Creating Curves and Surface Primitives

Curves and surface primitives are created by inputting a certain number of control points. The number of points that define a curve or a surface primitive is determined by the type and, in some cases, by the method of creation. A curve is constructed by inputting the control points in sequence. When the specified number of control points are entered, the curve is completed automatically, and the computer is ready for creating the next curve. However, the number of control points are not specified in case of parametric curves such as spline or Bezier curves. In such cases, the curve is completed when termination of the input is informed by entering key (Windows : key) or clicking the last point twice. To create a type of curve or surface primitive, the corresponding tool button should be pressed. As long as the button is pressed, inputting of the control points can proceed without interruption. For example, a view angle or rendering mode can be changed without terminating the input procedure.