20 Replies Latest reply on Apr 27, 2011 11:36 AM by Robert Stupplebeen

    Open surfaces: Loft versus Boundary question

    Christopher Thompson

      I have a question. When modeling a surface connection (curvature to face), what is the advantage of using a boundary surface instead of a loft to connect the sides of an open surface? From the feature statistics, the boundary surface requires more regeneration time (4X: 0.13s versus 0.03s) to create the surface, and the boundary surface also had a crown that was slightly more prominent than the loft considering both used curvature to face constraints.


      Refering to an example from the SolidWorks Surfacing and Complex Shape Modeling book on page 318 (chapter 17), a lofted surface was used on the open surface edges (figure 17.3) with guide curves to control the unconstrained edges of the loft surface. I suspect a boundary surface could have been used as a substitute for the loft, but what advantage would it offer as the model regeneration time would have been higher?



      When connecting the edges of two open surfaces (with or without guide curves), does a situation exist where it would be more advantageous to use a boundary surface instead of a loft?
        • Re: Open surfaces: Loft versus Boundary question
          Jerry Steiger



          Mark and Matt can do a better job of laying out the differences between Loft and Boundary Surface, but since they haven't responded yet, I'll give you the complete idiot's guide.


          Boundary Surfaces seem to be more consistent to me, but I often find myself trying both out to see which works best. On some of my models you will find them used seemingly at random on very similar shapes. I usually default to Boundary Surfaces first, but often end up switching when I run into problems. Sometimes I get better looking tangencies and continuity of curvature on Lofts when I turn off tangency completely. I haven't run into that kind of behavior with Boundary Surfaces, but I haven't been using them as long.


          I never looked at the Feature Statistics on Lofts and Boundary Curves as I was comparing them, I guess because I was much more worried about whether I could get a model to complete than how long it would take to rebuild. It would be interesting to compare a large number of cases to see if Boundary Surfaces do consistently take longer. Not that it would stop me from using them, but it would be good to know.


          Boundary Surfaces give you more control over the tangencies on all four edges, although that doesn't matter in the context of your question.


          Boundary Surfaces treat all the curves equally, whereas guide curves and loft profiles in Lofts are weighted differently when SW decides where the surface should run. It seems to me that guide curves are followed more closely than the profiles, but I may be mistaken about that. Boundary Surfaces may be more likely to fail when you push them, because they seem to try harder to match the curves and continuity conditions at the corners. Lofts are sloppier at the corners, so they may complete, but not give you what you were hoping for. They may also be more prone to failing later on, say in a shell or thicken operation, because they've gone a bit wacko at those corners.


          Boundary Surfaces are newer and the user interface seems to be a little more well thought out, or at least presented better.


          Remember, not just any idiot can use Lofts and Boundary Surfaces; it takes a very special kind of idiot.

          • Re: Open surfaces: Loft versus Boundary question
            Kevin De Smet

            I think the decision to add boundary, while keeping loft, is kind of sloppy. Maybe back in 2008 when boundary was a surface only feature but now that's it's not anymore it just raises confusion. Maybe have some of the loft specific functionality (guide curves having less influence, centerline) embedded as checkboxes inside of boundary.


            It's like Autodesk Alias, it's got an "Align" and "Align 2008" ... which do I choose!?

            • Re: Open surfaces: Loft versus Boundary question
              Matt Lombard

              Sorry I was slow to respond to your email. I'll respond here as well.


              Boundary in general is preferred. When the book was written, I was still trying to work out the differences between loft and boundary, and pretty much came to the conclusions I now hold in the course of writing the book. I understand that boundary produces a better surface with better tangency and continuity to neighboring surfaces.


              There was no particular reason to use boundary, and I was in the habit of using loft, and it was still early on in the lifetime of the boundary feature, so I used loft. Boundary might have been smoother. Model quality trumps rebuild time every time in my book.


              About eliminating loft, please don't. I remember there are a few loft situations that boundary still doesn't cover, possibly like nested loops, or closed loop lofts, or lofting to a point. I've forgotten. Before you kill off loft, make sure all my legacy models will still work, or like Dwight says kill off boundary, and rename the old boundary as loft.

              • Re: Open surfaces: Loft versus Boundary question
                Christopher Thompson

                Thanks to Jerry and Matt for your insight as well as everyone else who responsed. I agree that loft should be keep and not removed for these reasons:


                • Most SW users are already familiar with loft
                • Boundary requires a longer rebuild than loft (probably not an issue in most cases).
                • Loft may have the capability to create some surfaces that are not possible using a boundary surface, such as those that require the centerline parameter.

                What would be helpful is some case scenarios as a guide for when to choose between boundary or loft, especially for Pro-E users new to SW. As mentioned earlier, perhaps create one feature with the option of using either boundary or loft inside of that feature.


                Another useful application of the loft feature (solid) is to create a loft between two surfaces as a replacement for the thicken feature (page 200 figure 10.13). Not sure if boundary is a suitable replacement for that specific scenario and Matt was correct about the need to loft to a point.


                Perhaps the two features should remain separate after all, but I think clarification on when to choose between using the two features would be helpful. It is a little more obvious when to choose between using a fill feature or a boundary (surface) feature but that is another discussion.

                  • Re: Open surfaces: Loft versus Boundary question

                    I hear all of what you're saying:


                    • Do not take away any current capability
                    • Don't give us worse performance



                    Just for those of you that are not aware of it -


                    • Boundary can do solids
                    • Boundary can close loops
                    • Boundary can use points as a profile


                    • Boundary cannot do a centerline loft
                    • Yes, Boundary is slower with low no.# of curves, faster than Loft with large no.# of curves (10 or more)  (but low # is still the majority of cases)





                  • Re: Open surfaces: Loft versus Boundary question
                    Kieran Choy

                    Found another good reason to keep loft around (I think) - connectors.

                    Correct me if I'm wrong but you can manipulate the connectors in a Loft but not in a Boundary Surface (apart from flipping). See file and image of loft feature below.

                    • Re: Open surfaces: Loft versus Boundary question
                      Robert Stupplebeen

                      I have had no issues with flipping connectors for either Lofts or Boundaries.  I do always make sure to have a closed profile with any guide curves pierced to the other bounding edges.  I routinely have 100+ configurations with all upstream surfaces changing and the down stream swoopy stuff does not fail.  If my model is unstable I probably forgot one of these 2 steps.  I hope this helps.

                      Rob Stupplebeen