Flaws Found in MicroStation/AutoCAD Compatibility
Conversion of AutoCAD files to MicroStation V8 not quite perfect, reports Ralph Grabowski, CAD journalist and author of "AutoCAD for MicroStation Users"
reprinted by permission from Ralph Grabowski, editor, upFront.eZine, June 18, 2002
Back in the early 1990s, with MicroStation v4's Nexxus add-on, Bentley Systems was the very first non-AutoCAD product to read AutoCAD's .dwg files (until then, translation was via the documented .dxf format). Writing .dwg came with a later update.
At the time, there was a good deal of speculation whether Autodesk would allow this use of its proprietary .dwg file. Some software vendors have been hostile to the point of launching lawsuits over the manner in which their file formats had been reverse-engineered. Autodesk did not hinder Bentley from proceeding, perhaps recognizing that a translation is a flawed copy of the original. In addition Autodesk's "100% Pure DWG" campaign was probably effective in discouraging potential AutoCAD customers from defecting to competitors.
With MicroStation V8, Bentley Systems upped the ante, claiming that "the MicroStation V8 file format is a superset of DGN and DWG, allowing users to share data in hybrid workflows without translating files." Industry watchers feel, however, that V8 still translates .dwg, albeit through the use of a "schema" first developed for Bentley's ProjectBank product.
Benley's 'MicroStation Manager' magazine defines schema as helping "migrate design file elements to more capable components... Thus when you import a design file in ProjectBank, every element in the file is examined and a corresponding component is created that can be recreated faithfully when you extract the design file back from ProjectBank. ... the DWG Schema defines a superset of all AutoCAD elements in the AutoCAD DWG file."
I suppose the idea is to attract AutoCAD users, particularly those in dual-CAD shops -- firms that use both AutoCAD and MicroStation. To help convince them, Bentley hired eTesting Labs <http://etestinglabs.com> to prepare a whitepaper. The lab, a Ziff-Davis Media company, describes its logo as telling customers "that you cared enough about your test to go with the company whose objectivity, integrity, and reliability the entire industry recognizes." It's not clear, however, WHICH industry they mean.
The whitepaper, titled "Improving Teamwork and Collaboration in Design Environments", says it describes the "rigorous testing by eTesting Labs that shows the fidelity and consistency that Bentley provides in saving, modifying, and writing AutoCAD files." eTesting Labs performed tests using the following four sample drawings delivered with recent releases of AutoCAD:
- Opera.dwg - a 3D drawing of the Sydney opera house that features non-rectangular paperspace viewports, solid model components, and a rendered view.
- MKMplan.dwg - a 2D architectural floorplan.
- R300-200.dwg - an exploded view of an air cylinder made from 2D objects, 3D surface meshes, and 3D ACIS solids. Includes an embedded raster image and multiple rendered images of the cylinder and its sections.
- Truckmodel.dwg - similar to Opera.dwg but of a truck.
The whitepaper says four tests were conducted:
- Visual Fidelity - the drawings were viewed in both MicroStation V8 and AutoCAD 2002. Result: "No discernable visual difference found."
- Element Fidelity - CompareDWG2000 from Furix was used to examine the .dwg files after they were read and written by MicroStation V8. ("Element" is Bentley's term for objects.) Result: "...discrepancies ... of no consequence."
- Order Fidelity - Design files that ship with MicroStation V8 were "converted" to .dwg, and opened in AutoCAD. Result: Visual inspection and use of the Properties tool (Element Information tool) found "no discernable differences." ("Design file" is Bentley's term for a drawing file.)
- Data Integrity - AutoCAD's Layer command was used to "ensure full fidelity was achieved." Result: "No differences" found in the number of layers, nor their properties.
Unreported by the Testing Process
That MicroStation was able to handle the four drawings is impressive, considering their complexity -- varied objects, non-rectangular viewports, embedded images, and multiple renderings. Using the four drawings, however, does not tell the entire story:
- Visual Fidelity - eTesting Labs didn't use .dwg files known to cause visual problems in non-AutoCAD products. These test drawings include Budweiser (developed by CAD Studio), XrefDemo and Column (Autodesk sample drawings), and my own Layer300 test drawing. The whitepaper does not report whether drawings were plotted, and then checked for visual integrity.
- Element Fidelity - No significant problems were found by the Furix software, but it appears that the drawings were not edited. IntelliCAD, for example, preserves .dwg data that it is unable to edit, so that the written version looks identical to the read version. eTesting reported one problem it considered to be of "no consequence": arcs had their rotation reversed by MicroStation.
- Order Fidelity - The whitepaper did not list the MicroStation files used for testing. It's not reported whether ALL objects were checked with AutoCAD's Properties command; the MKMPlan drawing, for example, contains 7,993 objects in model space.
- Data Integrity - Layers are just one of many "table" structures stored in a .dwg file. The whitepaper did not report whether additional tables were checked: blocks, dimension styles, linetypes, text styles, named UCSs, viewports, and named views. (A tip to eTesting Labs: AutoCAD's Rename command provides a convenient overview of all table objects in the drawing.)
Other issues that eTesting Labs could consider: the difference in user interface between MicroStation and AutoCAD; the differences in command structure; differences in customization and programming; and the difference in jargon.
The whitepaper is available online at: http://etestinglabs.com/main/reports/microstation_v8.pdf.
What Do Newsgroups Report?
At Bentley's DWG newsgroup, <news://news.viecon.com/bentley.microstation.v8.dwg> the following problems were reported by users during June 1-17 (after Ziff Davis released its whitepaper):
- ACIS bodies don't convert consistently.
- Lineweight display toggled when applying a saved view.
- No command like AutoCAD's purge.
- .dwg imported read-only randomly.
- Multi-level cells (multi-layer blocks) are placed on incorrect layers.
- Geometry from a 3D solid created in MDT v5 does not export.
- Tags (attribute text) fail to display.
- Reference files with non-zero insertion base scale incorrectly.
- Color 9 is displayed as an almost unreadable light grey.
- Fails some aspects of the "Budweiser" test drawing.
- Does not translate the layer states file.
Some of these problems were "user" problems (i.e., the MicroStation user did not understand how AutoCAD works), but other problems were recognized by Ray Bentley as bugs that need to be fixed.