Docs · Exporter

Deliberate differences

BMC ports Autodesk’s exporter (revit-ifc version 27.4) and is compared against the exporter installed with Revit 2027 (27.3). These are the only places where BMC writes something different on purpose. Each is a recorded decision. Anything else that differs is a bug or a gap.

Most of them remove randomness: exporting the same model twice gives the same file.

1. Duplicate stored GUIDs

When two elements carry the same stored IFC GUID (for example, an element copied after an export that stored GUIDs):

  • Autodesk keeps the first, gives the later one its own export GUID, and only if that is taken too makes up a random GUID, different on every export.
  • BMC follows the same order and fallbacks, but where Autodesk is random it derives the GUID from the element (an MD5 of its export id). The IFC is always valid, with unique GUIDs.

After the export, the plugin asks whether to save the new GUIDs to the model. Only an explicit Save to model click writes them. This is the only time BMC ever changes a model, and only on that click.

2. A custom property set named like a standard one

A user-defined set named, say, Pset_WallCommon becomes a second set of that name. Autodesk gives it a random GlobalId; BMC a repeatable one.

3. Floor and beam extrusion choice

Whether some floors and members are written as extrusions depends on the order in which Revit returns their faces, which can change between Autodesk exports. BMC uses a fixed order (element and face ids), so the result never changes. It predicts Autodesk’s other possible outcome and reports it, and the comparator accepts Autodesk’s result only if it equals that prediction.

4. Two parameters with the same name

For example, a built-in Material and a shared Material, both feeding one property:

  • Autodesk 27.3 takes the first with any value, so an empty built-in can hide the parameter you filled.
  • BMC takes the first usable value, and when several are filled, a project or shared parameter wins over a Revit built-in.

The reason: you filled the IFC+SG parameter for the IFC, so an empty or unrelated built-in must not override it.

5. Detail level of view exports

Autodesk reads geometry at the view’s detail level. BMC always reads at Fine, so an element has the same shape whether you export the whole model or a view.

6. Empty text

Autodesk writes an empty text attribute (for example LongName) as ''; BMC writes $, unset, the IFC standard. The meaning is the same.