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[ This file is an excerpt from the AutoCAD Release 10 Reference Manual.
Copyright (C) 1988 Autodesk, Inc. All rights reserved. ]
Appendix C
Drawing Interchange and File Formats
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 <default>: (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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AutoCAD Reference Manual
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:
Enter decimal places of accuracy (0 to 16)/Binary <6>:
C.1.2 DXFIN Command - Loading a DXF File
A drawing interchange file can be converted into an AutoCAD drawing by
means of the DXFIN command. First enter the Drawing Editor using the
"Create new drawing" task from the Main Menu. Then issue the DXFIN com-
mand.
Command: DXFIN File name: (name)
Enter the name of the drawing interchange file to be loaded.
Full DXFIN
To load a complete DXF file, you must use DXFIN in an empty drawing,
before any entities have been drawn and before any additional Block
definitions, layers, linetypes, text styles, named views, named
coordinate systems, or named viewport configurations have been created.
(If your prototype draw- ing contains any such items, use Main Menu Task
1's "name=" technique to create a new drawing without a prototype.)
If any errors are detected during the input, the new drawing is
discarded. Otherwise, an automatic "ZOOM All" is performed to set the
drawing extents.
Partial DXFIN
If the current drawing is not empty, DXFIN loads only the ENTITIES
section of the DXF file, adding the entities found there to the current
drawing. In this case, DXFIN displays the message:
Not a new drawing -- only ENTITIES section will be input.
If errors are detected during such partial DXF input, the drawing is
returned to the state it was in before the DXFIN command. Otherwise, the
newly added entities are drawn.
2
(C) Drawing Interchange and File Formats
C.1.3 DXF File Format
This section describes the format of a DXF file in detail. It contains a
great deal of technical information that you need only if you're writing
your own program to process DXF files. Otherwise, you can skip this sec-
tion.
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 below.
C.1.3.1 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.
2. TABLES section -- This section contains definitions of named
items.
o Linetype (LTYPE) table
o Layer table
o Text style (STYLE) table
o View table
o User Coordinate System (UCS) table
o Viewport configuration (VPORT) table
o Drawing manager (DWGMGR) table (for future use)
3. BLOCKS section - This section contains Block Definition entities
describing the entities comprising 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 will
contain only the ENTITIES and END OF FILE sections, and the ENTITIES
section will reflect only the objects you select for output.
A DXF file is composed of a multiplicity of 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" format
(that is, right justified and blank filled in a three character field).
The second line of the group is the group value, in a format which
depends on the type of the group as specified by the group code.
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AutoCAD Reference Manual
The specific assignment of group codes depends upon 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:
Group code range Following value
0 - 9 String
10 - 59 Floating-point
60 - 79 Integer
210 - 239 Floating-point
999 Comment (string)
Thus a program can easily read the value following a group code without
knowing the particular use of this group in an item in the file. The
appearance of values in the DXF file is not affected by the setting of
the UNITS command: coordinates are always represented as decimal (or
possibly scientific notation if very large) numbers, and angles are
always repre- sented in decimal degrees with zero degrees to the east of
origin.
Variables, table entries, and entities are described by a group that
intro- duces 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.
Entities, table entries, and file separators are always introduced with
a 0 group code that is followed by a name describing the item.
C.1.3.2 Group Codes
Group codes are used both to indicate the type of the value of the
group, as explained above, and to indicate the general use of the group.
The spe- cific 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 "(fixed)" any that always have the same
function.
Group code Value type
0 Identifies the start of an entity, table entry, or file
separator. The text value that follows indicates which.
1 The primary text value for an entity.
2 A name; Attribute tag, Block name, etc.
3-4 Other textual or name values.
5 Entity handle expressed as a hexadecimal string.
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.
continued ...
4
(C) Drawing Interchange and File Formats
Group code Value type
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). Output only
if system variable FLATLAND is set to 1.
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 "Entities follow" flag (fixed).
70-78 Integer values, such as repeat counts, flag bits, or
modes.
210, 220, 230 X, Y, and Z components of extrusion direction.
999 Comments
C.1.4 Comments
The 999 group code indicates that the following line is a comment
string. DXFOUT does not currently include such groups in its 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:
999
This is a comment.
999
This is another comment.
C.1.5 File Sections
The DXF file is subdivided into four sections. 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.
0 (Begin HEADER section)
SECTION
2
HEADER
<<<<Header variable items go here>>>>
0
ENDSEC (End HEADER section)
0 (Begin TABLES section)
SECTION
2
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AutoCAD Reference Manual
TABLES
0
TABLE
2
VPORT
70
(viewport table maximum item count)
<<<<viewport table items go here>>>>
0
ENDTAB
0
TABLE
2
LTYPE, LAYER, STYLE, VIEW, UCS, or DWGMGR
70
(Table maximum item count)
<<<<Table items go here>>>>
0
ENDTAB
0
ENDSEC (End TABLES section)
0 (Begin BLOCKS section)
SECTION
2
BLOCKS
<<<<Block definition entities go here>>>>
0
ENDSEC (End BLOCKS section)
0 (Begin ENTITIES section)
SECTION
2
ENTITIES
<<<<Drawing entities go here>>>>
0
ENDSEC (End ENTITIES section)
0
EOF (End of file)
C.1.5.1 HEADER Section
The HEADER section of the DXF file contains settings of variables
associated with the drawing. These variables are set with various
commands and are the type of information displayed by the STATUS
command. Each variable is specified in the header section by a 9 group
giving its name, followed by groups that supply its value. The header
variables, the groups that follow, and their meanings are listed below.
Although this list is very similar to the list of system variables in
Appendix A, the two lists are not identical. Be sure you're referring to
the proper list.
6
(C) Drawing Interchange and File Formats
$ACADVER 1 the AutoCAD drawing database version number.
$ANGBASE 50 Angle 0 direction.
