The schemata of this type of spatial
subdivision include meshes and binary space partition trees (BSP trees).
The elements of the meshes, which define the boundaries of the object, are
polyhedra. BSP trees work by recursively subdividing space. Each
interior node separates space into two disjointed point sets. The leaves
of the tree are labelled 'in' or 'out'. The union of all the 'in' cells
model the object (Hoffman & Rossignac 1996, 5).
These schemata work either by grids
or octrees. Grid schemata subdivide space in conformance with a grid
system. The elements of the grid are typically hexahedra. Octree
schemata recursively divide a cube into eight further cubes. Each cube is
either black (if it is inside the object), white (outside the object), or
grey (on the boundary). Grey cubes are sub-divided. (Hoffman & Rossignac
1996, 5).
Computer-Aided Design.
The use of computers to help in design
work, for example by architects. The emphasis is on visual information.
The term is also used by some people to refer to Computer-Aided Drafting,
and is related to CADD - Computer-Aided Drafting and Design.
Constructive Solid Geometry.
A solid modeller which deals with an
object by treating it as the product of a series of boolean operations
(such as unions and subtractions) performed on a range of primitives (such
as spheres and boxes).
Graphical User Interface.
The human eye is better able to work
with a visual representation of an object than with a string of
co-ordinates (or similar method), and so GUI are used.
Computer Graphics Animation Language.
A surface modeller developed by Peter Comninos at Teesside Polytechnic,
primarily for graphical animation. It was run by Chapman
(1990) on an
Apollo Domain 4000 workstation to produce a model of the Hoffman limekiln.
Entity Relationship model.
The ER model describes data as entities, attributes and relationships.
That is to say, 'things', how they can be described in themselves, and
how they relate to one another.
Arithmetic carried out with numbers which are exact, rather than
approximations, as is the case with floating-point numbers. The results
are precise, rather than approximations.
Calculations involving floating point arithmetic. In addition to the
inaccuracies involved in storing floating point numbers, calculations
with them are also problematic when two mantissas with very different
exponents are used. The results of the arithetic are not exact.
These are used when the number is expected to be very large or
small. The
number is represented as a mantissa (or 'integer') plus an exponent (or
'characteristic'). For instance, 4,900,000 may be written as 0.49 x 10
7, where 0.49 is the mantissa nd 7 is the
value of the exponent. Problems arise when one attempts to store a number
such as 4,900,001, as the number is still recorded as 0.49 x 10
7 .
Geographic Information Systems.
These are designed to store, analyse and
display spatial data which exist along a single surface. They are used
for working with map data.
Non-uniform rational B-splines.
The representation of any curve requires
an algorithmic expression. Some algorithms allow a curve to be expressed
precisely, so that any point along it can be known. Other algorithms
provide ways in which the curve can be mathematically approximated, and
these have been the commonest in use; the approximation becomes greater as
the curve becomes more complicated. NURBS are of the former type, and so
are more accurate in their recording of the position of the curve. (A
spline is originally a flexible ruler used in draughting to produce curved
lines).
Used in solid modelling as a way of representing a solid. The space is
decomposed into cells. Each cell has a simple topological structure, and
also often a simple geometric structure. There are two forms of spatial
subdivision: boundary conforming and boundary approximating. (Hoffman
& Rossignac 1996, 5)
Computations involving vectors. Vectors are mathematical entities which
possess both value and direction, as opposed to scalars, which possess
simply value.