]

**expressiveness**

An expressive graph encodes all relevant information and only that information. [MAC86]

**front-to-back**

A volume viewing algorithm in which the traversal for viewing is performed from the voxel
closest to the viewer to the farthest one. Voxels are written only to pixels that are not
painted yet. [KAU91]

**glyph**

An object or symbol for representing data values. Glyphs are generally a way of
representing many data values and are sometimes called icons. A common glyph is the arrow,
often chosen to represent vector fields. The arrow depicts both speed and direction at a
point. [KEL93]

**Grand Tour**

The grand tour is a method for viewing multivariate statistical data via orthogonal
projections onto a sequence of two-dimensional subspaces. The sequence of subspaces is
chosen so that it is dense in the set of all two-dimensional subspaces. Desirable
properties of such sequences of subspaces are considered, and several specific types of
sequences are tested for rapidity of becoming dense. Tabulations are provided of the
minimum length of a grand tour sequence necessary to achieve various degrees of denseness
in dimensions up to 20. [ASI85]

**HLS (hue,lightness,saturation)**

This color model is defined in the double-cone subset of a cylindrical space. Hue (H) is
measured by the angle around the vertical axis, with red at 0 degree, green at 120 degree
and so on. The height of the cone represents lightness (L) in a range from 0 (black) at
the apex of the first cone to 1.0 (white) at the apex of the second one. Saturation (S) is
a ratio ranging from 0 on the center line (V axis) to 1 on the triangular sides of the
cones.

**HSV (hue,saturation,value)**

This colour model is user-oriented, being based on the intuitive appeal of the artist's
tint, shade and tone. The coordinate system is cylindrical and the HSV model is defined as
a cone within this cylinder. Hue (H) is measured by the angle around the vertical axis,
with red at 0 degree, green at 120 degree and so on. The height of the cone represents
value (V) in a range from 0 (black) at the apex to 1.0 (white) at the base of the cone.
Saturation (S) is a ratio ranging from 0 on the center line (V axis) to 1 on the
triangular sides of the cone.

**icons**

See glyphs.

**intensity (of color)**

The amount of measured light energy. Often a synonym for brightness. [KEL93]

**interactive**

Describes behavior of the computer and program designed to respond to the user's request
in a timely manner, generally a few seconds or milliseconds. [KEL93]

**interpolation**

The process of computing new intermediate data values between existing data values.
[KAU91]

**interpretation aims**

UserŐs goals when interpreting picture, such as identifying objects, comparing values of
objects, distinguishing objects, focusing on certain details in text

**isosurface**

Surfaces within a volume that have the same parameter value. [WOL93]

**Marching Cubes**

A method of visualizing 3-D data structures by looking for level surfaces in a 3-space
comprised of a lattice of points. In contrast to volume rendering, where one can see the
entire structure, marching cubes only allows a single surface to be rendered. [WOL93]

**nominal data types**

are unordered collections of symbolic names without units. For instance, the names of the
orbiters, such as Hubble, Magellan, Mariner, Viking and Voyager form a nominal data set.

**opacity**

A material property that prevents light from passing through the object. [KAU91]

**ordinal data types**

are rank-ordered only, where the ordering does not reflect the magnitudes of the
differences. A typical example of an ordinal data set is the sequence of names of the
calendar months, January through December.

**pixel**

Equivalently, picture element, a pixel is the smallest unit of a computer image and is
assigned a unique color after rendering. [WOL93]

**quantitative data types**

are usually expressed as REAL values in the data set. The precise numerical value has a
certain importance in the semantics of the data. have concrete values like reals . A
typically quantitative data set is the length of objects.

**radiosity**

In Computer Graphics, the rate at which light energy leaves a surface, which includes
transmission and reflection. Rendering techniques which compute the radiosity of all
surfaces in a scene have been termed radiosity methods. [WOL93]

**ray-casting**

A volume viewing algorithm in which sight rays are cast from the viewing plane through the
volume. The tracing of the ray stops when the visible voxels are determined by
accumulating or encountering an opaque value. [KAU91]

**ray-tracing **

The general technique of computing an image by projecting rays into a scene and using
their interactions with the contents of that scene to determine pixel colors. In surface-
rendering methods, rays are intersected and possibly reflected or refracted by objects in
the scene to determine visible colors. Ray-tracing is also used in volume visualization
and is a type of DVR. [WOL93]

**render**

The process of converting the polygonal or data specification of an image to the image
itself, including color and opacity information. [WOL93]

**renderer**

A software algorithm which renders an image, calculating a color at each pixel based on
object visibility and lighting and shading models. [WOL93]

**RGB**

Red-green-blue, the color standard employed by the most computer manufacturers and which
roughly corresponds, in frequency, to the three bands of colors sensitivity of the human
eye. [WOL93]

**scalar data types**

possess a magnitude, but not directional information other than a sign; they are simply
defined as single numbers. Same as quantitative data types in this text.

**surface modeling**

Techniques and tools for building up computer representations of objects by modeling their
surfaces, usually as a collection of polygonal facets. [WOL93]

**surface rendering**

an indirect technique used for visualizing volume primitives by first converting them into
an intermediate surface representation (see surface reconstruction) and then using
conventional computer graphics techniques to render them. [KAU91]

**surface reconstruction**

A procedure that converts a set of data points or cross sections into a surface
representation by identifying the surface and representing it with geometric surface
primitives. The reconstruction procedure may use one of several techniques, such as
contouring, tiling, marching cubes, surface detection, and surface tracking. [KAU91]

**thresholding**

A technique used primarily with surface rendering, in which a density value of the
interface between two materials in the dataset is selected so that the interface surface
can be identified for rendering. [KAU91]

**translucency**

describes the property that allows light to partially pass through and partially reflect.
Translucency has the effect of making the translucent area appear smoky or cloudlike, thus
revealing objects behind. [KEL93]

**transparency**

A material property that allows light to pass through the object. [KAU91]

**vectors**

have direction and magnitude. Quantitatively, their mathematical presentation requires a
number of scalar components equal to the dimensionality of the coordinate system. In
general, a vector is a unified entity, which implies the problem of displaying
independent, multivariate scalar fields.

**visualization idiom**

is a specific sequence of data enrichment and enhancement transformations, visualization
mappings and rendering transformations that produce an abstract display of a scientific
data set [HAB90]

**volume rendering**

Volume rendering is a direct technique for visualizing volume primitives without any
intermediate conversion of the volumetric dataset to surface representation. [KAU91]

**volume viewing**

The process of projecting the volumetric dataset onto the image space by determining which
voxels are visible and what their contribution to the final image is. [KAU91]

**volume visualization**

Volume visualization is a visualization method concerned with the representation,
manipulation, and rendering of volumetric data. [KAU91]

**volumetric dataset**

A volumetric dataset is represented as a 3D discrete regular grid of volume elements
(voxels) and is commonly stored in a volume buffer (or cubic frame buffer, like frame
buffer in 2D), which is a large 3D array of voxels. [KAU91]

**volumetric graphics**

Volume graphics is the subfield of computer graphics concerned with volume synthesis,
volume modeling and volume visualization, typically using a cubic frame buffer to store
the volumetric dataset. Volumetric graphics is the 3D conceptual counterpart of raster
graphics. [KAU91]

**voxel**

An abbreviation for "volume element" or "volume cell." It is the 3D
conceptual counterpart of the 2D pixel. Each voxel is a quantum unit of volume and has a
numeric value (or values) associated with it that represents some measurable properties or
independent variables of the real objects or phenomena. [KAU91]