dxf12.htm

刚刚看到本站有Visual C++数字图象处理（人民邮电出版社）的电子书

as decimal (or possibly scientific notation if very large) numbers, 
and angles are always represented in decimal degrees with zero 
degrees to the east of origin.</pre>
    <pre>variables, table entries, and entities are described by a group that 
introduces the item, giving its type and/or name, followed by 
multiple groups that supply the values associated with the item. in 
addition, special groups are used for file separators such as 
markers for the beginning and end of sections, tables, and the file 
itself.</pre>
    <pre>entities, table entries, and file separators are always introduced 
with a 0 group code that is followed by a name describing the item.</pre>
    <pre>note: the maximum dxf file string length is 256 characters. if your 
autocad drawing contains strings that exceed this number, those 
strings are truncated during dxfout. if your dxf file contains 
strings that exceed this number, dxfin will fail.
</pre>
    <pre>group codes
-----------
group codes are used both to indicate the type of the value of the 
group, as explained earlier, and to indicate the general use of the 
group. the specific function of the group code depends on the actual 
variable, table item, or entity description. this section indicates 
the general use of groups, noting as &quot;(fixed)&quot; any that always have 
the same function.</pre>
    <pre>table 11-2. autocad entity group codes (by number) 
+===============================================================+
| group code | value type                                       |
|------------|--------------------------------------------------|
| 0          | identifies the start of an entity, table entry,  |
|            | or file separator. the type of entity is given   |
|            | by the text value that follows this group        |
|------------|--------------------------------------------------|
| 1          | the primary text value for an entity             |
|------------|--------------------------------------------------|
| 2          | a name: attribute tag, block name, and so on.    |
|            | also used to identify a dxf section or table name|
|------------|--------------------------------------------------|
| 3-4        | other textual or name values                     |
|------------|--------------------------------------------------|
| 5          | entity handle expressed as a hexadecimal string  |
|            | (fixed)                                          |
|------------|--------------------------------------------------|
| 6          | line type name (fixed)                           |
|------------|--------------------------------------------------|
| 7          | text style name (fixed)                          |
|------------|--------------------------------------------------|
| 8          | layer name (fixed)                               |
|------------|--------------------------------------------------|
| 9          | variable name identifier (used only in header    |
|            | section of the dxf file)                         |
|------------|--------------------------------------------------|
| 10         | primary x coordinate (start point of a line or   |
|            | text entity, center of a circle, etc.)           |
|------------|--------------------------------------------------|
| 11-18      | other x coordinates                              |
|------------|--------------------------------------------------|
| 20         | primary y coordinate. 2n values always           |
|            | correspond to 1n values and immediately follow   |
|            | them in the file                                 |
|------------|--------------------------------------------------|
| 21-28      | other y coordinates                              |
|------------|--------------------------------------------------|
| 30         | primary z coordinate. 3n values always           |
|            | correspond to 1n and 2n values and immediately   |
|            | follow them in the file                          |
|------------|--------------------------------------------------|
| 31-37      | other z coordinates                              |
|------------|--------------------------------------------------|
| 38         | this entity's elevation if nonzero (fixed).      |
|            | exists only in output from versions prior to r11 |
|------------|--------------------------------------------------|
| 39         | this entity's thickness if nonzero (fixed)       |
|------------|--------------------------------------------------|
| 40-48      | floating-point values (text height, scale        |
|            | factors, etc.)                                   |
|------------|--------------------------------------------------|
| 49         | repeated value - multiple 49 groups may appear   |
|            | in one entity for variable length tables (such   |
|            | as the dash lengths in the ltype table). a 7x    |
|            | group always appears before the first 49 group   |
|            | to specify the table length                      |
|------------|--------------------------------------------------|
| 50-58      | angles                                           |
|------------|--------------------------------------------------|
| 62         | color number (fixed)                             |
|------------|--------------------------------------------------|
| 66         | &quot;entities follow&quot; flag (fixed)                   |
|------------|--------------------------------------------------|
