drawing interchange and file formats release 12(AutoCAD DXF 12.0版文件格式说明)
copyright (c) 1982-1990, 1992 autodesk, inc. all rights reserved autocad can be used by itself as a complete drawing editor. in some
applications, 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
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 and
then send the descriptions to a more powerful computer for finite-
element structural analysis. you can compute stresses and
displacements and send back information to display the deformed
structure as an autocad drawing. since the autocad drawing database (.dwg file) is written in a
compact format that changes significantly as new features are added
to autocad, 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) 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 specification
(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 the autocad internal format.
ascii drawing interchange (dxf) files
************************************* this section describes the autocad dxf (drawing interchange file)
format and the commands provided to read and write these files. dxf
files are standard ascii text files. they can easily be translated
to the formats of other cad systems or submitted to other programs
for specialized analysis. autocad can also produce or read a binary
form of the full dxf file. this feature is described in detail later
in this chapter.
dxfout command - writing a dxf file
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you can generate a drawing interchange file from an existing drawing
by means of the dxfout command: command: dxfout when autocad prompts you, respond with a filename or press 5 to
accept the default. 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 filename, you do not need to include a file type; .dxf is
assumed. if a file with the same name already exists, the existing
file is deleted. if you specify the file using a file dialogue box,
and a file with the same name already exists, autocad tells you;
allowing you to ok or cancel the deletion. next, dxfout asks what
precision you want for floating-point numbers and permits output of
a partial dxf file containing only selected objects. | 
