http://forum.openscad.org/file/n19677/cup.jpg
I thought that it should be possible to make in one sweep a hollow object
like a cup. And the following code works without the use of the difference
function
:Any suggestions how to make a tube without use of the difference function?

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I don't know how to do it with Parkinbot's sweep but, yes, it is possible:

http://forum.openscad.org/file/n19680/Tube_polyhedron.png

Tube_example.scad http://forum.openscad.org/file/n19680/Tube_example.scad

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Very nice example and I even succeeded in understanding it so well that I
could generate a tube or ring.So I have 4 ways now to make a ring:

• difference of two cylinders
• lineair extrude of a 2D ring
• difference of two sweeped cylinders (works also with skin)
• creating polyhedrons

I added this to one of my examples files called "lord_of_the_rings.scad"
;-).
lord_of_the_rings.scad
http://forum.openscad.org/file/n19688/lord_of_the_rings.scad
http://forum.openscad.org/file/n19688/lordoftherings.jpg Is there another
way to make the complete olympic serie??

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Method nr. 5:

color("green") translate([20,20,0])
{
rotate_extrude(angle=360, convexity=10)
translate([8,0,0])
square([2,10]);
}

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Nice summary, Johan. Conceptually, your sweep example is not distinct from
the difference of cylinders: it only generates the cylinders in an
alternative way. Essentially, sweep is a polyhedron construction.

You want a fifth one... make two half rings by rotate_extruding squares and
union them. In a snapshot version, you can do rotate_extrude specifying an
angle.

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It is not too difficult to write a sweep function that connects a dual
sequence of polygons into a hollow solid like a pipe. At best you commit
yourself to obey to some rules to ease triangulation.

inner and outer polygons must
0. not self-intersect or mutually intersect !!! (no intersection at all )

1. use a defined orientation
   (2. be coplanar - i.e. at best they use the same affine 2D->3D
   transformation)
   (3. have the same number of points)
   (4. use minimally distanced start points and suitable sequencing.)

2 can be omitted, if you know, what you do.
3 and 4 are good if you want to use a trivial connection scheme like quads
for the first and last face. If you find some other nice triangulation
scheme they can be omitted. For the extruded inner and outer surfaces you
can use a trivial connection scheme like quads.

But honestly, I don't see any progress beyond saving some CPU time against
just differencing an inner solid from an outer solid. If both solids are
proper manifolds, the result will also be. If not, your new scheme will not
be any better.

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Nice fifth solution, Johan. I don't see why I divided the ring in two parts
in proposal. Ahrgh.

Yes, you are right. It seems I have been unconciously influenced by your
smart solution to threads.