| 67         | identifies whether entity is in model space or   |
|            | paper space                                      |
|------------|--------------------------------------------------|
| 68         | identifies whether viewport is on but fully off  |
|            | screen, is not active, or is off                 |
|------------|--------------------------------------------------|
| 69         | viewport identification number                   |
|------------|--------------------------------------------------|
| 70-78      | integer values such as repeat counts, flag       |
|            | bits, or modes                                   |
|------------|--------------------------------------------------|
| 210,       | x, y, and z components of extrusion direction    |
| 220,       | (fixed)                                          |
| 230        |                                                  |
|------------|--------------------------------------------------|
| 999        | comments                                         |
|------------|--------------------------------------------------|
| 1000       | an ascii string (up to 255 bytes long) in        |
|            | extended entity data                             |
|------------|--------------------------------------------------|
| 1001       | registered application name (ascii string up to  |
|            | 31 bytes long) for xdata (fixed)                 |
|------------|--------------------------------------------------|
| 1002       | extended entity data control string (&quot;{&quot; or &quot;}&quot;) |
|            | (fixed)                                          |
|------------|--------------------------------------------------|
| 1003       | extended entity data layer name                  |
|------------|--------------------------------------------------|
| 1004       | chunk of bytes (up to 127 bytes long) in         |
|            | extended entity data                             |
|------------|--------------------------------------------------|
| 1005       | extended entity data database handle             |
|------------|--------------------------------------------------|
| 1010,      | extended entity data x, y, and z coordinates     |
| 1020,      |                                                  |
| 1030       |                                                  |
|------------|--------------------------------------------------|
| 1011,      | extended entity data x, y, and z coordinates of  |
| 1021,      | 3d world space position                          |
| 1031       |                                                  |
|------------|--------------------------------------------------|
| 1012,      | extended entity data x, y, and z components of   |
| 1022,      | 3d world space displacement                      |
| 1032       |                                                  |
|------------|--------------------------------------------------|
| 1013,      | extended entity data x, y, and z components of   |
| 1023,      | 3d world space direction                         |
| 1033       |                                                  |
|------------|--------------------------------------------------|
| 1040       | extended entity data floating-point value        |
|------------|--------------------------------------------------|
| 1041       | extended entity data distance value              |
|------------|--------------------------------------------------|
| 1042       | extended entity data scale factor                |
|------------|--------------------------------------------------|
| 1070       | extended entity data 16-bit signed integer       |
|------------|--------------------------------------------------|
| 1071       | extended entity data 32-bit signed long          |
+---------------------------------------------------------------+
</pre>
    <pre>comments 
========
the 999 group code indicates that the following line is a comment 
string. dxfout does not currently include such groups in a dxf 
output file, but dxfin honors them and ignores the comments. thus, 
you can use the 999 group to include comments in a dxf file you've 
edited. for example:</pre>
    <pre>   999
   this is a comment.
   999
   this is another comment.
</pre>
    <pre>file sections
=============
the dxf file is subdivided into four editable sections, plus the end 
of file marker. file separator groups are used to delimit these file 
sections. the following is an example of a void dxf file with only 
the section markers and table headers present:</pre>
    <pre>   0            (begin header section)
  section
   2
  header
               &lt;&lt;&lt;&lt;header variable items go here&gt;&gt;&gt;&gt;
  0
  endsec       (end header section)
   0           (begin tables section)
  section
   2
  tables
   0
  table
   2
  vport
   70
  (viewport table maximum item count)
               &lt;&lt;&lt;&lt;viewport table items go here&gt;&gt;&gt;&gt;
  0
  endtab
  0
  table
  2
  appid, dimstyle, ltype, layer, style, ucs, view, or vport
  70
  (table maximum item count)
               &lt;&lt;&lt;&lt;table items go here&gt;&gt;&gt;&gt;
  0
  endtab
  0
  endsec       (end tables section)
  0            (begin blocks section)
  section
  2
  blocks
               &lt;&lt;&lt;&lt;block definition entities go here&gt;&gt;&gt;&gt;
  0
  endsec       (end blocks section)
  0            (begin entities section)
  section
  2
  entities
               &lt;&lt;&lt;&lt;drawing entities go here&gt;&gt;&gt;&gt;
  0
  endsec       (end entities section)
  0
  eof          (end of file)</pre>
    <pre>