# AutoCAD DWG

OGR supports reading most versions of AutoCAD DWG when built with the Open Design Alliance Teiga library. DWG is an binary working format used for AutoCAD drawings. A reasonable effort has been made to make the OGR DWG driver work similarly to the OGR DXF driver which shares a common data model. The entire contents of the .dwg file is represented as a single layer named "entities".

DWG files are considered to have no georeferencing information through OGR. Features will all have the following generic attributes:

- Layer: The name of the DXF layer. The default layer is "0".
- SubClasses: Where available, a list of classes to which an element belongs.
- ExtendedEntity: Where available, extended entity attributes all appended to form a single text attribute.
- Linetype: Where available, the line type used for this entity.
- EntityHandle: The hexadecimal entity handle. A sort of feature id.
- Text: The text of labels.

A reasonable attempt is made to preserve line color, line width, text size and orientation via OGR feature styling information when translating elements. Currently no effort is made to preserve fill styles or complex line style attributes.

The approximation of arcs, ellipses, circles and rounded polylines as linestrings is done by splitting the arcs into subarcs of no more than a threshhold angle. This angle is the OGR_ARC_STEPSIZE. This defaults to four degrees, but may be overridden by setting the configuration variable OGR_ARC_STEPSIZE.

## DWG_INLINE_BLOCKS

The default behavior is for block references to be expanded with the geometry of the block they reference. However, if the DWG_INLINE_BLOCKS configuration option is set to the value FALSE, then the behavior is different as described here.

- A new layer will be available called blocks. It will contain one or more features for each block defined in the file. In addition to the usual attributes, they will also have a BlockName attribute indicate what block they are part of.
- The entities layer will have new attributes BlockName, BlockScale, and BlockAngle.
- block referencesd will populate these new fields with the corresponding information (they are null for all other entities).
- block references will not have block geometry inlined - instead they will have a point geometry for the insertion point.

The intention is that with DWG_INLINE_BLOCKS disabled, the block references will remain as references and the original block definitions will be available via the blocks layer.

## Building

Currently DWG building is somewhat adhoc. On linux the normal practice is to hand edit gdal/ogr/ogrsf_frmts/dwg/GNUmakefile, update paths, and then build the driver as a plugin using the "make plugin" target.