polyhedron() cannot do "faces with holes" currently as far as i know.

However there is a work-around. It is called the Keyhole Polygon...

Basically you take any polygon with a single hole... and pretend you
slice a knife down from the outside of the polygon, straight down, until
you cut into the hole. What you end up with is a single polygon without
any holes. Here is a picture

https://sozvyezdami.files.wordpress.com/2013/04/11.png?w=640

The caveat that the new edges you create by cutting will be coincident
with each other.... and the 'points' or 'nodes' of those new edges will
also be coincident, which can be a problem for some polygon processing
algorithms. However it appears to work on your basic example in
OpenSCAD. Here is your code rewritten to have two 'keyhole' polygons on
top and bottom.

pts =  [[0, 0, 0], [6, 0, 0], [6, 5, 0], [0, 5, 0], [2, 1, 0], [5, 2,
0],
[3, 4, 0], [0, 0, 3], [6, 0, 3], [6, 5, 3], [0, 5, 3], [2, 1, 3], [5, 2,
3], [3, 4, 3]];

faces1= [[0, 7, 8, 1], [1, 8, 9, 2], [2, 9, 10, 3], [3, 10, 7, 0],
[4, 5,12, 11], [5, 6, 13, 12], [6, 4, 11, 13],
[0, 1, 2, 3, 4, 6, 5, 4, 3],
[10, 9, 8, 7, 11, 12, 13, 11, 7]];
polyhedron( points = pts, faces = faces1  );

Also i had to re-order some of the vertices, as viewable in
'View/ThrownTogether' mode within OpenSCAD to make sure there were no
naughty purples. Hope this helps!

-don bright
hmbright@fastmail.fm

Hi Don, interesting approach. I seem to read similar one when reading
Computer Geometry or something a while back, but can't recall what it's for.

This approach seems to simplify the problem significantly. The
re-arrangement of points is not a big issue.

On the other hand, for my application, it might bring up other problems.
What I show in this thread is an extremely simplified example. The actual
tube in my lib could be a complicated one with random curving, etc. To make
a holed surface on the end of a long tube with this approach, the entire
tube will have to be sliced apart.

I also intend to use a holed surface as the merging face when cylinder of
different sides (in polyhedron) linked together. That is, to merge two
polyhedria. It will then be a budding, but not hole-punching/cutting,
process.

Anyway, thx for the idea. I believe it'd turn out useful in the future
development of my lib.

$  Runsun Pan, PhD

$ -- libs: doctest , faces ( git ), offliner ( git );

tips: hash( 1 , 2 ), sweep , var

$ -- Linux Mint 17.1 Rebecca x64  + OpenSCAD 2015.03.15/2015.04.01.nightly

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Regarding "faces with hole", following up the "keyhole polygon" solution
described by dob bright 3 yrs ago (which is also presented as an "
alternative face description
https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/The_OpenSCAD_Language#Alternate_Face_Descriptions
" in the official document ).I found that it could even be used to make a
polyhedron cube with multiple holes, like this:
pts = [[0, 0, 0], [0, 4, 0], [3, 5, 0], [2, 0, 0], [0, 0, 2], [0, 4, 2], [3,
5, 2], [2, 0, 2], [1, 0.5, 0], [0.4, 1.2, 0], [0.5, 2.1, 0], [2, 1.5, 0],
[1, 0.5, 2], [0.4, 1.2, 2], [0.5, 2.1, 2], [2, 1.5, 2], [1.5, 2.5, 0], [0.5,
3.5, 0], [2, 3.5, 0], [1.5, 2.5, 2], [0.5, 3.5, 2], [2, 3.5, 2]];faces =
[[4, 5, 6, 7, 4, 15, 14, 13, 12, 15, 4, 21, 20, 19, 21], [3, 2, 1, 0, 3, 8,
9, 10, 11, 8, 3, 16, 17, 18, 16], [0, 1, 5, 4], [1, 2, 6, 5], [2, 3, 7, 6],
[3, 0, 4, 7], [9, 8, 12, 13], [10, 9, 13, 14], [11, 10, 14, 15], [8, 11, 15,
12], [17, 16, 19, 20], [18, 17, 20, 21], [16, 18, 21,
19]];polyhedron(pts,faces);
http://forum.openscad.org/file/t602/20181209_runsun_multi_hole_surface_1.png
In this case, the top face uses pts[4] as the starting point to cut into the
two holes:
[ 4, 5, 6, 7, 4, 15, 14, 13, 12, 15, 4, 21, 20, 19, 21]
as shown in this figure:
http://forum.openscad.org/file/t602/20181209_runsun_multi_hole_surface_2.png
What surprises me is that the 2 cut lines, 4~15 and 4~21,  go across the
entire holes.Is this normal?

It seems that this approach works in more holes, too:
pts = [[0, 0, 0], [0, 4, 0], [3, 5, 0], [2, 0, 0], [0, 0, 1], [0, 4, 1], [3,
5, 1], [2, 0, 1], [1, 0.5, 0], [0.4, 1.2, 0], [2, 1.5, 0], [1, 0.5, 1],
[0.4, 1.2, 1], [2, 1.5, 1], [0.5, 1.8, 0], [0.5, 2.1, 0], [1.8, 2.2, 0],
[1.5, 2, 0], [0.5, 1.8, 1], [0.5, 2.1, 1], [1.8, 2.2, 1], [1.5, 2, 1], [1,
2.75, 0], [0.5, 3.5, 0], [2, 3.5, 0], [1, 2.75, 1], [0.5, 3.5, 1], [2, 3.5,
1]];faces = [[4, 5, 6, 7, 4, 13, 12, 11, 13, 4, 21, 20, 19, 18, 21, 4, 27,
26, 25, 27], [3, 2, 1, 0, 3, 8, 9, 10, 8, 3, 14, 15, 16, 17, 14, 3, 22, 23,
24, 22], [0, 1, 5, 4], [1, 2, 6, 5], [2, 3, 7, 6], [3, 0, 4, 7], [9, 8, 11,
12], [10, 9, 12, 13], [8, 10, 13, 11], [15, 14, 18, 19], [16, 15, 19, 20],
[17, 16, 20, 21], [14, 17, 21, 18], [23, 22, 25, 26], [24, 23, 26, 27], [22,
24, 27, 25]];polyhedron(pts,faces);
http://forum.openscad.org/file/t602/20181209_runsun_multi_hole_surface_3.png
The top face:
[ 4, 5, 6, 7, 4, 13, 12, 11, 13, 4, 21, 20, 19, 18, 21, 4, 27, 26, 25, 27]
I'd like to know what you experts think about this.

$  Runsun Pan, PhD $ libs: scadx , doctest , faces ( git ), offline doc ( git ), runscad.py ( 2 , git ), editor of choice: CudaText  ( OpenSCAD lexer ); $ Tips ; $ Snippets

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Very interesting stuff. Thanks for digging out this hack!
I immediately wanted this for sweep();

A sweep with a "close" attribute doing a single hole without difference() is
already nothing new - you sweep up and down.

sweep([circle(10, 0), circle(10, 20), circle(7, 20), circle(7, 0)], close =
true);
function circle(r=10, z = 0, N=10) = [for(i=[0:N-1]) [rcos(360/Ni),
rsin(360/Ni), z]];

I guess this approach will allow to implement a sweep() allowing for as many
holes as one likes.
I'll have to add a new interface parameter to my sweep() that lets one
supply a list of polygons describing the holes. The first and last face need
the hack, the rest is more or less straight forward.

Being able to sweep objects with multiple holes can speed up rendering of a
lot of stuff. I'm looking forward to have it.

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