Hi steve,

> Alibre or Solidworks create solids as opposed to surfaces.

This is true, but the way that solid modeling programs define a solid is by a set of surfaces that touch each other along common edges to form a "watertight" skin that encloses a volume.

So surfaces are one of the elements that make up a solid model.


> Is Moi also a solid modeler?

Yes - MoI can work on both surfaces and solids, which is now the most common method. Long ago "solid modelers" like SolidWorks would focus their user interface on only working with solids, but these days they typically also allow working on surfaces individually or solids in combination with one another.

That's also the way that MoI works - in MoI you can work on solids which are made up of surfaces joined at common edges, or you can also work on an individual surface level and then use the Edit/Join command to glue them together to make a solid.


There is a lot of misconception out there that the solids defined in a "solid modeler" are somehow "more solid" than ones defined by a set of surfaces. But actually the system used by 99.9% of solid modelers is the "boundary representation" method which is using surfaces that are connected together at edges to make a solid volume.


> And when it writes a file in igs format will the objects still be solids?

Essentially yes. But the details are a bit complex.

Typically igs format does not contain full solids but instead all the surfaces that make up the solid, and then programs that want to work on it as a solid will join those together to make a solid. So yes, the final result at the end will be a solid.

STEP format is similar, but contains not just surfaces but also the connection information in between them, so that stores a complete solid in a STEP file. It can be good to use this method instead of IGES because it means there is one less step for a receiving program to do - it does not have to go through the process of figuring out which edges to glue together like has to be done with IGES format transfers.

So I would probably recommend trying STEP, but like I mentioned, the end result of the IGES transfer will also be a solid at the end as well.


The major differences between file formats are actually not so much whether they contain surface or solids in them, but rather if they contain "NURBS surfaces", or "Polygons" in them.

Both STEP and IGES are NURBS formats, which mean they transfer true surface data using mathematically curved surface sheets for the definition of the objects. Like a sphere is a true sphere surface.

Polygon-based file formats are what are a lot different, where they store no smooth surface data at all and instead have a large number of small planar facets to make up an object. So with a polygon format a sphere is not defined as a sphere but instead there are a lot of flat pieces arranged to be close in shape to a sphere.

Typically to transfer data to another CAD program you want to send that as NURBS because whether it is NURBS surfaces or NURBS solids, the true smooth surfaces will go over.

However, polygon formats are used pretty frequently for different kinds of CAM tasks as well, like STL is a polygon format which is typically used for sending data more directly to a rapid prototyping machine.


> If I import say the igs file into Moi, make some changes and export
> back to igs, will the data still be of the same type.

Yes - the data in the igs file will still be the same kind of NURBS surface data.


Let me know if you need any more clarification on this stuff, I know there are a lot of details here.


- Michael