$ANGDIR 70 1=clockwise angles, 0=counterclockwise.
$ATTDIA 70 Attribute entry dialogues, 1 = on, 0 = off
$ATTMODE 70 Attribute visibility: 0=none, 1=normal, 2=all.
$ATTREQ 70 Attribute prompting during INSERT, 1 = on, 0 = off
$AUNITS 70 UNITS format for angles.
$AUPREC 70 UNITS precision for angles.
$AXISMODE 70 axis on if nonzero.
$AXISUNIT 10,20 axis X and Y tick spacing.
$BLIPMODE 70 blip mode on if nonzero.
$CECOLOR 62 entity color number; 0 = BYBLOCK, 256 = BYLAYER.
$CELTYPE 6 entity linetype name, or BYBLOCK or BYLAYER.
$CHAMFERA 40 first chamfer distance.
$CHAMFERB 40 second chamfer distance.
$CLAYER 8 current layer name.
$COORDS 70 0=static coordinate display, 1=continuous update,
2="d<a" format.
$DIMALT 70 alternate unit dimensioning performed if nonzero.
$DIMALTD 70 alternate unit decimal places.
$DIMALTF 40 alternate unit scale factor.
$DIMAPOST 1 alternate dimensioning suffix
$DIMASO 70 1=create associative dimensioning, 0=draw
individual entities.
$DIMASZ 40 dimensioning arrow size.
$DIMBLK 2 arrow block name.
$DIMBLK1 1 first arrow block name.
$DIMBLK2 1 second arrow block name.
$DIMCEN 40 size of center mark/lines.
$DIMDLE 40 dimension line extension.
$DIMDLI 40 dimension line increment.
$DIMEXE 40 extension line extension.
$DIMEXO 40 extension line offset.
$DIMLFAC 40 linear measurements scale factor.
$DIMLIM 70 dimension limits generated if nonzero.
$DIMPOST 1 general dimensioning suffix
$DIMRND 40 rounding value for dimension distances.
$DIMSAH 70 use separate arrow blocks if nonzero.
$DIMSCALE 40 overall dimensioning scale factor.
$DIMSE1 70 first extension line suppressed if nonzero.
$DIMSE2 70 second extension line suppressed if nonzero.
$DIMSHO 70 1=Recompute dimensions while dragging,
0=drag original image.
$DIMSOXD 70 suppress outside-extensions dimension lines
if nonzero.
$DIMTAD 70 text above dimension line if nonzero.
$DIMTIH 70 text inside horizontal if nonzero.
$DIMTIX 70 force text inside extensions if nonzero.
$DIMTM 40 minus tolerance.
$DIMTOFL 70 if text outside extensions, force line between
extensions if nonzero.
$DIMTOH 70 text outside horizontal if nonzero.
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AutoCAD Reference Manual
$DIMTOL 70 dimension tolerances generated if nonzero.
$DIMTP 40 plus tolerance.
$DIMTSZ 40 dimensioning tick size: 0=no ticks.
$DIMTVP 40 text vertical position.
$DIMTXT 40 dimensioning text height.
$DIMZIN 70 zero suppression for "feet & inch" dimensions.
$DRAGMODE 70 0=off, 1=on, 2=auto.
$ELEVATION 40 current elevation set by ELEV command.
$EXTMAX 10,20,30 XY drawing extents upper right corner (in WCS).
$EXTMIN 10,20,30 XY drawing extents lower left corner (in WCS).
$FILLETRAD 40 fillet radius.
$FILLMODE 70 FILL mode on if nonzero.
$FLATLAND 70 force compatibility with older versions if nonzero.
$HANDLING 70 handles enabled if nonzero.
$HANDSEED 5 next available handle.
$INSBASE 10,20,30 insertion base set by BASE command (in WCS).
$LIMCHECK 70 nonzero if limits checking is on.
$LIMMAX 10,20 XY drawing limits upper right corner (in WCS).
$LIMMIN 10,20 XY drawing limits lower left corner (in WCS).
$LTSCALE 40 global linetype scale.
$LUNITS 70 UNITS format for coordinates and distances.
$LUPREC 70 UNITS precision for coordinates and distances.
$MENU 1 name of menu file.
$MIRRTEXT 70 MIRROR text if nonzero.
$ORTHOMODE 70 ORTHO mode on if nonzero.
$OSMODE 70 running object snap modes.
$PDMODE 70 point display mode.
$PDSIZE 40 point display size.
$PLINEWID 40 default Polyline width.
$QTEXTMODE 70 quick text mode on if nonzero.
$REGENMODE 70 REGENAUTO mode on if nonzero.
$SKETCHINC 40 sketch record increment.
$SKPOLY 70 0=sketch lines, 1=sketch polylines.
$SPLFRAME 70 spline control polygon display, 1 = on, 0 = off.
$SPLINESEGS 70 number of line segments per spline patch.
$SPLINETYPE 70 spline curve type for "PEDIT Spline"
(see Appendix A).
$SURFTAB1 70 number of mesh tabulations in first direction.
$SURFTAB2 70 number of mesh tabulations in second direction.
$SURFTYPE 70 surface type for "PEDIT Smooth" (see Appendix A).
$SURFU 70 surface density (for "PEDIT Smooth") in M direction.
$SURFV 70 surface density (for "PEDIT Smooth") in N direction.
$TDCREATE 40 date/time of drawing creation.
$TDINDWG 40 cumulative editing time for this drawing.
$TDUPDATE 40 date/time of last drawing update.
$TDUSRTIMER 40 user elapsed timer.
$TEXTSIZE 40 default text height.
$TEXTSTYLE 7 current text style name.
$THICKNESS 40 current thickness set by ELEV command.
$TRACEWID 40 default Trace width.
$UCSNAME 1 Name of current UCS.
$UCSORG 10,20,30 origin of current UCS (in WCS).
