While trying to extrude a simple cornice molding using a profile and path as in jpg 1, I get a result as in jpgs 2 and3.
The result should turn out as in jpg 4 where I used sweep. The alignment, orientation and shape are perfect,
although the ends need to be capped. The sweep tool appears to function as a path extrude tool in a manner that I am used to.

My assumption was that "extrude using set path" would maintain the profile orientation as the sweep did in this instance.

Can anyone shine a light on this? Is the result I got using "extrude-set path" the one to be expected?

Many thanks in advance.

Rich

Hi Rich, that's just the way that "Set path" for Extrude is intended to work.

It builds what is called a "Sum surface", where the generated surface is a direct combination of the 2 input curves, there is no rotation of the path involved unlike Sweep.

If you want to generate a result where the profile rotates to stay perpendicular to the path, then use the Sweep command to get that type of result rather than Extrude.

So what you show there is actually the intended behavior for extrude - you usually should not use it on a path that has pieces of it that go in the same plane as the profile curve because that will make for squished-flat zones like you see there, it would be more normally used for something like this:



You can see there that Extrude with "Set Path" generates a kind of sheared final result where the profile is not rotated.

- Michael

Hi Rich,

> The sweep tool appears to function as a path extrude tool in a
> manner that I am used to.

So I guess you're running into some difference in terminology from whatever other modeling program you are used to.

In NURBS modeling, a straight regular extrusion is actually generated by doing a "sum surface" of the profile curve and a line segment.

The "Set path" option basically allows you to generate the NURBS "sum surface" using a custom curve rather than just a line.

This kind of "sum surface" has a particular property about it that the surface that is generated is an exact combination of the 2 curves, where the surface that is generated actually inherits the exact same control point structure of both the input curves. But along with that it means that there is no rotation of the profile.

Sweep goes about constructing things in a much different way internally - unlike extrude it goes through a kind of iterative refinement process where it moves and rotates profiles along the rails and uses as many generated sections as needed until the surface constructed through those sections is within a good enough tolerance of the path.

So anyway, that may help to explain why historically there is a difference between "Extrude with path" and "Sweep" in NURBS modeling... The one you want for the kind of result you are looking for would be Sweep.

Sweep also does some extra work to generate "mitered corners" when you're sweeping along a path that has sharp corners in it. That's done by the ends of each segment of the sweep actually being extended in a straight line a ways past the juncture point and then they are intersected at a common miter plane to cut off the excess parts.

Hope this helps!

- Michael

Michael,

My assumption was indeed based on terminology differences. Moi3d Sweep gives me the exact result
that I need.

One thing that I discovered by trial and error (perhaps it is explained elsewhere) is that in using Sweep, the profile
cannot be made of separate segments. All component segments must first be joined for it to function.

Thanks for the rapid and complete explanation.

Rich

Hi Rich,

> One thing that I discovered by trial and error (perhaps it is explained
> elsewhere) is that in using Sweep, the profile cannot be made of
> separate segments. All component segments must first be
> joined for it to function.

Yes - for sweep the profile curve must be a joined curve instead of a bunch of separate individual curves, which is different than Extrude - extrude has an "auto join" mechanism built into it.

The main reason why that is different in Sweep is that Sweep can actually take multiple input curves but it interprets each separate curve as belonging to a different "station" along the path, something like this:



So when you have a bunch of separate unjoined segments for the first profile, Sweep thinks that each one of those separate segments is supposed to be a separate profile at its own station along the rail and will probably make a really jumbled result as it tries to morph from one of those shapes to the other rather than taking the whole set as a single profile at a single station.

I may try to tune up Sweep in the future to do an auto-joining step to glue together touching segments before doing the sweep, but it might actually cause problems if some different stations actually had profiles that happened to touch ends, so I'm not entirely sure about doing that for sweep.

- Michael