July 1993

pp. 51-64

**Sidebar: Glossary
**The terms in this glossary are defined in the context of volume graphics.
Their general and formal definition is not necessarily reflected in the
definition given here. Words italicized in the definitions are defined
under their own entries in this glossary.

**adjacency:** a relation between two voxels
which are said to be 6-adjacent if they share a face, 18-adjacent if they
share a face or an edge, and 26-adjacent if they share a face, an edge,
or a vertex.

**connectivity:** a relation between two
voxels which are said to be 6-connected (18-connected, 26-connected) if
there exists a 6-path (18-path, 26-path) between them.

**hybrid (intermixed) voxel
model:** a volumetric dataset in which a synthetic voxelized model is
intermixed with sampled or computed data.

**neighborhood: **
the 6-neighborhood (18-neighborhood,
26-neighborhood) of a voxel is the set of all voxels that are 6-adjacent
(18-adjacent, 26-adjacent) to it.

**path:** a sequence of voxels is a 6-path (18-path,
26-path) if every two consecutive voxels along the sequence are 6-adjacent
(18-adjacent, 26-adjacent).

**raster graphics:** a subfield of computer
graphics that represents the scene by a 2D array (raster) of pixels stored
in a 2D frame-buffer.

**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 opaque value.

**ray tracing (3D raster ray tracing, discrete
ray tracing, volumetric ray tracing):** a volume viewing algorithm in
which light behavior is simulated by recursively tracing individual imaginary
rays of light as discrete lines through the scene represented in a volume
buffer (see Discrete Ray Tracing).

**surface rendering:** an indirect
technique used for visualizing volume primitives by first converting them
into an intermediate surface representation and then rendering them to
the screen by employing conventional computer graphics techniques (cf.
volume rendering).

**three-dimensional
discrete topology:** the formal topological characterizations of the
digital voxel space that conform as closely as possible to the corresponding
characterizations used in the topology of the continuous three-dimensional
space.

**tunnel:** a 3Dpath of certain connectivity
that penetrates a discrete surface. Surface thickness is defined by the
absence of particular tunnels in it.

**vector graphics:** a subfield of computer
graphics that represents the scene by a set of lines (vectors) that are
repeatedly redrawn to the screen by a random vector generator.

**visualization:** a method of extracting
meaningful information from complex datasets through the use of interactive
graphics and imaging.

**volume
buffer (3D raster, cubic frame buffer):** a 3D array of voxels used for
storing a regular volumetric dataset.

**volume modeling:**The synthesis, analysis,
and manipulation of sampled, computed, and synthetic objects contained
within a volumetric dataset.

**volume rendering:** a direct technique
for visualizing volume primitives without any intermediate conversion of
the volumetric dataset to surface representation (cf. surface rendering).

**volume visualization:**
a visualization
method concerned with the representation, manipulation, and rendering of
volumetric data.

**volumetric dataset:** the aggregate
of voxels tessellating the volume.

**volume graphics:** the subfield of
computer graphics that employs a volume buffer for scene representation
and is concerned with synthesizing, manipulating, and rendering such scenes.
Volume graphics is the 3D counterpart of raster graphics.

**voxblt** (**vox**el **bl**ock
**t**ransfer):
A set of operations by which a rectangular sub-volume of voxels (called
a "room") can be copied within the volume buffer with arbitrary
write modes and maskings. Voxblt is the 3D counterpart of the 2D bitblt
(**bit bl**ock **t**ransfer).

**voxel:** an abbreviation for 'volume element'
or 'volume cell.' 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 a real object or phenomenon. The voxel is the
3D conceptual counterpart of the 2D pixel.

**voxel space:** a 3D integer grid or lattice
of voxels in which the volumetric dataset (the object) resides.

**voxelization (volume synthesis,
3D scan-conversion):**
the process of converting a geometric representation of a synthetic model
into a set of voxels that "best" represents that synthetic model
within the discrete *voxel space* (see
Fundamentals of Voxelization sidebar
).