8
(C) Drawing Interchange and File Formats
$UCSXDIR 10,20,30 direction of current UCS's X axis
(in World coordinates).
$UCSYDIR 10,20,30 direction of current UCS's Y axis
(in World coordinates).
$USERI1 - 5 70 Five integer variables intended for use by
third-party developers.
$USERR1 - 5 40 Five real variables intended for use by
third-party developers.
$USRTIMER 70 0=timer off, 1=timer on.
$WORLDVIEW 70 1=set UCS to WCS during DVIEW/VPOINT,
0=don't change UCS
The header variables listed below existed prior to AutoCAD Release 10
but now have independent settings for each active viewport. They are not
output by DXFOUT unless system variable FLATLAND is set to 1. DXFIN
honors these variables when read from DXF files, but if a VPORT symbol
table with "*ACTIVE" entries is present (as is true for any DXF file
produced by Release 10 or higher), the values in the VPORT table entries
will override the values of these header variables.
$FASTZOOM 70 fast zoom enabled if nonzero.
$GRIDMODE 70 grid mode on if nonzero.
$GRIDUNIT 10,20 grid X and Y spacing.
$SNAPANG 50 snap grid rotation angle.
$SNAPBASE 10,20 snap/grid base point (in UCS).
$SNAPISOPAIR 70 isometric plane: 0=left, 1=top, 2=right.
$SNAPMODE 70 snap mode on if nonzero.
$SNAPSTYLE 70 snap style: 0=standard, 1=isometric.
$SNAPUNIT 10,20 snap grid X and Y spacing.
$VIEWCTR 10,20 XY center of current view on screen.
$VIEWDIR 10,20,30 viewing direction (direction from target, in WCS).
$VIEWSIZE 40 height of view.
The date/time variables ($TDCREATE and $TDUPDATE) are output as real num-
bers in the format:
<Julian date>.<Fraction>
The elapsed time variables ($TDINDWG and $TDUSRTIMER) have a similar
format:
<Number of days>.<Fraction>
C.1.5.2 TABLES Section
The TABLES section contains several tables, each of which in turn
contains a variable number of table entries. The order of the tables may
change, but the LTYPE table will always precede the LAYER table. Each
table is intro- duced with a 0 group with the label "TABLE". This is
followed by a 2 group identifying the particular table (VPORT, LTYPE,
LAYER, STYLE, VIEW, UCS, or DWGMGR) and a 70 group that specifies the
maximum number of table entries
9
AutoCAD Reference Manual
that may follow. The tables in a drawing may contain deleted items, but
these are not written to the DXF file. Thus, fewer table entries may
follow the table header than are indicated by the 70 group, so don't use
the count in the 70 group as an index to read in the table. It is
provided so that your program to read DXF files can allocate an array in
advance large enough to hold all the table entries that follow.
Following this header for each table are the table entries. Each table
item consists of a 0 group identifying the item type (same as table
name, e.g., "LTYPE" or "LAYER"), a 2 group giving the name of the table
entry, a 70 group specifying flags relevant to the table entry (defined
for each table below), and additional groups that give the value of the
table entry. The end of each table is indicated by a 0 group with the
value "ENDTAB".
If any table entry has bit value 64 set in its group 70 flags, the table
entry was referenced by at least one entity in the drawing the last time
the drawing editor was entered to edit this drawing. This "referenced"
flag is for the benefit of the PURGE command; it can be ignored by most
programs that read DXF files, and need not be set by programs that write
DXF files.
The following are the groups used for each type of table item. All
groups are present for each table item.
LTYPE 3 (descriptive text for linetype), 72 (alignment code), 73
(number of dash length items), 40 (total pattern length), 49
(dash length 1), 49 (dash length 2), . . .
LAYER 62 (color number, negative if layer is off), 6 (linetype
name). The 1 bit is set in the 70 group flags if the layer is
frozen.
STYLE 40 (fixed text height; 0 if not fixed), 41 (width factor), 50
(obliquing angle), 71 (text generation flags), 42 (last height
used), 3 (primary font file name), 4 ("bigfont" file name;
blank if none). If the third bit (4) is set in the 70 group
flags, this is a vertically-oriented text style.
A STYLE table item is used to record shape file LOAD requests
also. In this case the first bit (1) is set in the 70 group
flags and only the 3 group (shape file name) is meaningful
(all the other groups are output, however).
The "text generation flags" are a bit-coded field with the
following bit meanings:
Flag bit value Meaning
2 Text is backwards (mirrored in X)
4 Text is upside down (mirrored in Y)
10
(C) Drawing Interchange and File Formats
VIEW 40 and 41 (view height and width), 10 and 20 (view center
point), 11, 21, 31 (view direction from target, in WCS), 12,
22, 32 (target point, in WCS), 42 (lens length), 43 and 44
(front and back clipping planes-offsets from target point), 50
(twist angle), 71 view mode (see VIEWMODE system variable
Appendix A).
UCS 10, 20, 30 (origin), 11, 21, 31 (X axis direction), 12, 22, 32
(Y axis direction). All in World coordinates.
VPORT 10 and 20 (lower left corner of viewport; 0.0 to 1.0), 11 and
21 (upper right corner), 12 and 22 (view center point), 13 and
23 (snap base point), 14 and 24 (snap spacing, X and Y), 15
and 25 (grid spacing, X and Y), 16, 26, 36 (view direction
from target point), 17, 27, 37 (view target point), 40 (view
height), 41 (viewport aspect ratio), 42 (lens length), 43 and
44 (front and back clipping planes; offsets from target
point), 50 (snap rotation angle), 51 (view twist angle), 71
(view mode; see VIEWMODE system variable in Appendix A), 72
(circle zoom percent), 73 (fast zoom setting), 74 (UCSICON
setting), 75 (snap on/off), 76 (grid on/off), 77 (snap style),
78 (snap isopair).
