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Filled Surface vs. Boundary Surface vs. Lofted Surface

Question asked by Russell Jelinek on Aug 19, 2008
Latest reply on Aug 20, 2008 by 1-8MMYCK

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I've been repairing some surfacing models, and I end up in situations where a filled surface, boundary surface, and lofted surface would probably all work equally well.

Are there situations where one feature is more desirable than the others?

Does one affect performance more than the others?

The one I understand the least is boundary surface. What makes this different from a lofted surface?

Sorry I'm not more specific, I'm just kind of looking for rough general guidelines.

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Charles Culp Aug 19, 2008 12:14 PM
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Boundary gives you full curvature control on all four edges. If you are "fixing up" patches on a surface model, I would almost always use boundary surface over loft. I only use loft in very specific instances where I need something halfway between sweep and boundary surface.

Fill does not give you near as much control as boundary, but it works well to fill areas with more than 4 edges. It's also great if you don't need your surface to be exact, you just need a smooth transition between other constrained surfaces.

Thus my general principle is that if it needs to be well controlled I use boundary, and if it just needs to look smooth, I use fill.
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Russell Jelinek Aug 19, 2008 2:18 PM
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Great advice...thanks. Does a boundary surface require significantly more resources than a filled surface?
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1-8MMYCK Aug 19, 2008 2:20 PM
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i guess fill surface requires more resources than boundary surface because it's mathematically more complex.
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Matt Lombard Aug 19, 2008 2:38 PM
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This is a great question. It's one that I tried to tackle in my surfacing book: http://dezignstuff.com/blog/?page_id=75

I give examples of when each type of surface is most appropriate and maybe more importantly, when not to use certain things. I called loft, boundary, fill and sweep the primary surface creation tools.

SW was surprised that people tried to compare the boundary and fill surfaces. They had a lot of reasons why they thought these were apples and oranges, but I think there is a fair amount of overlap.
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Mark Matthews Aug 20, 2008 1:52 PM
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My first choice is to use the boundary tool as it has the most control, and curvature options on all four edges. What annoys me is that most of the time though, I get an error that one boundary edge is not curvature continuous when I know that I have set it up so all edges should be, either via a 3-D curve that has a curvature contraint to another edge, or a surface trim between two surfaces that were built with curvature continuity between them.

My wish down the road is that SW gets the boundaries between surfaces to consitstently be really tangent and curvature continuous as measured with the dihedral angle tool. Sometimes I get measurements that are as much as half a degree off with SW considering those to be tangent. The loft tool is the worst for this, with boundary next. The best results come from the fill tool, but it is also the most uncontrollable.
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Russell Jelinek Aug 20, 2008 3:03 PM
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After working with boundary surfaces for a couple days, I agree that's the way to go. I still seem to run into a lot of situations that require filled surfaces, though.

Thanks to all for the thoughts.
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1-8MMYCK Aug 20, 2008 3:16 PM
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my question here is whats the effect of choosing one of these methods when they overlap on the downstream operations?
the downstream operations on surfaces must be factor. a surface created with fill surface might have different iso-parameter curves with the surface created using boundary surface or loft. CAM softwares for example have toolpaths which are dependent on underlying UV curves.
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Charles Culp Aug 20, 2008 3:44 PM
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Boundary surfaces (at least those with 4 sides) will follow the ISO curves with their 4 sides. Fill seems to be a "best fit", and will not orient itself to any edge. My experience at least, I sure didn't program SW.
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1-8MMYCK Aug 20, 2008 4:31 PM
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thanks for the reply charles. another question why it's recommended to avoid a degenerate surface?
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- Charles Culp @ 1-8MMYCK on Aug 20, 2008 5:57 PM
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  Degenerate surfaces are typically surfaces with 3 sides, where the ISO curves run between two of the sides. As you can see from the attached image, this can be created by lofting or boundary surface from one edge to another, where the ends are coincident. In the corner, the ISO lines all converge together. Thus at one "point" the curvature is now in multiple directions. This is of course a mathematical impossibility (think of an equation where as x approaches zero, y approaches infinity), and Solidworks uses some sort of internal "fudge factor" to accommodate this.
  
  On their own they are not a problem, being able to loft between to coincident edges can make creating certain surfaces much easier. The problem is when you try and use other surfacing tools after this. In the example I have attached, you cannot create a fillet on either of the edges touching the degenerate point. You also cannot shell this solid.
  
  Basically, any time you have to create an offset, or modify that corner, it will probably fail. So it is best to always avoid degenerate surfaces when you can. Using a "surface fill" instead of a boundary surface (in my example) would orientate the ISO curves so that it will not create a degenerate point. As mentioned earlier in this thread, surface fill will not give you the control on the edges like boundary will, so it is give-and-take.
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  - DegenerateSurface.png116.1 KB
  - DegenerateSurface_cc.zip41.9 KB
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- Mark Matthews @ 1-8MMYCK on Aug 20, 2008 6:01 PM
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  Degenerate surfaces, while they can be made, can lead to problems later in the model, or with CAM software trying to mill that degenerate point. You might not be able to shell, or a round will not build. This is not always the case, but it can happen.
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1-8MMYCK Aug 20, 2008 6:13 PM
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Charles and Mark,great information. many thanks!
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Filled Surface vs. Boundary Surface vs. Lofted Surface

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