Glossary D

data region	(n) The region in a visualization where the actual data is represented. In a graph or plot, the region would be bounded by the scales (axes) where the point or line markers representing the data are drawn.
data visualization	(n or v) The method or end result of transforming numeric and textual information into a graphic format. Visualizations are used to explore large quantities of data holistically in order to understand trends or principles.
datum	(n) A theoretically exact point, axis, or plane used as a reference for tabular dimensioning. A datum marks the origin from which the location and orientation of geometric features are established.
datum feature	(n) A term used in geometric dimensioning and tolerancing to describe a real or theoretical feature selected to ensure the proper orientation or location of other features on a part. Controls define features on the part relative to the datum features.
datum reference frame	(n) A term used in geometric dimensioning and tolerancing to describe the theoretical reference frame used to evaluate the location and orientation of features of a part. The frame consists of three mutually perpendicular (orthogonal) planes, which typically correspond to the principal planes of projection in a multiview drawing or the global coordinate system in a 3-D modeling system. In inspections, a physical surface may be established to simulate one of the reference planes.
declarative information	(n) A term used to describe the current state of an object. This type of information can be thought of as descriptive nouns. The location of a point in a coordinate system (8,3,5) is an example of declarative information.
density slicing	(n) A visualization technique, used with area and volume renderings, in which a particular data value or small range of values is highlighted in contrasting color. This technique can be used either for a single static image or for sequential images highlighting different data values in the image.
dependent variable	(n) A variable dependent on the response of the model and not controlled by the experimenter. Every dependent variable value is paired with one or more independent variables and represents the model’s response to the independent variable values.
depth	(n) One of the three principal dimensions of an object. The depth dimension is described in the right side and top views of a multiview drawing. The depth dimension is often associated with the Z axis in a 3-D modeling system. Depth can also refer to the distance cut into an object using a machining process (e.g., the distance measured between the crest and root of a thread normal to the axis or the depth of a drilled hole).
descriptive geometry	(n) The fundamental basis of the science of projection drawing, attributed to the work of Gaspard Monge (1746–1818). It is the basis of all types of 3-D representations on 2-D media used today.
design analysis	(n) The evaluation of a proposed design during the refinement process, using the criteria established in the ideation phase. The refinement stage is normally the second of three major phases in the design process.
design for manufacturability (DFM)	(n) A technique in which ways are found to simplify a design and reduce manufacturing costs. Using CAD/CAM technology, DFM determines the manufacturability of a design before it is produced.
design intent	(n) A phrase describing the integration into the model of how feature dimensions and geometry relate to each other in order to satisfy the design constraints of the final product. Design intent is usually embodied in the model through the definition of features and their constraints.
design process	(n) The method used to create a solution to an engineering or technical problem. The result is often a product, either one of a kind or mass produced. The design process, though unique to each problem, typically includes standard stages and uses graphics to communicate information between and within these stages.
design visualization	(n) A process using the fundamental capabilities of a human visual system; the ability to perceive 3-D form, color, and pattern. Visualization is an analysis method used in the second stage of the design process, the refinement phase.
detail drawing	(n) A dimensioned multiview drawing of a single part, using ANSI standard conventions to describe the part’s shape, size, material, and finish. Detail drawings are sufficiently descriptive that the part can be manufactured using the drawing as the only communications device. Detail drawings are produced from design sketches or extracted from a 3-D computer model.
development	(n) An alternative description of the surfaces of a 3-D object as a 2-D pattern. The creation of a development is often thought of as an unfolding of the surfaces of an object. A developable surface, then, can be unfolded or unrolled onto a plane without distortion. Single-curved surfaces, such as cylinders and cones, are also developable.
deviation	(n) A tolerancing measure used to describe the difference between the size of a produced part and the basic size. DFM (n) See design for manufacturability.
diameter symbol	(n) A symbol used in ANSI dimensioning that precedes a numerical value, indicating that the value shows the diameter of a circle. The symbol used is the Greek letter phi Φ
difference (-)	(n) The Boolean operation that subtracts, or removes, the intersecting geometry from the specified solid. For example, in the Boolean operation A - B, the intersection of solids A and B is removed from A.
