Open DWG File

Computer Aided Design and Manufacturing

DWG file format is a proprietary file format used by Autodesk AutoCAD and several other CAD applications. Because DWG file format was so widely popular (it is estimated that more than 2 billion DWG files were available in 1998) it was used as a basis for many CAD applications. Autodesk as the biggest creator of DWG files only provided limited access to DWG format via RealDWG library for non-competitive uses. That is why many software vendors reverse engineered DWG file format in order to read and write it in their applications. The most successful effort was Open Design Alliance – a non-profit consortium consisting of many CAD software vendors. Still due to controversy of the DWG reverse engineering and trademark ownership there was a long chain of lawsuits between Autodesk, Open Design Alliance and multiple other companies. As a result of these lawsuits DWG trademark is not registered to any company, but Open Design Alliance also agreed not to use DWG in their marketing. Still DWG read/write libraries existence is not breaking any agreements as far as DWG trademark is not used. Below is a brief overview of DWG file format.

DWG is a binary file format. It means that data in it is not human readable and needs to be parsed by specific software. Below is a structure of R13/R14/R15 DWG file:
HEADER
    FILE HEADER
    DWG HEADER VARIABLES
    CRC
CLASS DEFINITIONS
TEMPLATE (R13 only, optional)
PADDING (R13C3 AND LATER, 200 bytes, minutes the template section above if present) IMAGE DATA (PRE-R13C3)
OBJECT DATA
    All entities, table entries, dictionary entries, etc. go in this section.
OBJECT MAP
OBJECT FREE SPACE (optional)
TEMPLATE (R14-R15, optional)
SECOND HEADER
IMAGE DATA (R13C3 AND LATER)


In general DWG file format organization is somewhat similar to IGES and STEP CAD data exchange formats with main difference of DWG being binary. Also performance of the software processing DWG file will be typically faster because of the binary format.

Full DWG file format specification is available at Open Design Alliance DWG specification document.