Evan on Engineering Software

There is an interesting thread about Autodesk's Trusted DWG over on the AutoCAD CAD manager's discussion group. It's full of lots of misinformation, conjecture, and apologist blather. (But it's still interesting.)

Trusted DWG, a new "feature" in AutoCAD 2007-based products, is a marketing term for a more aggressive variant of the DWGCHECK function that's been in AutoCAD since R14. In versions of AutoCAD from 2004 and up, the program writes an encrypted digital signature into DWG files. Upon opening a DWG file, AutoCAD 2007 checks this signature. If it isn't there (and correct), the program pops up a warning dialog box. (While it's possible that AutoCAD actually does some integrity checking in this process, my current information is that the only check it makes is for the validity of the digital signature.)

What "Trusted" means, in this context, is that AutoCAD trusts the file – not that the file is inherently more trustworthy. (That being said, 2007 DWG files do incorporate Reed-Solomon error correction, which is of particular use in correcting burst-errors resulting from media defects, such as scratched CDs. It's of no use in assuring that a DWG file is written correctly to start with, but is a real help in recovering DWG files that were damaged in storage.)

One of the significant causes of bad DWG files is poor consistency checking on the part of the application that writes the file. DWG files have many internal interrelations and critical values – and if these aren't correct when they're written, the resulting file may trip up an application that reads it later. This is true for AutoCAD (especially with ARX applications), or for any applications that can write DWG files.

The older OpenDWG Toolkit libraries (from the Open Design Alliance) relied upon application programmers to know the critical interrelations and values in the DWG files they were creating. In some cases, these programmers missed some details, and the resulting applications could write DWG files that required auditing before they could be opened. For the last several years, the Alliance's DWGdirect libraries have done this checking automatically – and as a result, applications using these newer libraries generally create very high quality DWG files.

I believe that the biggest problem with the Trusted DWG function in AutoCAD 2007 is that it is inherently anti-competitive, as it warns users about DWG files that were written by application programs from companies other than Autodesk – even if they are perfectly valid files.

Because of this, the Alliance plans to include functions for writing DWG digital signatures in the next version of our DWGdirect libraries. Applications using this version of DWGdirect will be able to write DWG files that AutoCAD-based applications will open with no error messages or warning screens.