diffuse	(adj.) A term describing a reflection of light that strikes a surface and reflects uniformly in all directions, creating no highlights or hot spots.
digital terrain model (DTM)	(n) A 3-D computer model of a land form generated from survey data. Such models are often used by civil engineers and land planners to explore possible placements of structures or changes to the terrain before actual construction begins.
dihedral angle	(n) The angle between two planes. Finding the true measurement of a dihedral angle requires an edge view of the two planes and is a common application for the use of auxiliary views.
dimension	(n or v) The physical size of an object, or the process of documenting physical sizes. The number of dimensions used to describe the object depends on whether length (1-D), area (2-D), or volume (3-D) is being measured. Dimensioning can also refer to the measurement of size, location, and orientation of particular features on an object.
dimension line	(n) A thin, solid line that shows the extent and direction of a dimension. Dimension lines are paired with the number representing the physical size of the feature, and extension lines are used to associate the dimension to the corresponding feature on the drawing.
dimetric projection	(n) An axonometric projection in which two of the three axes have the same scale and the same angle relative to the third axis. A dimetric drawing is less pleasing to the eye than a trimetric projection but is easier to draw.
direct data exchange	(n) A method of data exchange between CAD systems. Data is directly translated from the format native to one system to the format native to another system.
directrix	(n) A line (typically curved) that describes the path of a straight line (the generatrix). The path of the generatrix along the directrix describes ruled surfaces or solids.
direct view	(n) A descriptive geometry technique (sometimes referred to as the natural method) that places the observer at an infinite distance from the object, with the observer’s line of sight perpendicular to the geometry in question. In third-angle projection, a projection plane is placed between the observer and the object, and the geometry is projected onto the projection plane.
discretization	(v) The method of dividing more complex geometry into simpler geometry. A discretized model can be manipulated using techniques such as finite element analysis, in which all the simple geometric components of a model are evaluated simultaneously.
displacement	(n) The linear or angular distance moved by a part or mechanism. In cam design, a displacement diagram is used to chart the linear location of the follower relative to the angular position of the cam.
display devices	(n) The hardware used to display output text and graphics of computer software, such as CAD. The most common display device is the cathode ray tube, or CRT, used in computer monitors.
dither pattern	(n) A method of simulating varying value (gray) levels within an image when only one color of ink (usually black) is available. The patterns are achieved by grouping square matrices of pixels in an image (e.g., 3 ∞ 3) and coloring a pattern of pixels in the matrix, based on the average value level in the matrix region. Because of the small size of the pixels relative to the size of the image, the pixel patterns give the impression of value changes when viewed at a distance.
document sketches	(n) A more formal type of freehand sketch used during the refinement stage of the design process. These sketches often contain many of the conventions found in more formal drawings.
domain	(n) A term that describes the types of objects that can or cannot be represented by a computer modeler. The larger the domain, the greater the number of objects the CAD system can describe.
donut	(n) See feed-through.
double-curved surface/line	(n) A surface or line that curves in two orthogonal dimensions at the same time. A sphere is an example of a double-curved surface.
double-line drawing	(n) See piping drawing.
drawing exchange format	(n) See DXF.
drill drawing	(n) See fabrication drawing.
drilling	(v) A process that uses a rotating cutting tool to produce holes. It can also refer more specifically to a process that produces holes using twist drill bits.
DTM	(n) See digital terrain model.
DXF	(n) Acronym for drawing exchange format, a nonproprietary CAD data format developed by Autodesk to facilitate the exchange of CAD-based graphic information. It has become a de facto neutral data exchange format for the microcomputer CAD industry.
dynamic analysis	(n) The evaluation of a design as it changes over time. Unlike kinematic analysis, dynamic analysis assumes that forces (e.g., gravity, angular momentum) are both acting on and created by the mechanism being analyzed.