12 Replies Latest reply on May 29, 2009 7:31 AM by 1-AM5C9P

    working with splines

      There seems to have been much interest at SWW this year about how to better understand and work with splines. The SWW presentations will be made available later this month, but in the meantime, I put together this video this morning.

      I have to laugh at myself as I introduced it as a "short" video but it ends up being approximately 23 minutes long - there's nothing short about explaining and demoing splines I guess!

      Click Here for the download MP4.
        • working with splines
          Dwight Livingston
          Mark

          I ran your presentation and it looks very good, very helpful. I'm going to see how it applies to some of the spline work I do.

          In my work, I don't often try to match an existing curve, so you'd think it be easier for me. Usually though, I need tangency to construction lines, so that height and width come out to the exact dimensions I want. In my experience, that's where things get squirrelly. When I get time I'll try to do an example and post it here.
          • working with splines
            Matt Lombard
            Ok, I give. Any suggestion for players or codec?
            • working with splines
              Matt Lombard
              Mark,

              Please tell us other relevant information about the term "bicubic spline". Educated users are better users.

              Thanks for doing the video. I think it shows that getting nice splines in SW is a lot of work, even for people who know what they are doing. Even with all of that work, there are still no quantitative tools to tell you how good or how bad a spline is.
                • working with splines
                  Matt,

                  Oh, did I say bi-cubic? I was incorrect and meant to say cubic. Bi-cubic applies to surfaces not splines and means that the surface is cubic in both the U and the V by degree 3 polynomial but do you really want to know that? :-)

                  As far as what I was trying to convey with my spline "cubic" comment is that splines in SW will reshape themselves when spline points are repositioned or there magnitude or vectors are changed. The amount and portion of the spline that changes depends on how many "Knots - away" from the point that you're influencing. A rule-of-thumb is that it is generally three away from the repositioned point.

                  Again, I'm not a mathematician and I only have a rudimentary understanding of them. What's important is to know where behavior and find ways to work with them to get them to the shape we desire. If Andy Fraser is looking in on this post, he could certainly give a much better definition and understand to the subject.
                    • working with splines
                      Matt Lombard
                      Mark,

                      Thanks for the clarification. I'm not a mathematician either, but like a lot of other people here I did go through 4 years of engineering school, and had to take more math classes than I want to remember.

                      Degree 3 polynomial helps visualize the types of shape you can expect to get between knots. These are "piece wise", right? I think that means that there is a new set of parameters between each pair of knots.

                      I think for some people that kind of information is helpful. It helps you predict behavior and it helps align your expectations. It's not just a black box, splines follow real rules that people can understand.
                  • working with splines
                    Matt Wallace
                    I am getting practically no sound, both with Total Video Player and with a codec downloaded for WMP. Any ideas on what I am doing wrong? I have downloaded the video twice, with the same problem.

                    EDIT- working better with WMP, but I have volume all the way up and it is still really soft. Hope my computer doesn't make any sounds on its own while watching!
                    • working with splines
                      Matt Wallace
                      Interesting- you don't even mention the control polygon, let alone use it. It is what I have used the most with my very limited experience with splines. Is this just personal preference or is there an underlying reason to avoid it's use?

                        • working with splines
                          Matt Lombard
                          Control polygon is perfectly fine way of controlling splines. I find it a little more abstract than other methods, so I usually use handles and points, which are more direct. They are really just two different ways of controlling the same stuff.
                        • working with splines
                          Matt Wallace
                          MasterCAM gives you the choice of creating a cubic spline or a NURB spline (or used to, I only do very simple geometry creation in MasterCAM). A cubic spline went through the control points, and a NURB spline used the points at the control polygon vertices to generate the spline (MasterCAM didn't have a polygon, just the points). The advantage of a NURB spline was that it gave you more local control over the spline, i.e., moving a point around had a much smaller effect away from the point than moving a control point on a cubic spline. I used to wonder if SW used cubic splines or NURB splines, but it looks like it is able to toggle between the two, depending on how you want to control the spline. I haven't the interest to dive into to underlying mathematics to see if that is the case or even possible, but it strikes me as a neat trick if that is what it is doing.

                          I was watching the demo thinking that it would be easier to tweak the curve into shape with the control polygon, but it could easily be a case in which you have a good feel for how to use a certain method, and after a bunch of time using a different method you end up almost as good as you were with your original method.
                          • working with splines
                            For the codecs i recommend VLC Player. No messing around with codecs, it's light and free
                            http://www.videolan.org/vlc/