In some sense this is a bug of rotate_extrude(). OpenScad complains when
some edge of the polygon crosses the Y Axis because that would generate a
self-intersection 3D object. The current case is a limit case of the
forbiden crossing and OpenScad should issue an error as well.

A terça, 16/07/2019, 21:02, MichaelAtOz oz.at.michael@gmail.com escreveu:

Thanks guys.

If it's not my bug but is a kind of error in rotate_extrude I suggest 2
things:

1. in the documentation for rotate_extrude mention that all points must be

are skimming through you don't notice right away
"The 2D shape needs to lie completely on either the right (recommended) or
the left side of the Y-axis. More precisely speaking, each vertex of the
shape must have either x >= 0 or x <= 0. If the shape crosses"
I think it would be more clear if the y instructions were written in a
similar vein to the  x instructions ("y>0, or y<0 for all vertices").  I
tend to skim over things quickly and the x stuff jumps out whereas the y
stuff just sort of disappears into the background

2. if the exception message was different, perhaps the code could check for
   a bad vertex and tell the user.  It already seems to do that for x values.

I know everyone is busy, I'm just thinking if these suggestions are put on
a list they might get implemented at some point.

On Tue, Jul 16, 2019 at 8:02 PM MichaelAtOz oz.at.michael@gmail.com wrote:

DanS wrote

Well it sort of is your bug, you are asking CGAL to make something
impossible.
A zero thickness 3D object, at that single point at the top.

What Ronaldo says is if there is a bug it would be that OpenSCAD might be
able to detect such attempts and produce a better error message.

I'll review the wiki tho, there may be a place for some tips.

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Em qua, 17 de jul de 2019 às 03:07, MichaelAtOz oz.at.michael@gmail.com
escreveu:

The only situation the rotate_extrude() of a simple polygon is clearly a
manifold is when the polygon does not have any intersection with the Y axis
and it rests entirely at left or at right of the Y axis. Then the
rotate_extrude() will be a torus like object.

When the intersection of a simple polygon and the Y axis is a full polygon
edge, I would expect a non-manifold result because that particular edge
would be in the interior of the object. But CGAL does produce a valid
polyhedron which means, I suppose, that OpenSCAD smartly removes that
offending edge before calling CGAL. Then, the rotate_extrude() will not be
the union of torus like objects - it will not have a genus.

In the limit case, when just a simple polygon vertex is on the Y axis,
there is no smartness to avoid non-manifoldness and an error might be
issued.

However, when, the polygon is not simple, like in the example bellow, it is
harder to detect beforehand a non-manifoldness of rotate_extrude():

rotate_extrude(){
polygon([ [2,2],[3,3], [2,4] ]);
translate([1,0]) square(2);
}
cube();

Ronaldo wrote

So a test would be if polygon vertex.x=0 then at least one adjacent vertex
must have x=0 & a different y.

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Yes that sounds right. Each bit that touches the y axis can't be a single
point because that creates a singularity when rotated.

The zero thickness knife edge at the bottom is dodgy as well. Any CGAL
operations near that point are likely to create self intersections.

Both features are psychically impossible.

On Wed, 17 Jul 2019 at 07:43, MichaelAtOz oz.at.michael@gmail.com wrote:

On 17.07.2019 08:16, Ronaldo Persiano wrote:

I don't know how rotate_extrude is implemented in OpenSCAD, but I am
guessing CGAL does not provide such a feature, and instead
rotate_extrude is done by OpenSCAD directly to compute the coordinates
of a polyhedron. Implemented this way, there are no boolean operations
involved.

This is certainly the case in AngelCAD where rotate_extrude is not a
feature at all in Carve (Carve is the equivalent to CGAL), it is
computed directly by the application, not the library.

The implication is that the issue is not about manifoldness, but rather
about self intersection and collapsed faces. Manifoldness is about
topology (connectivity), self-intersection and collapsed faces is about
geometry (coordinates in this case). You can have a self-intersecting
polyhedron with collapsed faces which is 2 manifold, as in the below example

When you say "I would expect a non-manifold result because that
particular edge would be in the interior of the object. But CGAL does
produce a valid polyhedron". I would say CGAL that is correct, and the
expectation is incorrect.

