Hi rhodesy, offset happens relative to a single "up" direction of a plane - it won't be the right thing for a path that is moving around in all sorts of 3D directions.

There is a method you can use to get this type of offset that you want - start with the segmented result of your 1-rail sweep:



Select one common set of outside edges, shown highlighted above. Then hit Ctrl+C to copy those edges, use Undo to remove the sweep, and then hit Ctrl+V to paste in those edges:



Now select just 2 of those edges, and run Edit / Extend. At the Extend prompt it will say "Select boundary objects or push Done for mutual extend" - you want to do the mutual extend operation here, so just push Done or right-click in the viewport. That will extend these 2 pieces so that they touch. Repeat in sets of 2 until everything is touching:



I think that is the kind of offset that you wanted to generate.

However, this will probably not help you for doing the 2-rail sweep workaround, using 2-rail sweep for mitered corners only works well when the rails are on a single plane - when performing it on rails that are moving around in 3D, the sweep happens on each pair of segments separately and there won't be any guarantee that the pieces meet up properly (unless I guess the segments were of equal length).

But you may find that offset method useful for something anyway.


For now to get the kind of mitered corners you want you have to do it manually by creating an extension and trimming the pieces.

When you do the one-rail sweep, turn off end caps so the ends are open. Then you will have this situation:



Select one set of edges along one open end:



Then run Extrude, and set the Cap ends option to off:



Repeat with the other end:



Now select the 2 original pieces:



Then run Edit / Trim, at the "select cutting objects" prompt, press "Done" or right-click to do a mutual trim operation (that is, cutting each object with the other). Switch the trim mode to "Keep", and select the 2 long pieces as the pieces to keep (or alternatively you can keep the trim mode to "Remove" and do a careful window select using a left-to-right window to capture the inside pieces to discard).

Then select the 2 extension pieces:



And run Trim again with the mutual trim method, and this time set trim mode to "Remove" and click on the extra outside pieces.

The result will be this corner:



You can select those pieces and use Edit/Join to glue them together.


I do expect to improve 1-rail sweep to do all this automatically for you in v2.


A variant which you may find interesting is selecting 2 of the open edges at the corner:



And then running Construct / Blend to put in a smooth bit in there:




- Michael