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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 1 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. 3 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 5 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. 7 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 11 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 13 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) 14 (C) Drawing Interchange and File Formats 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. 15 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). 16 (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. 17 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: 18 (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). 19 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 20 (C) Drawing Interchange and File Formats 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. 21 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" 22 (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, 23 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. 24 (C) Drawing Interchange and File Formats 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