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Request for lofting tutorial/primer

julian_lelandjulian_leland Member, OS Professional, Mentor Posts: 59 PRO
As I've been playing around with the loft tool, something that I've realized is that I have no idea how lofting works (under the hood, at least). From the recent profusion of forum posts, it looks like there are a lot of other folks in the community who are in the same boat.

OS guys, would you consider putting together some kind of documentation - whether a static document on the help pages, a video tutorial, or a webinar - that not only goes over best practices for the loft tool, but also gives us at least a little insight into what's going on behind the scenes in the lofting algorithm? This type of info would be really helpful for "black magic" tools like the loft where getting the tool to output the forms you want is more art than science (unless you're a dev/understand what steps the loft algorithm goes through) - maybe it could be presented as a "To learn more..." link in the help documentation long-term?

(Finally, if anyone has any suggestions for resources like this that are already out there, please send them on! I looked, but not very hard - this probably already exists someplace, although I would still be interested to hear how OS specifically is dealing with this.)


  • pete_yodispete_yodis OS Professional, Mentor Posts: 666 ✭✭✭
    @Julian Leland If you are new to lofts, this may help (I hope the URL works for you...)  I haven't played too much with lofts in Onshape.


  • andrew_troupandrew_troup Member, Mentor Posts: 1,584 ✭✭✭✭✭
    One name which springs immediately to mind is Ed Eaton. He had a knack for explaining the "under the hood" concepts of the basic shape creation tools which was almost unparalleled.
    So you could try a Google search "Ed Eaton" tutorial (possibly include "Di Monte". too)

    For more up-to-date and detailed info on specifics, particularly for Solidworks (which inevitably shares some DNA with Onshape, and is broadly representative of midrange parametric history based MCAD solid modellers) it's probably hard to go past Matt Lombard.

    I'm away from base, where my copy of "SolidWorks® Surfacing and Complex Shape Modeling Bible" lives a satin cushion in a glass case ;) , but looking online at the http://samples.sainsburysebooks.co.uk/9780470386040_sample_410548.pdf

    which includes the table of contents and a bunch of pure gold info on the basics of surfacing from the first chapter, I'd guess that diving in at about p87 might give a good grounding on lofts and sweeps. Someone with a copy close to hand might be able to confirm or deny...

    Sweeps are really an automated way (Ed used to refer to them as a "macro") for creating a simple type of loft, namely one where the profile remains the same (at least in character) throughout.

    Behind the scenes, the software is allocating planes, normal to the path, at regular intervals, and positioning a sketch, based on your master sketch and mediated by your guide curves (in other words, moving key points on each sketch to where the guide curves pierce that sketch plane, while respecting the geometric constraints you set up in the master sketch, usually called "Profile")

    But other people do a much better job of explaining this than I can, and it probably doesn't hurt that they also know a lot more.
  • pete_yodispete_yodis OS Professional, Mentor Posts: 666 ✭✭✭
    Good call on the Ed Eaton stuff.  He was enormously helpful in explaining what was going on under the hood, in order to be smart enough to eventually win the fight and get the geometry you want.  I'll never forget the atomic bomb fillet method... I used to have several of his SolidWorks World presentation material saved somewhere.  I'll see if I can dig it up.
  • pete_yodispete_yodis OS Professional, Mentor Posts: 666 ✭✭✭
  • andrew_troupandrew_troup Member, Mentor Posts: 1,584 ✭✭✭✭✭
    edited May 2015
    Yup, the atomic bomb was an eye opener, for sure. I think Ed credited another brilliant guy, whose name escapes me right now#... but I seem to remember even he said it was someone else's original idea....

    Onshape needs to end up with an n-sided patch at least as good as Solidworks' legendary "Surface Fill", because otherwise there will always be some miscreant geometry we cannot 'nuke' 

    Even boundary surfaces require four sides, and that's just not always possible to arrange.

    # ON EDIT: Jason Pancoast was the name
  • pete_yodispete_yodis OS Professional, Mentor Posts: 666 ✭✭✭
    I think Pancoast credited Sluder.  Maybe Sluder credited someone else :D.  Whisper down the lane...
  • julian_lelandjulian_leland Member, OS Professional, Mentor Posts: 59 PRO
    Hi all - thanks for your feedback and thoughts! I appreciate the link to Ed Eaton's tutorials, as well as the mention of Lombard's Complex Surfacing Bible. I actually have a copy of that book which I've been working through slowly in my free time (about 2 chapters in so far) - it's helpful in giving a deeper understanding of how to wrangle the loft tool to work the way you want.

    With all that said, I still think there's something to be said for an OS-written "behind-the-scenes" loft tutorial. What I'm curious to learn about is the steps that the loft algorithm goes through to generate the surfaces that eventually make up the lofted solid; what assumptions are made by the tool; how it treats positive or negative sides of planes (if they exist in OS?); etc. If anyone wants to write one of these, that would be awesome - maybe an intern could get assigned something like this?

    Finally, +1 for a tool like Surface Fill. I just learned about this a few months ago, and it's my current go-to fix-anything tool.
  • billy2billy2 Member, OS Professional, Mentor, Developers, User Group Leader Posts: 2,008 PRO
    To help confuse or un-confuse.

    Surfaces alway have 4 sides and there is only one surface definition. There is a surface. How it's made varies: loft, ruled, sweep, radiate all produce a surface.

    The next step is to trim the boundaries of a surface. A cad system like OS won't show you the surface inside a trimming boundary and most people think there's a hole in the surface, but it's not a hole it's a trimming boundary and isn't displayed.

    If you want to learn more about surfaces, start with learning splines. 2 noded spline & 3 noded splines. A 3 noded spline has (qty 2) 2 node splines. The center node is blended to make a 3 noded spline look continuous. A 3 noded spline has 2 spline segments. Understanding how splines build up is the 1st step to surfacing.

    Think of a surface like:
    Drag an x direction line along the y direction and you'll get area. An xy plane.
    Drag a u direction curve along a v direction curve and you'll get a surface.

    Maybe this is too confusing....

    surface fill is a great way to create a surface, but they take a long time to compute. 

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