The VPORT table is unique in that it may contain several
entries with the same name (indicating a multiple-viewport
configuration). The entries corresponding to the active view-
port configuration all have the name "*ACTIVE". The first
such entry describes the current viewport.
DWGMGR For future use. Fields not yet defined.
C.1.5.3 BLOCKS Section
The BLOCKS section of the DXF file contains all the Block Definitions.
This section contains the entities that make up the Blocks used in the
drawing, including "anonymous" Blocks generated by the HATCH command and
by associative dimensioning. The format of the entities in this section
is identical to those in the ENTITIES section described below, so refer
to that section for details. All entities in the BLOCKS section appear
between BLOCK and ENDBLK entities. BLOCK and ENDBLK entities appear only
in the BLOCKS section. Block definitions are never nested (that is, no
BLOCK or ENDBLK entity ever appears within another BLOCK-ENDBLK pair).
C.1.5.4 ENTITIES Section
Entity items appear in both the BLOCK and ENTITIES sections of the DXF
file. The appearance of entities in the two sections is identical, with
the exception that entities in the BLOCK section never have handles. The
following gives the format of each entity as it appears in the file.
Some groups that define an entity always appear, and some are optional
and appear only if they differ from their default values. In the
following
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AutoCAD Reference Manual
discussion, groups that always occur are given by their group number and
function, while optional groups are indicated by "-optional N" following
the group description. "N" is the default value if the group is omitted.
Programs that read DXF files should not assume that the groups
describing an entity occur in the order given here. The end of the
groups that make up an entity is indicated by the next 0 group,
beginning the next entity or indicating the end of the section.
Remember that a DXF file is a complete representation of the drawing
data- base, and that as AutoCAD is further enhanced, new groups will be
added to entities to accommodate additional features. Writing your DXF
processing program in a table-driven way, making no assumptions about
the order of groups in an entity, and ignoring any groups not presently
defined, will make it much easier to accommodate DXF files from future
releases of AutoCAD.
Each entity begins with a 0 group identifying the entity type. The names
used for the entities are given in the table that follows. Every entity
contains an 8 group that gives the name of the layer on which the entity
resides. Each entity may have elevation, thickness, linetype, or color
information associated with it. If handles are enabled, every entity has
a 5 group containing its handle (as a string representing a hexadecimal
number). The following groups are included only if the entity has nonde-
fault values for these properties.
Group code Meaning
6 Linetype name (if not "BYLAYER"). The special name "BYBLOCK"
indicates a floating linetype.
38 Elevation (if nonzero). Output only if system variable
FLATLAND is 1. Otherwise, Z coordinates are supplied as
3x-groups as part of each of the entity's defining points.
39 Thickness (if nonzero).
62 Color number (if not "BYLAYER"). Zero indicates the
"BYBLOCK" (floating) color.
210, These groups are included for each Line, Point, Circle, Shape,
220, Text, Arc, Trace, Solid, Block Reference, Polyline, Dimension,
230 Attribute, and Attribute Definition entity if its extrusion
direction is not parallel to the World Z axis. The indicate
the X, Y, and Z components of the entity's extrusion direction.
The rest of the groups that make up an entity item are described below.
Many of the entities include "flag" groups. These are integer codes (6x
or 7x groups) that encode various pieces of information regarding the
entity, and are specific to the particular entity type. In the following
descrip- tions, the term "bit-coded" means that the flag contains
various true/false values coded as the sum of the bit values given. Any
bits not defined in the following section should be ignored in these
fields and set to zero when constructing a DXF file.
12
(C) Drawing Interchange and File Formats
LINE 10, 20, 30 (start point), 11, 21, 31 (end point).
POINT 10, 20, 30 (point), 50 (angle of X axis for the UCS in effect
when the Point was drawn -optional 0, for use when PDMODE is
nonzero).
CIRCLE 10, 20, 30 (center), 40 (radius).
ARC 10, 20, 30 (center), 40 (radius), 50 (start angle), 51 (end
angle).
TRACE Four points defining the corners of the trace: (10, 20, 30),
(11, 21, 31), (12, 22, 32), and (13, 23, 33).
SOLID Four points defining the corners of the solid: (10, 20, 30),
(11, 21, 31), (12, 22, 32), and (13, 23, 33). If only three
points were entered (forming a triangular solid), the third
and fourth points will be the same.
TEXT 10, 20, 30 (insertion point), 40 (height), 1 (text value), 50
(rotation angle -optional 0), 41 (relative X scale factor
-optional 1), 51 (obliquing angle -optional 0), 7 (text style
name -optional "STANDARD"), 71 (text generation flags
-optional 0), 72 (justification type -optional 0), 11, 21, 31
(alignment point -optional, appears only if 72 group is
present and nonzero).
The "text generation flags" are a bit-coded field with mean-
ings as follows:
Flag bit value Meaning
2 Text is backwards (mirrored in X)
4 Text is upside down (mirrored in Y)
The "justification type" value (not bit-coded) indicates the
text justification style used on this entity, as shown in the
following table.
Value Meaning
0 Text is left justified
1 Text is centered along its baseline
2 Text is right justified
3 Text is aligned between two points (height varies)
4 Text is "middle" (fully) centered
5 Text is fit between two points (width varies)
If the justification is anything other than 0 (left justi-
fied), 11, 21, and 31 groups will also appear in the entity
to specify the alignment point of the text (center, right-
most, or second alignment point).
DXFOUT handles ASCII control characters in text strings by
expanding the character into a "^" (caret) followed by the
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AutoCAD Reference Manual
appropriate letter. For example, an ASCII Control-G (BEL,
decimal code 7) is output as "^G". If the text itself con-
tains a caret character, it is expanded to "^ " (caret,
space). DXFIN performs the complementary conversion.
SHAPE 10, 20, 30 (insertion point), 40 (size), 2 (shape name), 50
(rotation angle -optional 0), 41 (relative X scale factor
-optional 1), 51 (obliquing angle -optional 0).
