Spline vs Arcs

[Dimitri Harvalias]

At the risk of angering the 'we need better modeling tools' crowd, and at the same time exposing my ignorance, please explain to me why one would need to use a spline curve instead of a series of tangent arcs or other curves that can approximate the spline geometry?

I realize a spline is easier to manipulate and appears smoother but once it's placed how do you go about dimensioning and annotating so it can be built on site?

This came up when a student of mine used a spline curve to create a simple, quarter circle, curved floor edge and was trying to offset it to create a sketch line for a stair boundary that would be parallel. Of course it couldn't be offset and caused no end of frustration until she figured out it couldn't be done. So why would the inclination be to reach for the spline tool instead of the arc tool?

[Calvn_Swing]

Some things in a spline are impossible to accurately describe with a series of tangent arcs. You'd need tangent ovals to hit the mark. I know you can get close, but when you want a smooth shape at a large scale you pretty much need the spline.

To dimension it, we set up a grid in a plan or section detail view, and then dimension to the intersections of the grid with the structure. This is a far better approximation of the spline than dimensioning arcs which get close. Once the studs are in the right place, in the field they use several thinner layers of gyp or whatever the finish material is, and surprise, you get a perfect spline. I find that if you do the "several arcs" technique that it is built as such, and that you can visibly see the joints between the different curvatures in the finished product, more so the larger the difference is, and the more so the larger the scale of the spline is.

So, I'm all for splines when you actually want a spline. When you actually want an arc, a spline is a pretty dumb thing to use. Build it like you want it to be built is a good motto. If you want it to look like a bunch of arcs strung together, go for it. Otherwise, use a spline...

[mattmols]

I agree with HCSL, splines are are only good for sketching or a guide for arcs. Arcs can be properly made by utilizing all the line tools...and without pinch points

[hand471037]

However, with all that said I'd really, really like some way to either split and trim splines, and/or auto-convert splines to actual arcs. Taking a sketch and/or lines draw using the spline or ellipse tools, and then having to re-draw it using arcs so that I can either dimension it or properly edit it, really can eat up a great deal of time.

And yeah, for things like wall layout and such sure it's worth it, for you'll be giving those dimensions over to someone else and it makes sense to break down those complex curves into properly laid out arcs. But for decorative elements, especially those of the kind we're able to produce directly from the Revit model via CNC, it sure would be nice for us NOT to have to manually convert splines and ellipses into arcs when we want to split, trim, or otherwise edit them.

[Calvn_Swing]

I agree with HCSL, splines are are only good for sketching or a guide for arcs. Arcs can be properly made by utilizing all the line tools...and without pinch points

Sorry, you can't. Mathematically, you cannot describe most spline forms using arcs. I'm not making this up. It's a mathematical law. You can only approximate them using arcs. To accurately describe a spline with a varying curvature you have to use ellipsoids. Revit does not give sufficient options to make and snap ellipsoids together to make a "spline." (Contractors tend to like ellipses about as much as they like splines.)

That being said, there are plenty of ways to describe and dimension proper splines, and the only reason Revit can't do this yet is because Autodesk hasn't programmed it. The reason splines are such a pain in Revit has nothing to do with splines and everything to do with how Revit works with them.

On top of that, with some of the complex interior forms we've done, it would take ages to approximate them closely enough using arcs and ellipses in Revit. Even as poorly used as splines are in Revit, it is still far faster to use them and then grid out the dimensions than it is to try and describe it with arcs.

Talking with the contractors at my firm, they much prefer getting the grid dimensions to having a bunch of arcs. They don't like going out on the site and trying to make 18 different arcs with 18 different radii to make a wall. They asked us to do it with a gridded system regardless of how we built the form. It makes it easier to build, and the materials are a lot better at making a smooth curve than any contractor is.

And then there's Jeffrey's point about CNC fabrication. When you have that kind of accuracy available to you, why wouldn't you use splines? It's crazy not to.

I don't know what kind of work your firm does. We have a wide range of styles at our office, and I can promise that there are people who would agree with HCSL and mattmols 100%. I'll also tell you those are the same guys who haven't drawn anything more complex than a simple arc or circle for any of their design work. If you go over and look at what our interiors department does, they aren't even using Revit yet because they cannot make the kind of forms with Revit that they're used to making. That is a major adoption issue right there. It's one thing to say that splines are useless for the kind of design you do. It's another entirely to make that statement as a generalization. It just isn't true at that level. Now, in Revit, they're not near where they need to be (trimming, editing, consistent usage in all sketch assemblies), but they are very useful for some people in their design work. We need them to get a lot better so that more people can use them, and do so effectively.

[Dimitri Harvalias]

And then there's Jeffrey's point about CNC fabrication. When you have that kind of accuracy available to you, why wouldn't you use splines? It's crazy not to.

I don't know what kind of work your firm does. We have a wide range of styles at our office, and I can promise that there are people who would agree with HCSL and mattmols 100%. I'll also tell you those are the same guys who haven't drawn anything more complex than a simple arc or circle for any of their design work. .Hold on there Kelly, let's not get snippy please. If you read my post more carefully I did not say splines were 'useless'. I suggested that the majority of this type of curve can be 'approximated' with arcs. I do know enough about math to understand that a spline is very different from an arc and in close to 30 years of doing this I've done my share of stuff that is not boxes and arcs.
My question was asked to try to understand the issues that of concern to those people who do this type of work more often than others.
I have used a grid approach for layout of this type of geometry. It makes sense and is easy to translate in the field. As for the CNC argument, I think if that technology is available for use it shouldn't be discounted and I'd be happy to send my Revit file to a sub-trade or contractor to assist them. I haven't met a large number of framing contractors that can utilize CNC for their portion of the work but fabricated components might be a different story.
In this day and age where qualified trades are getting harder to find I have trouble getting straight line designs constructed so walls are straight and plumb. I'm just trying to get a better understanding of what others are facing.

BTW, please call me Dimitri. Gotta get around to changing that user ID

[patricks]

I tend to stay away from splines myself. The only thing I've really used them for, that I can remember, is I fixed the factory toilet families by replacing the faceted revolves and sweeps with ones using spline curves, so you don't see all those facet lines when you see the toilets in elevations.