Hi Pilou,

> I have not follow but a simple Extrude by Dir for the internal cylinder
> don't make the trick ?

That works too, but it will create a surface that has a kind of slanted structure to it. That's not too bad but it's kind of nice for a cylinder to be represented by the extrusion of a circle that is perpendicular to the cylinder's axis just to have the most simple kind of end result that would match up with an analytic cylinder primitive.

- Michael

I had just reloaded the unrolled_patern.3dm file, so don't know that was a filled tube! :)

And the external section was composed of some arcs sections for some reasons (?)

And don't know in what direction must be the extrusion :)

At this subject :)

here I had made a Blend or a straight Loft then Boolean Union the 3 parts (it's surfaces) why the 2 sections don't disapear?
What is the rule ? If I have 3 same rectangular faces, Boolean Union them make only one face and edges common disapear!
It's because cylinder has been trimed not in perpendicular section? (same with a normal cylinder :)

Hi Pilou, boolean union does some special work when unioning co-planar planes together that will reform them into one single big plane.

It can also do some special work for the case of unioning together 2 pieces that have the same full underlying surface to combine those together into a single surface as well.

The situation you have there does not fit into either of those special cases, so there won't be any special handling for it to make it any different from the regular generic handling, and in general booleans work by potentially cutting up surfaces into smaller pieces but for gluing things together the generic way is for the pieces to become joined.


> It's because cylinder has been trimed not in perpendicular section?

No, it's because there is special extra work done in the plane/plane case.

- Michael

Thx for the explanation :this is always a little not evident result to accept :)

Hi Pilou, also consider that in order to remove the edges between pieces it is necessary to form just one single surface out of the different surfaces that you currently have.

That's difficult to do automatically in cases where the different surfaces have different control point structures in them or meet only at internal trim edges instead of only meeting at edges of the underlying surfaces.

Planes are a special case where it's easier for the code to know that 2 planes that touch each other and have the same plane normal can be represented ok by just one single larger plane. For other kinds of surfaces it is more difficult to construct code that would be able to analyze the surfaces to determine that.

- Michael

2 same primitive cylinder Bolean union have only the internal common faces dispears but not the common section :)
But I understand that must be an headache to resolve all cases!

Hi Pilou,

> 2 same primitive cylinder Bolean union have only the internal common
> faces dispears but not the common section :)

Yup, that's normal - notice that I did not list cylinder surface combining as one of the special cases that are currently handled ?

It could be possible for that particular case to happen in the future, it would require some modifications to some pretty complex areas of the geometry library though.

In the meantime usually it is possible to solve such things yourself if you want to eliminate internal edges by creating one larger cylinder and trimming it to the proper boundaries.

Also if your 2 cylinders were made up of pieces cut out from one single large cylinder, they will combine in that case since it then is handled by the "same underlying surface" special case. So for example if you draw a tall cylinder, and then cut it in half with a horizontal line to make 2 cylinders that way, if you then boolean union those pieces together it will reform into one single cylinder piece. This can be handy sometimes, because it can sometimes be convenient to carve off a chunk of a model using boolean difference, then do some work on the individual pieces and then when you union it back you will then get surface combining - that's done in the "Crown of clubs" tutorial video for example which is why the final unioned piece does not have extra edges at the bottom of each club where they connect to the bottom part that was carved off from the whole initial revolve.

- Michael

< then cut it in half with a horizontal line to make 2 cylinders that way, if you then boolean union those pieces together it will reform into one single cylinder piece.
It's that who is very pertubating! :)
2 copy cylinders don't union + disapear, 2 cylinders from a cutting one yes! :)

Hi Pilou,

> 2 copy cylinders don't union + disapear, 2 cylinders from a cutting one yes! :)

Yes - it's because there is special case handling used for the situation of "2 surfaces that are each trimmed surfaces using the exact same underlying surface" - that case is more straightforward for the code to detect that the surfaces can be combined together.

For that case and for planes there are mechanisms in place in the booleans to try and reform the unioned result with larger glued together surfaces. For other kinds of cases like the 2 copied cylinders - that is a different case and would require different handling in order for it to work, which is certainly possible but not implemented currently in the geometry kernel.

- Michael

Michael,
Thanks for the help. I think I've got it now.
Mark