By Randall S. Newton
When someone uses MicroStation, the information created is saved in a file format called DGN. If the same work were to take place in AutoCAD, that same data would be saved in a file format called DWG. Would the drawings or models look any different? Probably not. Then why, many creative professionals wonder, does every CAD vendor have to use their own file format?
The answer has little to do with architecture, engineering or other design functions and everything to do with the history and internal workings of the various CAD products. “There are natural reasons,” says Keith Bentley, co-founder of Bentley Systems. He cites the evolutionary development of a product, intent of use, and a developer's pace of innovation as factors that shape a CAD program's native file format. Bentley's DGN format, for example, has its roots in a product called pseudoStation, the predecessor to MicroStation from the early 1980's that was the first program to store and edit Intergraph CAD data on personal computers. The “pseudo” part of the name reflects its origins in the Pascal programming language, a programming environment that was abandoned by most software developers years ago. A second strong influence on DGN is Bentley's insistence that the file format should be stable over time. Bentley counts the world's largest AEC firms among its clients, creating projects that span years. Including the early pseudoStation, DGN has only changed three times.
AutoCAD was first created for computers that used the CP/M operating system, another environment that fell by the wayside long ago. Autodesk has modified the DWG format at least 11 times over the years, as a way to improve AutoCAD as both user requests and hardware matured. Most AutoCAD customers are smaller firms, doing work with a shorter design time; frequent changes to the native file format are less disruptive.
CAD-using professionals have three primary file types at their service. Autodesk COO Carl Bass refers to these three formats as native formats used to initially create and store data, interchange formats to share editable data between applications, and publishing formats to share design data with a wider audience. Keith Bentley agrees with these three, but adds a fourth specific to MicroStation, mapped file types. “We write schemas to 'map' the features of other file types to DGN,” Keith Bentley told attendees at the 2005 BE Conference earlier this year. “We did this for DWG, for DGN V7, and we will be introducing a schema for ESRI SHP files” in MicroStation V8 XM Edition, which was announced May 2005 and is expected to ship soon.
In his general session presentation at Autodesk University 2005, Autodesk's Bass talked about three processes for design data: create, manage, and share, on which he elaborated, “create smartly, manage securely, and share immediately.” Good file formats, he told me later in a private discussion, are written to match these objectives. All three objectives, and the file format behind them, are undergoing change. “Create smartly,” Bass says, now means the use of model based design. Keith Bentley thinks designs are getting better because of technology. “3D helps the visualization process in design,” he notes, adding that today's digital mock-ups are models. Now that architects and engineers can quickly study “thousands of permutations, not just tens, you can get a better design.”
“Manage smartly” requires tools that can hold and organize data for use throughout the project or product lifecycle. “Share immediately” requires the use of tools and formats that allow quick transit of information to non-technical users, so widgets can be built, roads can be paved and buildings can rise into place.
In his main stage presentation at this year's BE Conference, Keith Bentley discussed the purposes of each primary file type. Native file formats are designed for quick and complete ability to edit data. They should be compact, so that they open and close quickly. They must allow direct accessibility to the data, they must be extensible (easy to add new data types). They are always specific to the originating program.
Interchange file formats (including STEP, IGES, DXF, the IFC specifications, and the various dialects of XML) are defined by standards bodies. They are generally program neutral (Autodesk's DXF is an exception), flexible, and verbose. Interchange formats are all about import/export. The fidelity of these various interchange formats vary with both the format and the specific implementation.
The disagreement between Autodesk and its competitors about the use of DWG speaks to the heart of a file format's suitability to purpose. Because of its dominance in the market, DWG has come to be used by design professionals as an interchange format as well as a native file format, despite its lack of suitability for such a purpose. The Open Design Alliance exists to create a shadow DWG independent of Autodesk. The ODA believes DWG must be freed from the control of any single vendor, even though in doing so they change the primary purpose of DWG from data creation to data interchange. Autodesk continues to resist the notion of DWG for interchange, preferring the use of Autodesk-specified DXF (the specifications are published) or any of the external, standards-based interchange formats.
At this year's Autodesk University, a main stage presentation showed the use of building data exported from Autodesk Architectural Desktop (ADT) to Autodesk Revit. Instead of using its own DWG or DXF formats, Autodesk demonstrated the use of IFC's, the architectural data meta-format created by the International Alliance for Interoperability. ADT is based on Autodesk, and uses DWG; Revit is a next-generation CAD modeler that has a completely different native file format. It was both prudent and courageous for Autodesk to turn to a neutral format for this particular data exchange. Prudent because it works, courageous because there must have been internal pressure to create an in-house solution for such an important data transfer.
By design, Keith Bentley says, publishing file formats are meant to be immutable, repeatable, and universal. Such format support printing and markup. These formats are created by multiple sources, not just the authoring software. Such formats include Adobe's popular PDF, Autodesk's DWF, a new 3D format called Universal 3D promoted by Intel, JT Open from UGS, 3DXML from Dassault Systèmes, various printing formats, and the new Metro publishing format coming in the next version of Microsoft Windows. Battle lines are being drawn by the vendors who support these various formats; AECnews will detail this new file format war in another article.
One thing to keep in mind: “File formats aren't everything,” Bass notes. “Applications are key.”