.. _vector.dwg:

AutoCAD DWG
===========

.. shortname:: DWG

.. build_dependencies:: Open Design Alliance Teigha library

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
threshold 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 referenced 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.
