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RP

Ronaldo Persiano

Mon, Apr 10, 2017 8:36 PM

If we change the test in lhs_faces() to:

cosine_between(normal(points), points[0]-centroid(vertices)) < 0

it works for both cases. Anyway, this strategy of using centroid will
possibly work for all convex objects but it will not work, for instance,
for a holed cube.

BTW, your example is not a manifold.

2017-04-10 16:44 GMT-03:00 kitwallace kit.wallace@gmail.com:

Odd - works for me.

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If we change the test in lhs_faces() to: cosine_between(normal(points), points[0]-centroid(vertices)) < 0 it works for both cases. Anyway, this strategy of using centroid will possibly work for all convex objects but it will not work, for instance, for a holed cube. BTW, your example is not a manifold. 2017-04-10 16:44 GMT-03:00 kitwallace <kit.wallace@gmail.com>: > Odd - works for me. > > > > -- > View this message in context: http://forum.openscad.org/ > polyhedron-to-a-3D-printing-from-tp18692p21164.html > Sent from the OpenSCAD mailing list archive at Nabble.com. > > _______________________________________________ > OpenSCAD mailing list > Discuss@lists.openscad.org > http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org >

K

kitwallace

Mon, Apr 10, 2017 9:46 PM

Still don't see why your second case doesn't work for you - maybe it's a
version thing - I'm on 2017.01.20 but its hard to see why a simple
translation would affect anything.

BTW It's not my example , its the OP's

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Still don't see why your second case doesn't work for you - maybe it's a version thing - I'm on 2017.01.20 but its hard to see why a simple translation would affect anything. BTW It's not my example , its the OP's -- View this message in context: http://forum.openscad.org/polyhedron-to-a-3D-printing-from-tp18692p21171.html Sent from the OpenSCAD mailing list archive at Nabble.com.

R

Ronaldo

Mon, Apr 10, 2017 10:43 PM

Sorry, but I got the same result with both 2017.01.20 and 2015.03-2 running
under Windows 7 what is really odd.

http://forum.openscad.org/file/n21176/TT.png

And the reason of this failure is simple: in lhs_faces, your test uses the
angle between a vector (the face normal) and a point (the centroid). This
has no geometrical meaning.

Even correcting this, the strategy may fail for non convex objects, as I
said before.

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Sorry, but I got the same result with both 2017.01.20 and 2015.03-2 running under Windows 7 what is really odd. <http://forum.openscad.org/file/n21176/TT.png> And the reason of this failure is simple: in lhs_faces, your test uses the angle between a vector (the face normal) and a point (the centroid). This has no geometrical meaning. Even correcting this, the strategy may fail for non convex objects, as I said before. -- View this message in context: http://forum.openscad.org/polyhedron-to-a-3D-printing-from-tp18692p21176.html Sent from the OpenSCAD mailing list archive at Nabble.com.

K

kitwallace

Tue, Apr 11, 2017 12:10 AM

Ronaldo - you are right of course -my apologies for wasting your time - I was
over confident and must have slipped out of thrown together mode . There
must have been a regression in the code because I use this to clean up
Johnson solids which have occasional reversed faces

<red-face>

Chris

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Ronaldo - you are right of course -my apologies for wasting your time - I was over confident and must have slipped out of thrown together mode . There must have been a regression in the code because I use this to clean up Johnson solids which have occasional reversed faces <red-face> Chris -- View this message in context: http://forum.openscad.org/polyhedron-to-a-3D-printing-from-tp18692p21178.html Sent from the OpenSCAD mailing list archive at Nabble.com.

K

kitwallace

Tue, Apr 11, 2017 12:31 AM

OK, I was looking in the wrong place - it isn't the code, it's the data.
I've tested the code and it is correct when the solid is convex (a known
limitation). The problem is that this example is not convex - I think the
apex of the bottom facet is wrong , and hence its not manifold.

Re cosine - the centroid point defines a line from the origin to the
centroid so the angle is well-defined

Chris

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OK, I was looking in the wrong place - it isn't the code, it's the data. I've tested the code and it is correct when the solid is convex (a known limitation). The problem is that this example is not convex - I think the apex of the bottom facet is wrong , and hence its not manifold. Re cosine - the centroid point defines a line from the origin to the centroid so the angle is well-defined Chris -- View this message in context: http://forum.openscad.org/polyhedron-to-a-3D-printing-from-tp18692p21179.html Sent from the OpenSCAD mailing list archive at Nabble.com.

