drawing interchange and file formats release 12(AutoCAD DXF 12.0版文件格式说明)

dxf file format
===============
this section describes the format of a dxf file in detail. it 
contains technical information that you need only if you write your 
own programs to process dxf files or work with entity information 
obtained by certain autolisp and ads functions.

it would probably be helpful to produce a dxf file from a small 
drawing, print it out, and refer to it occasionally while reading 
the information presented next.

general file structure
----------------------
a drawing interchange file is simply an ascii text file with a file 
type of .dxf and specially formatted text. the overall organization 
of a dxf file is as follows:

1. header section - general information about the drawing is found 
   in this section of the dxf file. each parameter has a variable 
   name and an associated value (see table 11-3 for a list of the 
   header variables).

2. tables section - this section contains definitions of named 
   items.

   o linetype table (ltype)
   o layer table (layer)
   o text style table (style)
   o view table (view)
   o user coordinate system table (ucs)
   o viewport configuration table (vport)
   o dimension style table (dimstyle)
   o application identification table (appid)

3. blocks section - this section contains block definition entities 
   describing the entities that make up each block in the drawing.

4. entities section - this section contains the drawing entities, 
   including any block references.

5. end of file

if you use dxfout's entities option, the resulting dxf file contains 
only the entities section and the end of file marker, and the 
entities section reflects only the objects you select for output.

note: if you select an insert entity, the corresponding block 
definition is not included in the output file.

a dxf file is composed of many groups, each of which occupies two 
lines in the dxf file. the first line of a group is a group code, 
which is a positive nonzero integer output in fortran i3 - that is, 
right-justified and blank filled in a three-character field (the 
exception to this is the four-digit extended entity data group 
codes, which are output in fortran i4). the second line of the group 
is the group value, in a format that depends on the type of group 
specified by the group code. although dxfout output has a fixed 
format, the dxfin format is free.

the specific assignment of group codes depends on the item being 
described in the file. however, the type of the value this group 
supplies is derived from the group code in the following way:

table 11-1. group code ranges
+================================+
| group code  | following value  |
| range       |                  |
|-------------|------------------|
| 0 - 9       | string           |
|-------------|------------------|
| 10 - 59     | floating-point   |
|-------------|------------------|
| 60 - 79     | integer          |
|-------------|------------------|
| 140 - 147   | floating-point   |
|-------------|------------------|
| 170 - 175   | integer          |
|-------------|------------------|
| 210 - 239   | floating-point   |
|-------------|------------------|
| 999         | comment (string) |
|-------------|------------------|
| 1010 - 1059 | floating-point   |
|-------------|------------------|
| 1060 - 1079 | integer          |
|-------------|------------------|
| 1000 - 1009 | string           |
+--------------------------------+
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