Native IFC/de

Although FreeCAD already supported opening and saving IFC files through the Arch workbench for a long time, it did so, like most other BIM applications, by translating back and forth between the IFC file format and FreeCAD's internal data model. This means two translations, one when opening and another one when saving, which cause data loss, and a complete rewrite of the file each time. This turns it unsuitable for version control systems like Git, and very hard to locate changes.

Native IFC means that the IFC file is itself the data structure in FreeCAD. When opening an IFC file in FreeCAD, what you see on the screen is a direct rendering of the contents of the IFC file. Anything you modify will directly modify the IFC file. If you open a file, modify one element and save the file, the only thing that will have changed in the file is the line concerning that element. The rest of the file will be 100% identical to how it was before you saved. No data loss, and very identifiable changes.

The Native IFC idea itself comes from this paper by Bruno Postle. It is heavily inspired by BlenderBIM and tries as much as possible to use the same concepts and reuse the code. The final goal is to offer in FreeCAD the same level of functionality and performance found in BlenderBIM, and to upstream as much as possible to IfcOpenShell, the common IFC engine used by both applications. It started as a separate project by Yorik

Working with native IFC files in FreeCAD is simple: Open an existing IFC file or create a new project using the Project tool from the BIM Workbench. Create new objects with any of the FreeCAD workbenches or tools. Add these objects to your IFC project by dragging them into the project structure. Save the file, and you are done.

In locked mode, all the objects you create are immediately converted to IFC, and might therefore loose some of their original properties and some of their parametric behavior when these are not supported by the IFC format. This mode is more restrictive, which is a disadvantage at some points in project development, but becomes an advantage when data integrity and reliability become essential.

It is also possible to create native IFC projects inside an unlocked document. In that case, strict IFC elements coexist with non-IFC elements in your FreeCAD document. Objects within an IFC project are linked to its attached IFC file. Working with IFC projects is often not needed at early stages of a project, but if you work with others and use IFC files as your main communication medium, using native IFC projects can become an advantage, as the changes in each version of the file are much smaller.

You can always export your model to IFC whenever needed, using IFC projects or not. The difference is that when you are using an IFC project, very minimal changes are written to the IFC file. When not, the whole file is rewritten at each export. This might or might not be an issue for you

The general idea is to work in unlocked mode when working on your own projects, so you can have both IFC and non-IFC elements in your documents, and use the locked mode when working on other people's IFC files, so you can keep strict control over what you are changing in those files.

In unlocked mode, you have IFC project objects attached to IFC files, and the FreeCAD document is dependent on these IFC files. If you wish to transmit your FreeCAD file to someone else, you need to given them the IFC files too. Alternatively, you can also set the Shape mode of all the objects to "Shape" (see below). This will make the FreeCAD file retain the shape of all the IFC objects, and render them correctly even without the attached IFC file. However, the IFC objects inside will no longer be editable.

Once an object has become an IFC object, it is entirely controlled by the IFC data. It looses its former FreeCAD parametric object, and might loose any parametric properties and features that are not supported by the IFC format. The aim here is in the future to render all this seamless and keep the same editing possibilities for both IFC and non-IFC objects.

Opening and importing IFC files

• If you open, a new document will be created and you will automatically be in locked mode
• If you import, a native IFC project object will be added to the current document, but you can still create non-IFC objects outside that project. You will still be in unlocked mode

To open an IFC file, use menu File → Open and select an IFC file. To import an IFC file, use menu File → Import and select an IFC file.

Importing or opening an IFC file does not require the setting of any option. However, some defaults can be tweaked under menu Edit → Preferences → BIM → Native IFC.

You can always export any FreeCAD model to IFC, even if it was not built with BIM tools or contains non-BIM elements. The non-BIM objects will simply be exported as generic IfcBuildingElementProxy entities, while BIM objects can have their class (IfcWall, IfcDoor,...) defined.

Native IFC projects, be it in locked or unlocked mode, always have an attached IFC file. There is no need to export anything anymore, you just need to save that IFC file. The way you save IFC contents differs depending on the mode:

• When working in locked mode, all of what happens in your FreeCAD document is saved to the IFC file. There is no need to save the FreeCAD file anymore, and the menu entries File → Save and File → Save As are replaced with File → Save IFC file and File → Save IFC file As.... Saving the IFC file is all that's needed, there is no more .FCStd file.

• When working in unlocked mode, each IFC project object in your document will be attached to a separate IFC file. A project object keeps track of changes and its icon will show a red dot when it contains unsaved changes. Saving your FreeCAD document will also save all the IFC files attached to IFC projects found in the document.

All IFC objects always have at least two properties: Class and Step ID. The class is the IFC class (IfcWall, IfcWindow, etc...) and the Step ID is the ID of the element in the IFC file. The Step ID is actually the sole data needed to locate the object in the IFC file. You can change the class of an object anytime, it won't have any impact over the geometry or property sets but might change available attributes (see below). You can only change to a sibling class, a parent class or a descendant class. If you want to change to a class that is very distant from the current class, you will need to know the IFC class structure and repeat the operation several times.

Attributes are additional hard-coded properties of IFC objects, such as Name, Description or ID. Attributes depend on the class. For example, IfcWindows also have Length and Width, IfcSites have Longitude and Latitude, etc. Attributes are always grouped under the IFC group and are always loaded together with an object.

When changing the class of an object, the attributes might change. You can edit the value of attributes, but you cannot add or remove attributes, as those are mandatory and defined by the IFC standard.

IFC properties, which are grouped under property sets, are custom properties that can be attributed to any IFC object. They are not loaded by default, but get loaded when you double-click an object. The property sets, which are property packs that get attached to objects, get rendered as property groups inside the FreeCAD properties tree.

By the IFC standards, all IFC files must contain one and only one IfcProject (or IfcProjectLibrary) object, which constitutes the root of your IFC model. All other IFC objects can be freely aggregated under this root object (an idea we call a "flat file"), but the IFC standards require to have at least one building container object (IfcSite, IfcBuilding or IfcBuildingStorey) under the root IfcProject. In FreeCAD, with the default options set, exported IFC files will always contain the IfcProject and at least one building container (default ones will be created if you don't have one).

You can also use groups inside your IFC projects. However, the IFC standard does not allow to nest groups inside a building structure. So these groups will be converted to IfcAssemblies. However, since most BIM applications will happily work with nested groups, you can turn off this conversion in IFC export preferences.

One of the main advantages of the native IFC system, is that only the things you change in FreeCAD get reflected in the IFC file attached to a project. As a result, that IFC file contains only very few changes, as opposed to traditional BIM applications where the IFC file is completely rewritten at each export.

This allows to precisely locate the changes inside the file. Moving a wall, for example, changes only one line: The placement of that wall. This in turn allows version control systems like Git to precisely identify those changes, and only save the increments of each new version.

You can get a visual representation (diff) of the changes in a selected IFC project by using menu Utils -> IFC Diff or, in unlocked mode, right-clicking an IFC project and selecting IFC -> Diff.

Editable geometric properties such as "Height" or "Width" are then added to the object. This is a work in progress, not all properties are supported yet. Changing the value of these properties will change the shape of the object.