19 Replies Latest reply on Sep 9, 2012 5:35 PM by Pet Peever

    Curvature Comb Kink

    Kevin De Smet

      I have noticed now for quite some time, that there are sometimes kinks in the curvature comb of a spline with more than 4 points, when it creates a span. This is noticeable in the curvature comb plot, but, is it of much concern?

       

      I haven't made much real products yet, so I'm wondering. Now to a purist, the comb with the span looks bad. But would you ever really see this? Unless it's high end auto or aero, for which Solidworks does not market itself anyway. Would you notice this on, say, a consumer product design?

       

      I personally don't think so... but I would like to get opinions on this

       

      The spline on the left has 5 spline points, and the spline on the right  has 4.

        • Re: Curvature Comb Kink
          Roland Schwarz

          This is OK. As long as the curvature comb doesn't "leap" from one value to another, like a vertical step, you have C2 continuity.

           

          C2 continuity is usually important for visual reasons.  Breaks in C2 continuity are also breaks in reflectivity, which can show up as creases in reflective parts even if the surface is C1 ontinuous (tangent continuous) at that place.

          • Re: Curvature Comb Kink

            Kevin,

             

            What you are seeing is, by definition, C2 - which is to say that equal curvature is met. What I think you're wanting is C3 or better and you can achieve that by pulling the tangency magnitude handle at that spline's interim point to make it C3.

             

            Also, when using Control Polygon method of of editing, SW turns off curve re-parameterization - meaning that as you move the interim points closer to each other you can create knots (in point mode editing - re-parameterization is automatic)  If you "Relax Spline" you reparameterize the spline.

             

            Regards

             

            Mark

              • Re: Curvature Comb Kink
                Kevin De Smet

                Oh wow, by tweaking the magnitude handles you can get it C3, that's quite impressive! Relax spline being used for knot removal is also very interesting!

                 

                Solidworks uses splines of degree 3, am I correct?

                I thought this was only possible with degree 4, 5 or higher?

                 

                What do the magnitudes do, precisly?

                Do they alter the weighting (default = 1) of the points?

                 

                But I would agree - this is rediculously perfectionistic here and, in your opinion, does C2 do the job for like, 95% of the time?

                  • Re: Curvature Comb Kink
                    Roland Schwarz
                    C2 does the job for even the most discriminating industrial designers.  It might be an issue in aero- or hydrodynamic situations.
                    • Re: Curvature Comb Kink

                      Hi Kevin,

                       

                      Cut and Paste from my SWW presentation:

                       

                      • What kind of splines are used in SW?  - SW curves and surfaces are B-splines
                      • What degree are SW splines? - 3rd degree cubic B-splines, Fill Feature uses 5th degree quintic
                      • What is the accuracy of SW surfaces? - Generally for surface features 1.0e-5(m) or .01mm
                        Surface fill – 1.0e-6(m) or .001mm
                      • What is the maximum allowed distance between surfaces edges to be knitted? <2010 – 1.0e-3(m) or 1mm 2010> 1.0e-4(m) or .1mm
                        • Re: Curvature Comb Kink
                          Chris Kamery

                          Just out of curiosity, what was the reason behind the change from 1mm to .1mm in the max distance for knitting?

                           

                          Personally I don't think I have ever tried to knit with a gap anywhere near 1mm, just interested in the reason.

                            • Re: Curvature Comb Kink

                              Chris,

                               

                              Precisely! Prior to 2010 the  maximum allowable distance was 1mm and was causing all kinds of problems for users knitting things that were not intended to knit. The knit algorithm was also replaced/enhanced to make it more accurate and squash a number of other issues we had with knit. In 2010, the user gets to decide with what precision to close gaps - so I think it's way you'll want.

                               

                              Regards

                               

                              Mark

                      • Re: Curvature Comb Kink
                        Roland Schwarz

                        A good experiment to do would be as follows:

                         

                        Extrude shapes with various continuities: C0 (sharp corner), C1 (tangency, ie. fillet), C2 (curvature continuous), C3.  Turn on the zebra stripes and rotate the model.  Watch what happens to the stripes over various continuity conditions.

                        • Re: Curvature Comb Kink
                          Jerry Steiger

                          Kevin,

                           

                          It's all in the eye of the beholder. As others pointed out, a fair number of people will notice when edges or surfaces aren't C2 continuous. Not very many will pick up on a lack of C3 continuity like your one example. If you were to turn off the curvature combs, I might be able to tell which was which. I have no doubt that there are people who would notice immediately. The question is, are you trying to sell to those people? Perhaps an even more important question, is who decides whether it is good enough? Managers love to decide on appearance questions, cheerfully overriding the judgement of the industrial designers they pay for their expertise.

                           

                          Jerry Steiger