BLOCK 2 (Block name), 70 (Block type flags), 10, 20, 30 (Block base
point). Appears only in BLOCKS section. The "Block type
flags" are bit-coded, with the following bit meanings:
Flag bit value Meaning
1 This is an "anonymous" Block generated by
hatching, associative dimensioning, or
other internal operations.
2 This Block has Attributes.
ENDBLK No groups. Appears only in BLOCKS section.
INSERT 66 ("Attributes follow" flag -optional 0), 2 (Block name),
10, 20, 30 (insertion point), 41 (X scale factor -optional
1), 42 (Y scale factor -optional 1), 43 (Z scale factor
-optional 1), 50 (rotation angle -optional 0), 70 and 71
(column and row counts -optional 1), 44 and 45 (column and
row spacing -optional 0).
If the value of the "Attributes follow" flag is 1, a series
of Attribute (ATTRIB) entities is expected to follow the
INSERT, terminated by a sequence end (SEQEND) entity.
ATTDEF 10, 20, 30 (text start), 40 (text height), 1 (default value,
see TEXT above for handling of ASCII control characters), 3
(prompt string), 2 (tag string), 70 (Attribute flags), 73
(field length -optional 0), 50 (text rotation -optional 0),
41 (relative X scale factor -optional 1), 51 (obliquing angle
-optional 0), 7 (text style name -optional "STANDARD"), 71
(text generation flags -optional 0, see TEXT above), 72 (text
justification type -optional 0, see TEXT above)), 11, 21, 31
(alignment point -optional, appears only if 72 group is
present and nonzero).
The "Attribute flags" are a bit-coded field in which the bits
have the following meanings:
Flag bit value Meaning
1 Attribute is invisible (does not display)
2 This is a constant Attribute
4 Verification is required on input of this
Attribute.
8 Attribute is preset (no prompt during
insertion)
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ATTRIB 10, 20, 30 (text start), 40 (text height), 1 (value, see TEXT
above for handling of ASCII control characters), 2 (Attribute
tag), 70 (Attribute flags; see ATTDEF above), 73 (field
length -optional 0), 50 (text rotation -optional 0), 41 (rel-
ative X scale factor -optional 1), 51 (obliquing angle
-optional 0), 7 (text style name -optional "STANDARD"), 71
(text generation flags -optional 0, see TEXT above), 72 (text
justification type -optional 0, see TEXT above), 11, 21, 31
(alignment point -optional, appears only if 72 group is
present and nonzero).
POLYLINE 66 ("vertices follow flag"), 70 (Polyline flags), 40 (default
starting width), 41 (default ending width), 71 and 72 (poly-
gon mesh M and N vertex counts -optional 0), 73 and 74
(smooth surface M and N densities -optional 0), 75 (smooth
surface type -optional 0). The default widths apply to any
vertex that doesn't supply widths (see below).
The "vertices follow" flag is always 1, indicating that a
series of VERTEX entities is expected to follow the POLYLINE,
terminated by a sequence end (SEQEND) entity. The "polyline
flags" group is a bit-coded field with bits defined as fol-
lows:
Flag bit value Meaning
1 This is a closed Polyline (or a polygon
mesh closed in the M direction)
2 Curve-fit vertices have been added
4 Spline-fit vertices have been added
8 This is a 3D Polyline
16 This is a 3D polygon mesh. Group 75 indi-
cates the smooth surface type, as follows:
0 = no smooth surface fitted
5 = quadratic B-spline surface
6 = cubic B-spline surface
8 = Bezier surface
32 The polygon mesh is closed in the N direc-
tion
VERTEX 10, 20, 30 (location), 40 (starting width -optional, see
above), 41 (ending width -optional, see above), 42 (bulge),
70 (vertex flags), 50 (curve fit tangent direction
-optional). The bulge is the tangent of 1/4 the included
angle for an arc segment, made negative if the arc goes
clockwise from the start point to the end point; a bulge of 0
indicates a straight segment, and a bulge of 1 is a semicir-
cle. The meanings of the bit-coded "vertex flags" are shown
in the following table.
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AutoCAD Reference Manual
Flag bit value Meaning
1 Extra vertex created by curve fitting
2 Curve fit tangent defined for this vertex.
A curve fit tangent direction of 0 may be
omitted from the DXF output, but is signif-
icant if this bit is set.
4 Unused (never set in DXF files)
8 Spline vertex created by spline fitting
16 Spline frame control point
32 3D Polyline vertex
64 3D polygon mesh vertex
SEQEND No fields. This entity marks the end of vertices (VERTEX
type name) for a Polyline, or the end of Attribute entities
(ATTRIB type name) for an INSERT entity that has Attributes
(indicated by 66 group present and nonzero in INSERT entity).
3DLINE 10, 20, 30 (start point), 11, 21, 31 (end point).
3DFACE Four points defining the corners of the face: (10, 20, 30),
(11, 21, 31), (12, 22, 32), and (13, 23, 33). 70 (invisible
edge flags -optional 0). If only three points were entered
(forming a triangular face), the third and fourth points will
be the same. The meanings of the bit-coded "invisible edge
flags" are shown in the following table.
Flag bit value Meaning
1 First edge is invisible
2 Second edge is invisible
4 Third edge is invisible
8 Fourth edge is invisible
DIMENSION 2 (name of pseudo-Block containing the current dimension pic-
ture), 10, 20, 30 (definition point for all dimension types),
11, 21, 31 (middle point of dimension text), 12, 22, 32
(insertion point for clones of a dimension (for BASELINE and
CONTINUE), 70 (Dimension type; 0=rotated, horizontal, or ver-
tical; 1=aligned; 2=angular; 3=diameter; 4=radius - the value
128 is added to this field if the dimension text has been
positioned at a user-defined location rather than at the
default location), 1 (dimension text explicitly entered by
the user. If null, the dimension measurement is drawn as the
text. Otherwise, this text is drawn (but if it includes the
sequence "<>", the dimension measurement is drawn in place of
the "<>")), 13, 23, 33 (definition point for linear and angu-
lar dimensions), 14, 24, 34 (definition point for linear and
angular dimensions), 15, 25, 35 (definition point for diame-
ter, radius, and angular dimensions), 16, 26, 36 (point
defining dimension arc for angular dimensions), 40 (leader
length for radius and diameter dimensions), 50 (angle of
rotated, horizontal, or vertical linear dimensions).
