I have a problem. I have 2D objects, which might be concave, and which has
holes in them. These objects are pretty complex, done in several stages, and
the holes are made in a pretty early part of the process, and the holes may
(or may not) also affect the shape of the outline.

What I want is to get just the outer contour of the object, without the
holes contained fully within, to create masks for other stages in the
production.

If I knew the objects were convex, it wouldn't be a problem, then hull()
would work, but I cannot rely on them to be convex (in fact, they most
certainly aren't).

If the objects were simple, I could do it by union() with some "cover
objects", but they aren't.

If I could do the holes last, it would (partly, but not guaranteed) help,
but I can't as I have other later stages which requires them to be there.

Likewise, bouncing an offset out and back won't work, as it'll affect
corners.

So, any idea on how one could do an outline() module?

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Position the object and then do projection()?
William

Wouldn't projection() just give me the same 2D object as I had, with the same
holes in it?

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I don't know if this would be practical for your situation, but BOSL2 has
some functions for doing boolean operations on 2d objects expressed as point
lists.  I haven't really experimented with that code much, but I believe if
you were to construct your object with that code the final output would be
the outline you desire plus a list of (entirely internal and disjoint) holes
that need to be subtracted.

Simple example below shows some boolean operations to make a 2d object.
Full 2d object on the left, and then just the outline object on the right.

include<BOSL2/std.scad>

$fn=32;

obj1 = circle(5);
obj2 = apply(move([1,2]), circle(1));
obj3 = apply(move([2.5,-4]), square(3));
obj4 = apply(move([-3,-3.1]), circle(2));
obj5 = apply(move([-2,-2.5]), square(2));

obj = difference([ [obj1],[obj2], [obj3], [obj4], [obj5]]);

region(obj);      // Display compound object

right(12)
polygon(obj[0]);  // obj[0] is the desired outline

http://forum.openscad.org/file/t2477/geom.png

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Normally, when I have this problem, I open the file in Inkscape, Ctrl+Shift+K
to break paths apart, then just grab the outside shape.  But to do this in
OpenSCAD:

Here is a simple proof of concept using offset and no external libraries.

In this case there is an interplay between the $fn and the "interior"
variable; with $fn high (like 200) interior can be between 10 and 20 (on
this example) without the outline of the external shape varying.  If $fn was
low... for example $fn=40, then you would start getting artifacts on some of
the outer curves.  But this technique works well enough for certain shapes.

$fn=200;
interior = 20;

module shape()
{
// Make a Complex Shape
difference()
{
square (100);
translate ([50, 50, 0]) circle (10);
translate ([100, 100, 0]) circle (20);
translate ([30, 70, 0]) circle (20, $fn=3);
translate ([0, 0, 0]) circle (40);
translate ([80, -1, ]) square (20);
translate ([80, 80, 0]) circle (5)
translate ([85, 40, 0]) circle (10);
}
translate ([80,20,0]) circle (20);
}

//Create the Outline
color ("blue") offset(r=-interior) offset (r=interior) shape();

//Original Shape
color ("yellow") translate ([0, 0, 10]) shape();

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