Design Engineering

CAD Report: Evolving DWG

By Ralph Grabowski   

CAD/CAM/CAE Autodesk Bricsys DWG

Bricsys bets on the DWG file format for CAD (and all else).

Drawings stored in AutoCAD’s DWG format are so prevalent that Belgium-based CAD vendor Bricsys is pinning its success on using the aging format for every aspect of its CAD software. In the opinion of the company’s officers, users shouldn’t endure more than one file format just to use CAD systems specialized for MCAD, ECAD, BIM or GIS.

For Bricsys CEO Erik de Keyser, DWG is sufficiently flexible to handle all the extra data needed by 3D MCAD and BIM models (see Figure 1). His BricsCAD software accomplishes this by storing the unique data sets in these areas of the DWG file:

  • Extended entity data (a.k.a. xdata)
  • Xrecords (like xdata but not limited in size
  • Extension dictionaries (for document-level definitions)
  • Named object dictionaries (for object-level definitions, a.k.a. tables)
  • Custom entities (for defining entities not available in plain AutoCAD)

These parts of DWG are not secret; Autodesk provides APIs (application programming interfaces) so that third-party developers can write and read data into these locations. It’s a simple matter for Bricsys to combine the data locations with documented API calls to extend DWG to many disciplines of CAD.

Bricsys embraced and extended AutoCAD’s APIs, specifically LISP, DCL (for designing dialog boxes), Diesel (used for menu macros and the command line), the all-powerful ARx (which Bricsys calls Brx), Tx (licensed from the Open Design Alliance), .Net, COM, ADS (obsoleted by Autodesk but still supported by Bricsys as SDS), and VBA (licensed from Microsoft).

BricsCAD Bricsys - DWG

Figure 1: BricsCAD modeling a tripod through an assembly of 3D parts

Bricsys figures it has the solution to a problem Autodesk created for itself and its MCAD and BIM customers. Users who deal with a half-dozen file formats would prefer a single format, he thinks. At one time, Autodesk recognized the problem when it showed a nascent universal file format that held data from many of its CAD programs. It was based on Navisworks, but then work stalled as Autodesk acquired and launched more products with their own file formats.

Autodesk’s Move Away from DWG

It wasn’t always this complex. Autodesk’s first releases of MCAD ran on AutoCAD – Mechanical Desktop (MDT) and AutoCAD Mechanical. They employed DWG files with custom objects, for which Autodesk provided “object enablers” so that non-MDT users could view and edit the specialized drawings.

But then Autodesk developed Inventor to replace MDT, and an entirely new file format – well, four of them, actually: .ipt files to store 3D parts in Inventor; .iam for assemblies; .idw for 2D drawings; and .ipn for presentations.

The formats were designed to make a fundamental break from AutoCAD. That was okay at the time because, in the late 1990s, DWG was seen as yesteryear’s file format, destined to fade away. Years later, the company realized it misread DWG’s decline; a multiplicity of emerging AutoCAD clones made DWG more prominent. Autodesk spent years giving Inventor and AutoCAD an ability to read each other’s drawings. The same sequence of events befell Revit, also developed in the late 1990s.

More recently, Autodesk decided to begin replacing the Windows-bound Inventor with a new MCAD program that could operate online, Fusion. Users faced new file formats designed for today’s multi-user, multi-server world.

The same compatibility crisis affected Dassault Systèmes, whose mid-range Solidworks software is data-incompatible with its high-end 3DExperience line. The compatibility problem is sufficiently complex that after ten years of effort, Dassault has not solved it to its satisfaction.

Bricsys management saw the contortions taking place among their competitors and wondered if the solution could be the reverse: Not abandoning DWG but extending it to meet the needs of parts and assemblies, BIM, sheet metal, GIS and more.

A single file format meant no disruption due to translation between CAD systems and project engineers. Modeling and editing techniques developed for one discipline (MCAD, say) could be reused for others, such as BIM. No need for multiple teams of programmers working on silo’ed software.