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(C) Drawing Interchange and File Formats
In addition, all dimension types have an optional group (code
51) that indicates the "horizontal" direction for the Dimen-
sion entity. This determines the orientation of dimension
text and dimension lines for horizontal, vertical and rotated
linear dimensions. The group value is the negative of the
ECS angle of the UCS X axis in effect when the Dimension was
drawn. In other words, the X axis of the UCS in effect when
the Dimension was drawn is always parallel to the XY plane
for the Dimension's ECS, and the angle between the UCS X axis
and the ECS X axis is a single 2D angle. The value in group
51 is the angle from "horizontal" (the effective X axis) to
the ECS X axis. Entity Coordinate Systems (ECS) are
described later in this section.
For all dimension types, the following groups represent 3D
WCS points, regardless of the FLATLAND setting.
10, 20, 30
13, 23, 33
14, 24, 34
15, 25, 35
For all dimension types, the following groups represent ECS
points, and are 2D or 3D depending on the FLATLAND setting.
11, 21(, 31)
12, 22(, 32)
16, 26(, 36)
Linear (13,23,33) The point used to specify the first extension line.
(14,24,34) The point used to specify the second extension line.
(10,20,30) The point used to specify the dimension line.
Angular (13,23,33) and (14,24,34) The endpoints of the first line
(10,20,30) and (15,25,35) The endpoints of the second line
(16,26,36) The point used to specify the dimen-
sion line arc
Diameter (15,25,35) The point used to pick the circle/arc to dimension
(10,20,30) The point on that circle directly across from the
pick point.
Radius (15,25,35) The point used to pick the circle/arc to dimension
(10,20,30) The center of that circle.
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AutoCAD Reference Manual
Entity Coordinate Systems (ECS)
To save space in the drawing database (and in the DXF file), the points
associated with each entity are expressed in terms of its own Entity
Coor- dinate System (ECS). The Entity Coordinate System allows AutoCAD
to use a much more compact means of representation for entities. With
ECS, the only additional information needed to describe its position in
3D space is the 3D vector describing the Z axis of the ECS, and the
elevation value.
For a given Z axis (or extrusion) direction, there is an infinite number
of coordinate systems, defined by translating the origin in 3D space and
by rotating the X and Y axes around the Z axis. However, for the same Z
axis direction, there is only one Entity Coordinate System. It has the
follow- ing properties:
o Its origin coincides with the WCS origin.
o The orientation of the X and Y axes within the XY plane are calcu-
lated in an arbitrary, but consistent manner. AutoCAD performs
this calculation using the "arbitrary axis" algorithm described
below.
For some entities, the ECS is equivalent to the World Coordinate System and
all points (DXF groups 10-37) are expressed in World coordinates. See the
following table.
Entities Notes
LINE, POINT, 3DFACE, 3D These entities do not lie in
Polyline, 3D Vertex, 3D a particular plane. All
Mesh, 3D Mesh vertex points are expressed in
World coordinates. Of these
entities, only Lines and
Points can be extruded;
their extrusion direction can
differ from the World Z axis.
CIRCLE, ARC, SOLID, TRACE, These entities are planar in
TEXT, ATTRIB, ATTDEF, SHAPE, nature. All points are
INSERT, 2D Polyline, 2D expressed in Entity coordi-
Vertex nates. All these entities
can be extruded; their
extrusion direction can
differ from the World Z axis.
DIMENSION Some of a Dimension's points are
expressed in WCS, and some in ECS.
Others The remaining entities have
no point data and their
coordinate systems are
therefore irrelevant.
Once AutoCAD has established the ECS for a given entity, here's how it
works:
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(C) Drawing Interchange and File Formats
o The elevation value stored with an entity indicates how far along
the Z axis to shift the XY plane from the WCS origin to make it
coincide with the plane that the entity is in. How much of this
is the user-defined elevation is unimportant.
o Any 2D points describing the entity that were entered through the
UCS are transformed into the corresponding 2D points in the ECS,
which (more often than not) is shifted and rotated with respect to
the UCS.
A few ramifications of this process are:
o You can not reliably find out what UCS was in effect when an
entity was acquired. You can only find out where the entity is in
the current UCS if the current UCS has the same Z axis direction
as the original UCS (i.e., they both reduce to the same ECS).
o When you enter the XY coordinates of an entity in a given UCS and
then do a DXFOUT, you probably won't recognize those XY coordi-
nates in the DXF file. You'll have to know the method by which
AutoCAD calculates the X and Y axes in order to work with these
values.
o The elevation value stored with an entity and output in DXF files
will be a sum of the Z coordinate difference between the UCS XY
plane and the ECS XY plane, and the elevation value that the user
specified at the time the entity was drawn.
Arbitrary Axis Algorithm
The arbitrary axis algorithm is used by AutoCAD internally to implement
the "arbitrary but consistent" generation of Entity Coordinate Systems
for all entities except Lines, Points, 3D Faces, and 3D Polylines, which
contain points in World coordinates.
Given a unit-length vector to be used as the Z axis of a coordinate
system, the arbitrary axis algorithm generates a corresponding X axis
for the coor- dinate system. The Y axis follows by application of the
right hand rule.
The method is to examine the given Z axis (also called the normal
vector) and see if it is close to the positive or negative World Z axis.
