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THE DXF FORMAT ************** The DXF file (Drawing Interchange File) is an ASCII text file with a file type of ".dxf" and specially formatted text. The overall organisation 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 - Linetype (LTYPE) table - Layer table - Style table - View table 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 A DXF file is composed of multiple 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 "13" format (ie, right justified and blank filed in a three character field). The second line of the group is the 'group value', in a formatwhich depends on the type of the group as specified by the group code. The specific assignment of group codes depends on the item being described in the file. However the type of the value in 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 999 Comments within DXF file The appearance of values in the DXF file is not affected by the settings of the UNITS command; coordinates are always represented as decimal (or possibly scientific notation if very large) numbers and angles are always represented in decimal degrees with zero degrees to the east of the origin. Entites, table entries, and file separators are always introduced with a 0 group code followed by a name descibing the item. GROUP CODES *********** Group codes are used both to indicate the type of value of the group and to indicate the general use of the group. The specific function of the group 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. 0 Identifies the start of an entity, table entry or file separator The text value that follows indicates which 1 The primary text value of an entity 2 A name; Attribute tag, Block name etc. 3-5 Other textual or name values 6 Line type name (fixed) 7 Text style name (fixed) 8 Layer name (fixed) 9 Variable name identifier (used only in HEADER section of DXF file) 10 Primary X coordinate (start point of a Line or Text entity, centre of a Circle etc.) 11-18 Other X coordinates 20 Primary Y coordinate. 2n values always correspond to 1n values and follow them in a file. 21-28 Other Y coordinates 30 Primary Z coordinate. 3n values always correspond to 1n and 2n values and follow them in a file. 31-36 Other Z coordinates (future) 38 The entity's elevation if nonzero (fixed) 39 The 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 Colour number (fixed) 66 "Enities follow" flag (fixed) 70-78 Integer values such as repeat counts, flag bits, or modes 999 Used to include comments (DXFIN ignores the following line) 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 where only the section markers and table headers are present. 0 SECTION (Begin HEADER section) 2 HEADER <<<< Header variables go here >>>> 0 ENDSEC (End HEADER section) 0 (Begin TABLES section) SECTION 2 TABLES 2 LTYPE 70 (Line type table maximum item count) <<<< Line type items go here >>>> 0 ENDTAB 0 TABLE 2 LAYER 70 (Layer table maximum item count) <<<< Layer table items go here >>>> 0 ENDTAB 0 TABLE 2 STYLE 70 (Text style table maximum item count) <<<< Text style table items go here >>>> 0 ENDTAB 0 TABLE 2 VIEW 70 (View table maximum item count) <<<< View table type 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) 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 as follows: $ACADVER 1 (The AutoCAD drawing database version number) $ANGBASE 50 (Angle 0 direction) $ANGDIR 70 (1 = clockwise, 0 = counterclockwise) $ATTMODE 70 (Attribute visibility: 0 = none, 1 = normal, 2 = all) $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 spacing) $BLIPMODE 70 (Blip mode on if nonzero) $CECOLOR 62 (Entity colour 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, 1 = continuous update, 2 = "d<a") $DIMALT 70 (Alternate unit dimensioning performed if nonzero) $DIMALTD 70 (Alternate unit precision, number of decimal places) $DIMALTF 40 (Alternate unit scale factor) $DIMAPOST 1 (Alternate text suffix) $DIMASO 70 (Associative dimensioning mode 1 = on, 0 = off) $DIMASZ 40 (Dimensioning arraow size) $DIMBLK 2 (Arrow block name) $DIMCEN 40 (Size of centre marks/lines) $DIMDLE 40 (Dimension line extension) $DIMDLI 40 (Dimension line increment) $DIMEXE 40 (Extension line extension) $DIMEXO 40 (Extension line offset) $DIMLFAC 40 (Linier measurements scale factor) $DIMLIM 70 (Dimension limits generated if nonzero) $DIMPOST 1 (General text suffix) $DIMRND 40 (Rounding value for dimension distances) $DIMSCALE 40 (Overall dimensioning scale factor) $DIMSE1 70 (First extension line suppressed if nonzero) $DIMSE2 70 (Second extension