Curve Manipulation

The shape of curves can be changed by two means:

• Editing via data points

• Editing via vertices

The geometry of a curve is determined principally by the positions of its control points. Editing via data points means that the curve is fitted to the data points, a process which alters the control points. However, this relationship is not reciprocal - moving control points directly does not cause the data points to move, so the curve is no longer fitted. The curve can be refitted to the data points, but the changes made by editing the control points will be lost.

To avoid confusion, it is strongly recommended that the user adopt one of the following approaches:

• Manipulate fitted curves via data points only

• Manipulate non-fitted curves via control points only (perhaps using data points as a visual guide)

There are different merits to both approaches, however Lines is strongly geared toward curves that are fitted to data points, so the former approach is recommended. The correspondence between a curve and its associated data points is specified in the Correspondence column of the Curves grid in the Data Bar:

Figure 3:44. Correspondence between curve and data points set in the Curves Grid

Fitted and Fitted(centripetal) fit slightly differently shaped curves through the data points. Which of these gives the fairest result will vary according to the particular arrangement of the data points.

Slight curve colour differences are used to inform the user of the mode last used for editing - curves modified via vertices have a lighter colour than those modified via their data points:

Figure 3:45. Curves modified by data points (left) and control points (right)

All operations that affect the geometry of curves can be undone using EDIT \ UNDO, or <Ctrl+Z>. Operation that have been undone can be redone using EDIT \ REDO, or <Ctrl+Y>. Currently the last 30 edits can be undone.

The curves in Lines are updated using FILE \ UPDATE or . It is good practice to update every time a stage of operations has been completed satisfactorily. Beware that updating does not cause Lines to save any data to disc. To do this it is necessary to return to Lines and save the design in the conventional manner.