If it is, cross the World Y axis with the given Z axis to arrive at the
arbitrary X axis. If not, cross the World Z axis with the given Z axis
to arrive at the arbitrary X axis. The boundary at which the decision is
made was chosen to be both inexpensive to calculate and completely
portable across machines. This is achieved by having a sort of "square"
polar cap, the bounds of which is 1/64, which is precisely specifiable
in 6 decimal frac- tion digits and in 6 binary fraction bits.
In mathematical terms, the algorithm does the following (all "vectors"
are assumed to be in 3D space, specified in the World Coordinate
System).
Let the given normal vector be called N.
Let the World Y axis be called Wy, which is always (0,1,0).
Let the World Z axis be called Wz, which is always (0,0,1).
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AutoCAD Reference Manual
We are looking for the arbitrary X and Y axes to go with the normal N.
They'll be called Ax and Ay. N could also be called Az (the arbitrary Z
axis).
If (Nx < 1/64) and (Ny < 1/64) then
Ax = Wy * N (where "*" is the cross-product operator).
Otherwise,
Ax = Wz * N.
Scale Ax to unit length.
The method of getting the Ay vector would be:
Ay = N * Ax.
Scale Ay to unit length.
C.1.6 Writing DXF Interface Programs
Writing a program that communicates with AutoCAD via the DXF mechanism
often appears far more difficult than it really is. The DXF file
contains a seemingly overwhelming amount of information, and examining a
DXF file manually may lead to the conclusion that the task is hopeless.
However, the DXF file has been designed to be easy to process by
program, not manually. The format was constructed with the deliberate
intention of making it easy to ignore information you don't care about
while easily reading the information you need. Just remember to handle
the groups in any order and ignore any group you don't care about, and
you'll be home free.
As an example, the following is a Microsoft BASIC program that reads a
DXF file and extracts all the LINE entities from the drawing (ignoring
lines that appear inside Blocks). It prints the endpoints of these lines
on the screen. As an exercise you might try entering this program into
your com- puter, running it on a DXF file from one of your drawings,
then enhancing it to print the center point and radius of any circles it
encounters. This program is not put forward as an example of clean
programming technique nor the way a general DXF processor should be
written; it is presented as an example of just how simple a DXF-reading
program can be.
1000 REM
1010 REM Extract lines from DXF file
1020 REM
1030 G1% = 0
1040 LINE INPUT "DXF file name: "; A$
1050 OPEN "i", 1, A$ + ".dxf"
1060 REM
1070 REM Ignore until section start encountered
1080 REM
1090 GOSUB 2000
1100 IF G% <> 0 THEN 1090
1110 IF S$ <> "SECTION" THEN 1090
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1120 GOSUB 2000
1130 REM
1140 REM Skip unless ENTITIES section
1150 REM
1160 IF S$ <> "ENTITIES" THEN 1090
1170 REM
1180 REM Scan until end of section, processing LINEs
1190 REM
1200 GOSUB 2000
1210 IF G% = 0 AND S$ = "ENDSEC" THEN 2200
1220 IF G% = 0 AND S$ = "LINE" THEN GOSUB 1400 : GOTO 1210
1230 GOTO 1200
1400 REM
1410 REM Accumulate LINE entity groups
1420 REM
1430 GOSUB 2000
1440 IF G% = 10 THEN X1 = X : Y1 = Y : Z1 = Z
1450 IF G% = 11 THEN X2 = X : Y2 = Y : Z2 = Z
1460 IF G% = 0 THEN PRINT "Line from (";X1;",";Y1;",";Z1;") to (";X2;
",";Y2;",";Z2;")
1470 GOTO 1430
2000 REM
2010 REM Read group code and following value
2020 REM For X coordinates, read Y and possibly Z also
2030 REM
2040 IF G1% < 0 THEN G% = -G1% : G1% = 0 ELSE INPUT #1, G%
2050 IF G% < 10 OR G% = 999 THEN LINE INPUT #1, S$ : RETURN
2060 IF G% >= 38 AND G% <= 49 THEN INPUT #1, V : RETURN
2080 IF G% >= 50 AND G% <= 59 THEN INPUT #1, A : RETURN
2090 IF G% >= 60 AND G% <= 69 THEN INPUT #1, P% : RETURN
2100 IF G% >= 70 AND G% <= 79 THEN INPUT #1, F% : RETURN
2110 IF G% >= 210 AND G% <= 219 THEN 2130
2120 IF G% >= 20 THEN PRINT "Invalid group code";G% : STOP
2130 INPUT #1, X
2140 INPUT #1, G1%
2150 IF G1% <> (G%+10) THEN PRINT "Invalid Y coord code";G1% : STOP
2160 INPUT #1, Y
2170 INPUT #1, G1%
2180 IF G1% <> (G%+20) THEN G1% = -G1% ELSE INPUT #1, Z
2190 RETURN
2200 CLOSE 1
Writing a program that constructs a DXF file is more difficult, because
you must maintain consistency within the drawing in order for AutoCAD to
find it acceptable. AutoCAD allows you to omit many items in a DXF file
and still obtain a usable drawing. The entire HEADER section can be
omitted if you don't need to set any header variables. Any of the tables
in the TABLES section can be omitted if you don't need to make any
entries, and in fact the entire TABLES section can be dropped if nothing
in it is required. If you define any linetypes in the LTYPE table, this
table must appear before the LAYER table. If no Block Definitions are
used in the drawing, the BLOCKS section can be omitted. If present,
however, it must appear before the ENTITIES section. Within the ENTITIES
section, you can refer- ence layer names even though you haven't defined
them in the LAYER table.
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AutoCAD Reference Manual
Such layers will be automatically created with color 7 and the
CONTINUOUS linetype. The EOF item must be present at the end of file.