RP

Ronaldo Persiano

Tue, Apr 11, 2017 1:16 AM

2017-04-10 21:31 GMT-03:00 kitwallace kit.wallace@gmail.com:
Re cosine - the centroid point defines a line from the origin to the
centroid so the angle is well-defined

Sorry to disagree again. Translating the model I can put the centroid
anywhere I like. So the angle between the normal of a face (invariant to
translation) and the vector centroid-[0,0,0] may be any value I want. So
the test is not invariant to translation. But the face winding is.

Besides, what happens when the centroid happens to be the origin? All
faces, independent of its winding will pass the test.

2017-04-10 21:31 GMT-03:00 kitwallace <kit.wallace@gmail.com>: Re cosine - the centroid point defines a line from the origin to the centroid so the angle is well-defined Sorry to disagree again. Translating the model I can put the centroid anywhere I like. So the angle between the normal of a face (invariant to translation) and the vector centroid-[0,0,0] may be any value I want. So the test is not invariant to translation. But the face winding is. Besides, what happens when the centroid happens to be the origin? All faces, independent of its winding will pass the test.

K

kitwallace

Tue, Apr 11, 2017 1:24 AM

Yes of course. I recall this was a quick solution which works for the
polyhedra I was working on which are centred on the origin . i agree, it's
not a general solution at all.
On 11 Apr 2017 2:17 a.m., "Ronaldo [via OpenSCAD]" <
ml-node+s1091067n21181h3@n5.nabble.com> wrote:

2017-04-10 21:31 GMT-03:00 kitwallace <[hidden email]
http:///user/SendEmail.jtp?type=node&node=21181&i=0>:
Re cosine - the centroid point defines a line from the origin to the
centroid so the angle is well-defined

Sorry to disagree again. Translating the model I can put the centroid
anywhere I like. So the angle between the normal of a face (invariant to
translation) and the vector centroid-[0,0,0] may be any value I want. So
the test is not invariant to translation. But the face winding is.

Besides, what happens when the centroid happens to be the origin? All
faces, independent of its winding will pass the test.

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Yes of course. I recall this was a quick solution which works for the polyhedra I was working on which are centred on the origin . i agree, it's not a general solution at all. On 11 Apr 2017 2:17 a.m., "Ronaldo [via OpenSCAD]" < ml-node+s1091067n21181h3@n5.nabble.com> wrote: > 2017-04-10 21:31 GMT-03:00 kitwallace <[hidden email] > <http:///user/SendEmail.jtp?type=node&node=21181&i=0>>: > Re cosine - the centroid point defines a line from the origin to the > centroid so the angle is well-defined > > Sorry to disagree again. Translating the model I can put the centroid > anywhere I like. So the angle between the normal of a face (invariant to > translation) and the vector centroid-[0,0,0] may be any value I want. So > the test is not invariant to translation. But the face winding is. > > Besides, what happens when the centroid happens to be the origin? All > faces, independent of its winding will pass the test. > > > > _______________________________________________ > OpenSCAD mailing list > [hidden email] <http:///user/SendEmail.jtp?type=node&node=21181&i=1> > http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org > > > ------------------------------ > If you reply to this email, your message will be added to the discussion > below: > http://forum.openscad.org/polyhedron-to-a-3D-printing- > from-tp18692p21181.html > To unsubscribe from polyhedron to a 3D printing from, click here > <http://forum.openscad.org/template/NamlServlet.jtp?macro=unsubscribe_by_code&node=18692&code=a2l0LndhbGxhY2VAZ21haWwuY29tfDE4NjkyfDE3Nzk2Mzg2MzQ=> > . > NAML > <http://forum.openscad.org/template/NamlServlet.jtp?macro=macro_viewer&id=instant_html%21nabble%3Aemail.naml&base=nabble.naml.namespaces.BasicNamespace-nabble.view.web.template.NabbleNamespace-nabble.view.web.template.NodeNamespace&breadcrumbs=notify_subscribers%21nabble%3Aemail.naml-instant_emails%21nabble%3Aemail.naml-send_instant_email%21nabble%3Aemail.naml> > -- View this message in context: http://forum.openscad.org/polyhedron-to-a-3D-printing-from-tp18692p21182.html Sent from the OpenSCAD mailing list archive at Nabble.com.

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