Hi delic, I'm sorry but I'm not understanding much of what you write here...

> I was thinking about the "extra work", does the exporter merge only the joined edges,
> and leave open the others even if merge is set at export ?

Well there is no export option labeled "Merge" - do you mean the option for "Weld vertices along edges" ? That controls whether faces on either side of a joined edge will have their own separate vertices that are stacked on top of each other or whether the faces will share a single vertex. There's some illustration of that in the help file, see the section labeled "Weld vertices along edges option:" here: http://moi3d.com/3.0/docs/moi_command_reference11.htm#meshdialog


> Something is also unclear for me, should we consider joined surfaces the same as when
> they are displayed as one surface, or as a solid ?

Sorry I'm not sure if I understand this, but basically you can join several surfaces together at places where the surfaces have edges that touch each other. If you have several surfaces joined together in that way such that they form a completely enclosed skin then that will be considered a solid. If it has any empty areas in it so that it's not a solid it will be called a "joined surface".

Joined surfaces (and solids) will have extra work done at mesh export time to ensure that there is one single unified mesh vertex structure along the joined edge, to make sure that the generated mesh does not have any cracks between pieces. If 2 surfaces get meshed which are totally independent from each other rather than joined to each other it's easy for them to get slightly different vertex structures along the edges (due to different refinement in the surfaces that come off the edges), and that will result in cracks between the different independent surface pieces.

Additionally when you have a joined surface the polygons for the entire joined surface will have a consistent normal direction. If it's a solid it will additionally have the normals pointing to the outside of the volume. If it's not a solid it will have a unified normal direction but which specific side will be the positive side can vary depending on the particular way you constructed the object. There is a "Flip" command which can flip the positive normal direction for a non-solid joined surface: http://moi3d.com/3.0/docs/moi_command_reference10.htm#flip


> Sometimes the edges can be joined to a solid or a unique surface, sometimes not,
> what is the rule have in mind there ?

Well, usually you will want to join surfaces that are sitting next to one another so you'll get no cracking and a unified normal direction at mesh export time. It can also be a bit easier to have things be solids rather than open surfaces because it then also locks down the positive normal direction automatically as well, but that does not really matter if you just turn on double-sided rendering in your rendering program.

- Michael