line suppressed if nonzero) $DIMSHO 70 (Update dimension entities while dragging if nonzero) $DIMTAD 70 (Text above dimension line if nonzero) $DIMTIH 70 (Text inside horizontal if nonzero) $DIMTM 40 (Minus tolerance) $DIMTOH 70 (Text outside horizontal if nonzero) $DIMTOL 70 (Dimension tolerances generated if nonzero) $DIMTP 40 (Plus tolerance) $DIMTSZ 40 (Dimensioning ticksize, 0 = no ticks) $DIMTXT 40 (Dimensioning text height) $DIMZIN 70 (Include zero inches in text if nonzero) $DRAGMODE 70 (0 = off, 1 = on, 2 = auto) $DRAGP1 70 (Regen-drag input sampling rate) $DRAGP2 70 (Fast-drag input sampling rate) $ELEVATION 40 (Current elevation set by ELEV commannd) $EXTMAX 10 20 (Drawing extents upper right corner) $EXTMIN 10 20 (Drawing extents lower left corner) $FASTZOOM 70 (0 = off, 1 = on) $FILLETRAD 40 (Fillet radius) $FILLMODE 70 (Fill mode on if nonzero) $GRIDMODE 70 (Gridmode on if nonzero) $GRIDUNIT 10 20 (Grid X and Y spacing) $HIGHLIGHT 70 (1 = highlight selected objects, 0 = don't highlight) $INSBASE 10 20 (Insertion base set by BASE command) $LIMCHECK 70 (Nonzero if limits checking is on) $LIMMAX 10 20 (Drawing limits upper right corner) $LIMMIN 10 20 (Drawing limits lower left corner) $LTSCALE 40 (Global linetype scale) $LUNITS 70 (Units format for coordinates and distances) $LUPREC 70 (Units precision for coordinates and distance) $MENU 1 (Name of menu file) $MIRRTEXT 70 (Mirror text if nonzero) $ORTHOMODE 70 (Ortho mode on if nonzero) $OSMODE 70 (Running object snap mode) $PDMODE 70 (Point display mode) $PDSIZE 40 (Point display size) $PLINEWID 40 (Default polyline width) $QTEXTMODE 70 (Quick text mode if nonzreo) $REGENMODE 70 (RegenAuto mode on if nonzero) $SKETCHINC 40 (Sketch record increment) $SKPOLY 70 (0 = sketch lines, 1 = sketch polylines) $SNAPANG 50 (Snap grid rotation angle) $SNAPBASE 10 20 (Snap grid base point) $SNAPISOPAIR 70 (Isometric plane 0 = left, 1 = top, 2 = right) $SNAPMODE 70 (Snap mode on if nonzero) $SNAPSTYLE 70 (0 = standard, 1 = isometric) $SNAPUNIT 10 20 (Snap grid X and Y spacing) $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) $USRTIMER 70 (0 = timer off, 1 = timer on) $VIEWCTR 10 20 (Centre of current view on screen) $VIEWDIR 10 20 30 (Current viewpoint set by VPOINT command) $VIEWSIZE 40 (Height of current view on screen) The date / time variables ($TDCREATE and $TDUPDATE) are output as real numbers in the format: <Julian date>.<Fraction> The elapsed time variables ($TDINDWG and $TDUSRTIMER) have a similar format: <Number of days>.<Fraction> TABLES SECTION ************** The Tables section of the DXF file contains up to four tables each of which in turn contains a variable number of table entries. The tables always appear in the order given above (Ltype, Layer, Style, View). Each table in the TABLES section is introduced with a 0 group with the label "TABLE". This is followed with a 2 group naming the table ("LTYPE", "LAYER", "STYLE" or "VIEW"), and a 70 group that specifies the maximum number of table entries that may follow. The tables in a drawing may contain deleted items but these are not written to a DXF file.Thus the actual number of tables may be less than the number given in the 70 group, so the count cannot be used as an index. It is provided so additional programs can allocate an array in advance to hold all the table items which follow. Following the header for each table are the table entries. Each table item consists of a 0 group identifying the item type (same as the table name, eg. "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". The following are groups used for the 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 line items), 40 (Total pattern length), 49 (Dash length 1), 49 (Dash length 2),.... LAYER 62 (Colour number, negitive 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 orientated 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 other groups are output, however). VIEW 40 41 (View height and width) 10 20 (View centre point) 11,21,31 (View direction from the origin) BLOCKS SECTION ************** The BLOCKS section of the DXF file contains all the Block Definitions. This section contains all the entities that make up the Blocks used in the drawing. 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 the BLOCK and ENDBLK entities. BLOCK and ENDBLK only appear in the BLOCKS section. Block definitions are never nested (ie. no BLOCK or ENDBLK entity appears within another BLOCK-ENDBLOCK pair). ENITIES SECTION *************** Entity items appear in both the BLOCK and ENTITIES sections of the DXF file. The appearance of the entities in the two sections is identical. 