Autodesk Will Do It Differently 

The reason Autodesk didn’t stick with DWG is probably technology-related; it’s simply easier to write a new file system from scratch than to adapt 22-year-old DWG. Even so, Autodesk now says it will also go with a single file format for its future design software, although to call it a “file format” is inaccurate. At Autodesk University 2016, the company said its future lies in what it calls Project Quantum, currently in pre-alpha. In terms of data storage, that translates to a database that runs in the cloud. On the application side, design software like Revit will be rewritten as a group of collaborative “apps” that perform specific functions. These apps would then access the data they need from a shared, online storage system, while users have their own workspaces.

For Bricsys, it’s also technology but for different reasons. In part, the company is showing off; by adapting DWG, the company doesn’t need to work out new file formats and solve the subsequent tedious translation issues. On the marketing side, it uses the sole-source DWG to lure the users of millions of DWG files, saying, “It doesn’t matter what you do, we can edit it with one program” – albeit in some cases with extra-cost add-ons.

The most important concept in MCAD and BIM is assemblies, and Bricsys figured out how to do them with DWG: attach xrefs (parts) in 3D with a 3D constraint system, which the company wrote for itself. To show what’s possible, Bricsys built its own add-on app for doing sheet metal design ($300). Much of its MCAD technology is being applied to their BIM add-on module ($240).

Bricsys DWG Files Not Necessarily AutoCAD-compatible

Being compatible with AutoCAD is not a concern of de Keyser. He wants new customers bringing their AutoCAD drawings to BricsCAD, not the other way around – as do most other CAD vendors. All the stuff that BricsCAD adds to DWG files, however, is incompatible with AutoCAD. While both CAD systems use the ACIS solid modeling kernel (well, Autodesk uses a variant that it modified on its own), the two have incompatible constraint management systems, for instance.

I found that AutoCAD has no problems opening and displaying models constructed in Bricsys. The 3D parts look accurate, (See Figure 2). I tested a BIM model of a house, a sheet metal part and an assembly of 3D parts. Editing is limited, however, as AutoCAD lacks the ability to define BIM slabs, bend sheet metal or constrain parts in 3D.

CAD Report Bricsys

Figure 2: BricsCAD-made 3D model opened in AutoCAD.

For accessing files from other CAD systems, Bricsys licensed translators to handle all the usual neutral formats (STEP, IGES, and so on) and many MCAD systems, like Creo, Inventor and CATIA. The company has also added intelligence to BricsCAD to analyze incoming 3D models using design intent and direct editing – similar to what SpaceClaim is famous for. The Bricsys Communicator translation add-on is US$610; Autodesk translates models in the cloud and does not charge for the service.

Anything can be stored in DWG, but it is not necessarily the best place for everything. On the AutoCAD side, shared data like hatch patterns, linetypes and block libraries are stored externally. The same happens in BricsCAD, where it stores shared data like bending tables for sheet metal and material definitions for BIM slabs in their own files.

The ODA also is modernizing DWG by adapting it to multi-user streaming environments, along with change management and project archiving. This will let several people work on a single DWG file online. The organization credits Onshape for some of its inspiration.

Bricsys executives are betting the company on a file format they do not control. Whatever Autodesk does to DWG in the years to come, however, is immaterial to the millions of CAD users not employing AutoCAD. Their security rests on the one or two billion DWG files that are already in place. Firms will always need to access them, update them and even start anew.

A unified file system is becoming important to the future of the CAD industry, as we see electrical diagrams become part of MCAD models housed in BIM buildings situated on GIS/DTM terrains being viewed in VR and AR. History shows that a more convenient product often overcomes a superior one, and DWG is sufficiently a common denominator that it could become the one pervasive file format for all CAD.

Ralph Grabowski writes on the business of CAD in his weekly upFront.eZine newsletter. He is the author of many articles and books about AutoCAD, BricsCAD, Visio and other graphics software. He also maintains the WorldCAD Access blog at


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