The General Cylinder Function

A general cylinder is a special type of surface defined in the following way. Let C be a curve in one of the co-ordinate planes XY, XZ or YZ. Furthermore, let L be a straight line perpendicular to this coordinate plane. The surface generated when the line L is moving along the curve C is called a general cylinder. The figure below shows a general cylinder where the plane is XZ.

Figure 2:37. A general cylinder where the plane is XZ.

The curve C is said to be the directrix of the general cylinder and the line L is said to be the generator. The generator L can be restricted to its length by specifying two co-ordinates on the axis parallel to L defining the lower and upper boundaries of the generator. The directrix curve C is a spline curve defined by a number of points. The spline curve can be controlled by tangent angles in each end point.

The necessary parameters for defining a general cylinder are given in a form, except for the directrix points which can be picked directly on the screen as well. Defining a general cylinder is done in three steps:

1. Selecting the generator direction along the x, y or z axis

2. Defining the directrix, either by keying in the exact co-ordinates of the points, or by pointing in an appropriate view

3. Setting general cylinder data.