The following Microsoft BASIC program constructs a DXF file representing
a polygon with a specified number of sides, leftmost origin point, and
side length. This program supplies only the ENTITIES section of the DXF
file, and places all entities generated on the default layer "0". This
may be taken as an example of a minimum DXF generation program. Since
this pro- gram doesn't create the drawing header, the drawing limits,
extents, and current view will be invalid after performing a DXFIN on
the drawing gener- ated by this program. You can do a "ZOOM E" to fill
the screen with the drawing generated. Then adjust the limits manually.
1000 REM
1010 REM Polygon generator
1020 REM
1030 LINE INPUT "Drawing (DXF) file name: "; A$
1040 OPEN "o", 1, A$ + ".dxf"
1050 PRINT #1, 0
1060 PRINT #1, "SECTION"
1070 PRINT #1, 2
1080 PRINT #1, "ENTITIES"
1090 PI = ATN(1) * 4
1100 INPUT "Number of sides for polygon: "; S%
1110 INPUT "Starting point (X,Y): "; X, Y
1120 INPUT "Polygon side: "; D
1130 A1 = (2 * PI) / S%
1140 A = PI / 2
1150 FOR I% = 1 TO S%
1160 PRINT #1, 0
1170 PRINT #1, "LINE"
1180 PRINT #1, 8
1190 PRINT #1, "0"
1200 PRINT #1, 10
1210 PRINT #1, X
1220 PRINT #1, 20
1230 PRINT #1, Y
1240 PRINT #1, 30
1250 PRINT #1, 0.0
1260 NX = D * COS(A) + X
1270 NY = D * SIN(A) + Y
1280 PRINT #1, 11
1290 PRINT #1, NX
1300 PRINT #1, 21
1310 PRINT #1, NY
1320 PRINT #1, 31
1330 PRINT #1, 0.0
1340 X = NX
1350 Y = NY
1360 A = A + A1
1370 NEXT I%
1380 PRINT #1, 0
1390 PRINT #1, "ENDSEC"
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(C) Drawing Interchange and File Formats
1400 PRINT #1, 0
1410 PRINT #1, "EOF"
1420 CLOSE 1
The DXFIN command is relatively forgiving with respect to the format of
data items. As long as a properly formatted item appears on the line on
which the data is expected, DXFIN will accept it (of course, string
items should not have leading spaces unless these are intended to be
part of the string). The above program takes advantage of this
flexibility in input format, and does not go to great effort to generate
a file appearing exactly like one generated by AutoCAD.
In the case of error loading a DXF file using DXFIN, AutoCAD reports the
error with a message indicating the nature of the error detected and the
last line processed in the DXF file before the error was detected. This
may not be the line on which the error occurred, especially in the case
of such errors as omission of required groups.
C.2 Binary Drawing Interchange Files
The ASCII DXF file format described in the preceding sections of this
appendix is a complete representation of an AutoCAD drawing in an ASCII
text form easily processed by other programs. In addition, AutoCAD can
produce or read a binary form of the full DXF file, and accepts limited
input in another binary file format. These binary files are described in
the following sections.
C.2.1 Binary DXF Files
The DXFOUT command provides a "Binary" option that writes binary DXF
files. Such a file contains all of the information present in an ASCII
DXF file, but in a much more compact form that takes, typically, 25%
less file space and can be read and written more quickly (typically 5
times faster) by AutoCAD. Unlike ASCII DXF files, which entail a
trade-off between size and floating-point accuracy, binary DXF files
preserve all of the accuracy in the drawing database. AutoCAD Release 10
is the first version to support this form of DXF file; it cannot be read
by older versions.
A binary DXF file begins with a 22-byte sentinel consisting of:
"AutoCAD Binary DXF<CR><LF><SUB><NUL>"
Following the sentinel are (group,value) pairs as in an ASCII DXF file, but
represented in binary form. The group code is a single-byte binary value,
and the value that follows is one of the following:
o a two-byte integer with the least significant byte first and the
most significant byte last,
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AutoCAD Reference Manual
o an eight-byte IEEE double precision floating-point number stored
with the least significant byte first and the most significant
byte last, or
o an ASCII string terminated by a zero (NUL) byte.
The type of the datum following a group is determined from the group
code according to the same rules used in decoding ASCII DXF files.
Translation of angles to degrees, and dates to fractional Julian date
representation, is performed for binary files as well as for ASCII DXF
files. The comment group, 999, is not used in binary DXF files.
DXFOUT writes binary DXF files with the same file type (".dxf") as for
ASCII DXF files. The DXFIN command automatically recognizes a binary
file (by means of its sentinel string) and loads it. There is no need
for you to identify it as a binary file.
If DXFIN encounters an error in a binary DXF file, it reports the byte
address within the file where the error was detected.
C.3 Binary Drawing Interchange (DXB) Files
The DXF file formats described earlier in this appendix are complete
repre- sentations of an AutoCAD drawing that can be written and read by
AutoCAD and other programs. However, AutoShade(tm) and programs executed
via the "external commands" facility (Appendix B) often have a need to
supply simple geometric input to AutoCAD. For these purposes, another
file format even more compact than the binary DXF format is supported.
This format, called DXB (for "drawing interchange binary") is limited in
the entities it can represent. Furthermore, AutoCAD has a command to
read such files, but no direct method of writing them. (The ADI plotter
driver can plot to a file in DXB format.)
C.3.1 DXBIN Command
To load a DXB file produced by a program such as AutoShade, enter the DXBIN
command:
Command: DXBIN
DXB file:
enter the name of the file you wish to load. Don't include a file type;
".dxb" is assumed.
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C.3.2 DXB File Format
This information is for experienced programmers, and is subject to change
without notice.
The format of a DXB file is as follows:
Header: "AutoCAD DXB 1.0" CR LF ^Z NUL (19 bytes)
Data: . . . Zero or more data records . . .
Terminator: NUL (1 byte)
Each data record begins with a single byte giving its type, followed by
data items. Th
