9 Replies Latest reply on Sep 8, 2011 2:33 PM by Robert Jost

    Spline Control Polygon

    Robert Jost

      Hi Everyone,

       

      I’m currently doing some research on spline control and wanted to ask a few questions.

       

      When working with splines, how many of you use the control polygon?  Currently, we can drag and reposition the nodes of the polygon in order to adjust the curvature of a spline. 

       

      1. If you use the control polygon, why?  Do you use it for simple, subtle adjustments, or as your primary tool to shape the spline?
      2. If you do not use the control polygon when working with splines, why not?
      3. If you haven’t already noticed, the control polygon does not adhere to standard sketch behavior (constraints, display in drawings etc).  Therefore, is there anything the control polygon cannot do that you wish it would do?

       

      All feedback is welcome.

       

      Thanks,

      Rob Jost

      SolidWorks Product Definition

        • Re: Spline Control Polygon
          Douglas French

          Quick reply:  I use it as my primary spline shaping tool.  I find it much easier to get the smooth shape that I want.  I love it!  I'll think about how I specifically utilize it and reply with a more detailed response ASAP.

          • Re: Spline Control Polygon
            Charles Culp

            When I am designing, it is typically to match a curve I already have in mind, or it is overlayed on a sketch picture. Thus, I will be trying to make my spline follow an existing curve. For this, I will lay out spline points on top of the curve, and then use handles to make the curve follow the shape I need.

             

            If I were to use the control polygon, it would move the control points. That does not work for me, because the control points are already placed where I need them to be.

             

            Also, I learned spline control years ago from Illustrator and Freehand, which use the control point and handle method. I'm sure if I was trained early with polygons, I might see things differently. But I don't.

            • Re: Spline Control Polygon
              Jerry Steiger

              Robert,

               

              Sorry to be so slow to reply to you, but I haven't been able to spend any time on the forum for the last couple of months and don't have much time now to catch up. I'm also not doing any design work right now that involves splines.

               

              I typically use the control polygon when I have trouble adjusting the spline using the spline points. It's also easier to use than setting the rather arbitrary tangency numbers. (As I recall, the tangency numbers are somehow scaled to the size of the spline. If I make the spline twice as big, then the tangency number needs to twice as large to get the shapes similar.)

               

              I use two point splines whenever I can. I usually end up using the control polygon to control the shape after setting the tangency or curvature continuity restraints.

               

              Jerry Steiger

                • Re: Spline Control Polygon
                  Robert Jost

                  Hi Jerry,

                   

                  No worries on the timing, I’m always looking for feedback. 

                   

                  Do you ever wish the polygon had more functionality?  Currently it is just a manipulator. 

                   

                  Thanks to all by the way, the responses so far have been valuable. 

                   

                  Rob Jost

                  SolidWorks Product Definition

                    • Re: Spline Control Polygon
                      Jerry Steiger

                      Rob,

                       

                      I just spent a little time playing with some splines to refresh my understanding of how I use the control polygon. It seems that the polygon isn't as useful as I remembered. I almost always aim for curvature continuity on both ends of my splines. Once I enforce that condition, the control polygon is locked for two point and three point splines. But I can still manipulate the spline shape by changing the strength of the tangency conditions, which causes the control polygon points to move. On the three point spline, I can still move the mid point, which also changes the control polygon. It seems "wrong" that I can't manipulate the control polygon when it is in reality still flexible.

                       

                      It does seem reasonable that if I am going to use the control polygon to control the shape of a spline that I could dimension the control points to lock it down. I would probably prefer to work that way, as I find the "value" of the tangency factor to be not at all intuitive.

                       

                      It has also seems very odd to me that splines will show that they are fully defined when the strength of the tangency is not set.

                       

                      Jerry Steiger

                        • Re: Spline Control Polygon
                          Robert Jost

                          Thanks Jerry, I appreciate you looking a little further into this.  I have also noted some of the inconsistencies you mentioned as well. 

                          It does seem reasonable that if I am going to use the control polygon to control the shape of a spline that I could dimension the control points to lock it down. I would probably prefer to work that way, as I find the "value" of the tangency factor to be not at all intuitive.

                           

                          Great feedback, and I have heard this before.  The polygon can also be used to adjust tangency weight, and degree of curvature, by adjusting the length and position of the "sides" relative to each other.  Therefore, ability to constrain the nodes and sides relative to each other can perhaps introduce even more value than just locking down the spline.  Food for thought.

                           

                          Thanks again,

                          Rob

                    • Re: Spline Control Polygon
                      Stephen Blizzard

                      Hi Robert,

                       

                      I regularly use the polygon to control my splines and I find it to be the fastest method of getting the spline shape where I need it to be.  I use splines as original contruction for lofted surfaces and also to trace on an existing sketch.

                       

                      I roughly put the spline points where I need them and then use the polygon to get the spline just right and sometimes if needed I turn on the combs and use the polygon to make the curves uniform according to the combs.

                       

                      I don't really have a need to constrain them, I usually constrain the handles at each end.

                       

                      Stephen Blizzard.

                        • Re: Spline Control Polygon
                          Robert Jost

                          Thanks Stephen,

                           

                          If you had the ability to constrain the nodes, do you think that might give you better control over the curvature combs?  Or, are you satisfied with just eyeballing it?

                           

                          Thanks,

                          Rob

                            • Re: Spline Control Polygon
                              Stephen Blizzard

                              Hi Rob,

                               

                              I think the "fix" constraint could be helpful to isolate part of a spline while working on another part, but I can see how too many contstraints on a spline could be a problem given the relationships they share.

                               

                              Yes I think having the ability to constrain the nodes would make controlling the combs easier, partly because it would be an option to use this so if it's not helpful on certain splines we could just not constrain them.

                               

                              Regards,

                               

                              Stephen.