Consider the example

rotate_extrude(){
polygon([ [0,-2],[2,0], [0,2] ]);
}

This produces no error by OpenSCAD, and you could argue it is ok at this
stage.

Top level object is a 3D object:
Facets:         42

The visible external faces are only 14. If you export to OFF format and
then analyze it with polyfix...

OFF export finished: O:/STL/rotate1.off

...you will find that OpenSCAD appears to have merged overlapping
vertices, thus creating a non-manifold model and containing collapsed
faces. After repair, the collapsed faces are removed and thus the model
is again 2-manifold.

===
$ polyfix rotate1.off

Parameters:
input_file = rotate1.off

polyhedron 0 ================= volume=14.5942, dtol=0.01, atol=1e-06,
maxiter=10
iteration 0: vertices=9 faces=42
warning: 28 zero area faces.
warning: nonmanifold edges: uc(4)=14 uc(14)=2 uc(28)=1
removed 28 collapsed or zero area faces
total changes=28
no warnings

iteration 1: vertices=9 faces=14
total changes=0
no warnings

Summary:
polyhedron 0: vertices=9 faces=14 : no warnings

Writing: rotate1_1.off

We can repeat the same exercise in AngelCAD (coordinates are a bit
different, because AngelCAD rotates around global Y).

// AngelCAD code.
shape@ main_shape()
{
array<pos2d@> p = { {0,-2},{2,0},{0,2} };
return rotate_extrude(polygon(p),deg:360);
}

void main()
{
shape@ obj = main_shape();
obj.write_xcsg(GetInputFullPath(),secant_tolerance:0.2);
}

This creates the output

xcsg processing: /media/nas_openbzr/STL/xcsg/test1.xcsg
processing solid: rotate_extrude
...completed CSG tree: 0 boolean operations to process.
...Info: rotate_extrude angle>=2*PI implies a torus
...completed boolean operations in 0 [sec]
...result model contains 1 lump.
...lump 1: 21 vertices, 15 polygon faces.
...Polyhedron is water-tight (edge use-count check OK)

...Polyhedron has 15 non-triangular faces
...Triangulating lump ...
...Triangulation completed with 28 triangle faces in 0 [sec]
...Exporting results
Created STL file    : /media/nas_openbzr/STL/xcsg/test1.stl
xcsg finished using 0h 0m 0.01s

i.e. 28 faces makes sense = 7x3, of which one third will be collapsed,
due to the way rotate_extrude is implemented.

=====

$ polyfix xcsg/test1.off

Parameters:
input_file = xcsg/test1.off

polyhedron 0 ================= volume=14.5942, dtol=0.01, atol=1e-06,
maxiter=10
iteration 0: vertices=21 faces=28
warning: 14 zero area faces.
warning: nonmanifold edges: uc(1)=14
merged 12 vertices
removed 14 collapsed or zero area faces
total changes=26
no warnings

iteration 1: vertices=9 faces=14
total changes=0
no warnings

Summary:
polyhedron 0: vertices=9 faces=14 : no warnings

Writing: xcsg/test1_1.off

=====

Here, polyfix assumes overlapping vertices is undesired and merges them,
the end result is the same as via OpenSCAD, 14 faces and 9 vertices.

Carsten Arnholm

I think rotate extrude does its work in PolySets which, just like STL, is a
polygon soup and cannot represent a 2 manifold with self intersections.
Then when that is given to CGAL it barfs.

Since OpenSCAD is aimed at 3D printing and nearly all output is via STL it
gets away with using PolySets and should give an error message for geometry
with self intersections. If an output format other than STL ever becomes
popular and people wanted to export self intersection geometry OpenSCAD
would have to be rewritten to not use PolySets. I don't really see the
point as such geometry only exists in maths, not in physical reality.

On Wed, 17 Jul 2019 at 12:31, Carsten Arnholm arnholm@arnholm.org wrote: