Power Tip

From CADD Master Ray

Digitizing Curves
 
Recently, while on top secret assignment for ARP (formerly GSI), I had the opportunity to digitize a number of free-form curves (more later, as it becomes declassified). This led to the inevitable question: Beziers or Splines?

While there are general arguments to be made for either, the question of digitizing would seem to favor Spline (CV) curves, as they flow freely through points, which can be relatively widely spaced. It's harder to hit a known curve with Beziers, even though they are easier to edit. It is difficult to judge whether the parts between the endpoints in fact follow the curve.

The final factor, however, was that these particular digitized drawings needed to be imported into Generic 3D, and Splines don't import. So, a digitizing method needed to be developed which didn't use tones of tiny Beziers to follow the curve.

The following, then, is the method that I worked out, which makes the digitizing of Beziers easy.

First, digitize a number of Standard Points along the curve that you want to trace. Put more where the curve is tighter or more gnarly.

Next, place individual Beziers (BW) at the approximate locations of the control points by eye. Don't worry if you miss initially, just remember that a Bezier can have only two significant curvatures. You'll get the hang of this pretty fast.

Finally, tweak the Beziers into place using Move Point (MP) and Bezier Edit (BE), using the Standard Points as markers. This solves the difficulty in visualizing the curve, since you how have targets on the screen.

When you are done, the Standard Points can easily be erased by filtering them for layer, color, and entity type in an Erase (ER) command.