Modeling 1: Polygons - An Introduction to Computer Graphics for Artists - page 69

Fig. 6.2 The curve in ( 1 ) is revolved in the Y axis to make surface ( 2 ), and in the X axis to make
surface ( 3 )

method. Objects with curvature require careful control of vertex distribution along
curves. This is very diffi cult to do manually, even with measurements.

6.2.1.3 Spline Controls

Splines are the same as curves, but they are a special type of curve that allows
adjustment by manipulating tangents and control points . A tangent affects the
direction of a curve at a control point, and the weight of the tangent relative to the
direction. To illustrate this, imagine an arc made of iron wire on a table. Then
picture a weak magnet placed near the wire. The wire would move towards the
magnet and distort the shape of the arc. This is what a tangent control can do.
With spline objects, a curve is defi ned and then used as the basis for the shape of
some polygonal geometry (Fig. 6.2 ). Numerous tools work with these to defi ne how
the spline will be used. For example, to make an arch, two arcs can be made fi rst, and
then bridged with polygons. Because the arcs are defi ned by curves, the vertex spac-
ing of the bridged polygons will be even across the curved boundaries of the arch.
Splines are a good way to build geometry that must follow a specifi c curved
shape, but to build a complex assembly of curved parts out of splines can be time-
consuming. Splines are used as needed, but not gratuitously because they can take
time to build correctly. Spline-based polygon modeling is related to NURBS model-
ing, a type of curve-based modeling discussed in Chap. 14 .
Tip: When working with curves do not expect to be able to click your way to a
perfect curve. Like polygonal editing, curve editing takes some getting used to before
you can do it well. Even if you get the shape right, you still have to learn how to break
it down properly for building either polygonal or NURBS geometry from it.