Measurements and Their Relationship to CG - An Introduction to Computer Graphics for Artists - page 53

Fig. 4.5 This carton has been built out of 2.5º sections, with 5º sections highlighted

Most often, angle measurements are of use in architectural and industrial design
subjects. These are more easily ascertained than for characters or organic subjects,
where angular measurements are rarely feasible due to surface complexity. For
non-planar or non-planar 3D subjects, angular measurements may be very diffi cult
to obtain with any degree of accuracy. For extremely complex objects, these measure-
ments are best taken after digitizing the object with a 3D scanner or digitizer.
CG modeling applications do not accept angles as a dimension for objects,
but angles are used during the modeling process. The local axis of a model can be
adjusted by typing in a specifi c angle, angle snaps can be set for radial transforma-
tions, and some editing tools depend on angles for a correct result.

4.2.4 Surface Curvature

Surface curvature is defi ned as the angle of inclination of a tangent at a location on
a surface combined with arc length. In CG, this term is not ordinarily applied to
polygons or planar NURBS surfaces. Instead, it is used when describing surfaces
that have a regular modifi cation of tangent direction and value along the surface of
an object. A car hood, the blade of a propeller, a leaf from a tree, or any form of human
or animal anatomy are all examples of objects that have these types of surfaces.
Most often, surfaces described this way are known as organic .
Estimating surface curvature of an object is very diffi cult to accomplish accurately
without using tools (Ware 2006 ). For objects with a high degree of surface curvature,
a number of techniques are available to describe them. For engineering subjects,