Spline Curve Drawing Technique (Tips n Tricks)

[RobDraw]

As for laying out a free form pool shape it seems it could be about the same amount of work to locate the center point for a bunch of arcs as it would be to layout a grid with some defining points through which the curve passes but you made the pool so I will default to your experience.

 

Think about it. An arc requires one point and a radius to define it. I could have an arc with a length of say 20' that could be laid out from that one point. If that 20' were a spline, I might need 5 or more points with at least two measurements to define each and we added a third as a check.

[lrm]

Think about it. An arc requires one point and a radius to define it. I could have an arc with a length of say 20' that could be laid out from that one point. If that 20' were a spline, I might need 5 or more points with at least two measurements to define each and we added a third as a check.

 

True, but the shape you are trying to reproduce may require several arcs thus closely matching the number of spline points needed. In addition, the center of some of the arcs you may need could be off the property boundary or obstructed by an adjacent building making the use of arcs problematic. Your turn!

[RobDraw]

We had ways of dealing with the exceptions like center points being inaccessible. Let's leave those instances out for the purposes of this discussion.

 

Most of these pools had a number of arcs. They were dimensioned using a series of center points. These points were numbered using a strategy that made it easy to layout these pools. We would start with a "center" line that was usually staked out by triangulating from two corners of the house. The center point of the first arc would be located from these two points. The next using the center point of the first arc and one of the endpoints of the "center" line. The next would usually be from the two centers. That's two measurements for each arc. When we needed to use the alternative strategy, the points defining the shape of the pool were all located using the endpoints of the line and a perpendicular for verification. Depending on how intricate the shape was, we had points every few feet. That would mean the person laying out the pool would have to walk the entire length of the pool multiple times to get what one arc center got them. The number of points using that method was many times the number using arcs and the arcs were much easier to locate. A huge difference in time in the field. CADwise not a big difference in time, but it was tedious work. The other method was actually fun to do. I miss that job.

[Dadgad]

Yes, back in the day splines were free form. Google "drafting spline weights" then click "images" and you will see thin wood and steel splines that were used to draft smooth curves such as this:

As for which guitar shape is better that is a matter of taste. I know I am biased here and you all have been so kind but I feel the spline-based shape I show in post #13 is sweeter than the arc/line-based shape of post #16 (no offense intended). Examine each shape for discontinuities in smoothness. All taste is taste so I know I may be out voted.

 

BTW, is there any interest in a layman's introduction to spline technology? There seems to be a hesitancy to use splines. The fundamental concepts behind Bezier splines is very cool and elegant. Bezier splines is the foundation for B-Splines and NURBS curves and surfaces. My experience with splines goes back to the early 70's when I wrote a program to convert a smooth curve to a series of arcs and lines. At that time I needed to define a smooth curve for a program I was writing that created a smooth curve that could be rolled along a given curve to yield a desired shape. I didn't know about Bezier, B-splines or NURBS so I used arcs and lines. Perhaps that experience has soured me on them. I later wound up working for a CAD company and met the guy who invented B-splines. The development of curves in CAD from arcs to NURBS is an interesting history.

 

LRM, I like those spline weights, I had never seen those before, little whales, nice.

I agree with you, your spline was much sweeter than my totally free hand non-traced sketch effort.

In light of your post, I went back and googled up an image for an actual attempt at a reasonable trace, and I uploaded that new polyline version to the earlier thread.

Not bad, but likely not as good as a state of the art spline.

 

I knew that you had somewhat more than a passing knowledge of spline technology.

I think it would be great if you created the layman's Spline guide, as you suggested, and posted it in the Tips & Tricks forum, or on the Tutorials Tab, as I would enjoy reading it, and learning more, and I am sure there are plenty of others who would be of like mind.

As clever as they are, I think most people who don't feel they really need them, probably tend to avoid them, and that they are a great tool which is under utilized.

Edited May 26, 2016 by Dadgad