drawing interchange and file formats(AutoCAD DXF 10.0版文件格式说明)

[ this file is an excerpt from the autocad release 10 reference manual.
  copyright (c) 1988 autodesk, inc.  all rights reserved. ]

autocad can be used by itself as a complete drawing editor.  in some appli-
cations, however, other programs must examine drawings created by autocad
or generate drawings to be viewed, modified, or plotted with autocad.

for example, if you've made an architectural drawing with autocad, using
inserted parts to represent windows, doors, and so on, you can process the
drawing file and produce a bill of materials of all the items used in the
drawing, or even make energy use calculations based on the area and the
number and type of windows used.  another possible application is to use
autocad to describe structures that are then sent to a large computer for
finite element structural analysis.  you can compute stresses and displace-
ments and send back information to display the deformed structure as an
autocad drawing.

since the autocad drawing database (.dwg file) is written in a very compact
format that changes significantly from time to time as new features are
added, we do not document its format and do not recommend that you attempt
to write programs to read it directly.  to assist in interchanging drawings
between autocad and other programs, a "drawing interchange" file format
(dxf(tm)) has been defined.  all implementations of autocad accept this
format and are able to convert it to and from their internal drawing file
representation.

autocad also supports the initial graphics exchange standard (iges) file
format.  the information comprising an autocad drawing can be written out
in iges format, and iges files can be read and converted to autocad's
internal format.

c.1  ascii drawing interchange (dxf) files

this section describes autocad's dxf (drawing interchange) file format and
the commands provided to read and write these files.  dxf files are stan-
dard ascii text files.  they can easily be translated to the formats of
other cad systems, or submitted to other programs for specialized analysis.

c.1.1  dxfout command - writing a dxf file

you can generate a drawing interchange file from an existing drawing by
means of the drawing editor's dxfout command.  the command format is:

    command:  dxfout   file name :  (name or return)

the default name for the output file is the same as that of the current
drawing, but with a file type of ".dxf".  if you specify an explicit file
name, do not include a file type; ".dxf" is assumed.  if a file with the
same name already exists, it is deleted.  next, dxfout asks what precision

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you want for floating-point numbers and permits output of a partial dxf
file containing only selected objects.

    enter decimal places of accuracy (0 to 16)/entities/binary <6>:

the "binary" option is described later in this appendix. if you respond
with "entities" (or just "e"), dxfout will ask you to select the objects
you want written to the dxf file.  only the objects you select will be
included in the output file - symbol tables (including block definitions)
will not be included.  once you've selected the desired objects, autocad
will prompt again for the numeric precision:
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