< 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