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 discussion, groups that always occur are given by their group number and function, while optional groups are indicated by "-optional N" following the group desciption. "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 a DXF file is a complete representation of the drawing database, 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 orders 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 name 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 colour associated with it. The following groups are included only if the entity has non default values for these properties. 6 Linetype name (if not "BYLAYER"). the special name "BYBLOCK" indicates a floating line type. 38 Elevation (if nonzero) 39 Thickness (if nonzero) 62 Colour number (if not "BYLAYER"). 0 indicates the "BYBLOCK" (floating) colour. The rest of the groups that make up an entity are as follows: LINE 10 20 (start point) 11 21 (end Point) POINT 10 20 CIRCLE 10 20 (centre) 40 (radius) ARC 10 20 (centre) 40 (radius) 50 (start angle) 51 (end) TRACE 10 20 11 21 12 22 13 23 (Four points defining the corners of the trace) SOLID 10 20 11 21 12 22 13 23 (Four points defining the corners of the solid. If the solid has only 3 sides, then the coordinates 12 22, 13 23 are equal) TEXT 10 20 (insertion point) 40 (height) 1 (text value) 50 (rotation angle -optional 0) 41 (relative X value -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 (alignment point -optional, appears only if the 72 group is present and nonzero) SHAPE 10 20 (insertion point) 40 (size) 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) Appears only in BLOCK section 10 20 (Block base point) ENDBLK No groups Appears only in BLOCK section INSERT 66 ("Attributes follow" flag -optional 0) 2 (Block name) 10 20 (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 71 (column and row counts -optional 1) 44 45 (column and row spacing -optional 0) ATTDEF 10 20 (text start) 40 (text height) 1 (default value) 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) 72 (justification type -optional 0) 11 21 (alignment point -optional, appears only if the 72 group is present and nonzero) ATTRIB 10 20 (text start) 40 (text height) 1 (value) 2 (Attribute tag) 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) 72 (justification type -optional 0) 11 21 (alignment point -optional, appears only if the 72 group is present and nonzero) POLYLINE 70 (Polyline flags) 40 (default starting width) 41 (default ending width) The flags currently have the bit values 1 (closed polyline) and 2 (curve fit information has been added) The default widths apply to any vertex that does not supply widths (see below) VERTEX 10 20 (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 semi-circle. Currently the vertex flags are 1 (extra vertex created by curve fitting) and 2 (curve fit tangent defined). A curve fit direction of 0 may be omitted from the DXF output, but is significant if the "tangent defined" bit is set in the vertex flags. SEQEND No fields This entity marks the end of the 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). ENTITY FLAG DEFINITIONS *********************** The entity items listed above use various "flag" values. These are integer codes (6x or 7x groups) that encode various pieces of information regarding the entity. The following paragraphs describe the meaning of the various flag groups used in entities. In this discussion, the term "bit coded" means that the flag contains various true/false values coded as the sum of the bit values given. If any of the group 70 flags of all symbol table entries (Layers, Linetypes, Text Styles, Block Definitions and Named Views) is set to 64, the table entry was referenced by at least one entity in the drawing the last time the drawing was entered to edit the drawing. This "referenced" flag is for the benefit of the PURGE command; it can be ignored by most programs that read DXF file, and need not be set by programs that write DXF files. Any bits not defined in the following section should be ignored in these fields and set to zero when constructing a DXF file. "Attibutes Follow" Flag ----------------------- This is an optional 66 group presently used only in an INSERT entity. If the value that follows is 1, Attributes (ATTRIB) entities are expected to follow the INSERT entity. Attribute Flags --------------- This is a 70 group flag that appears in ATTDEF and ATTRIB entities. This is a bit coded field in which 1 means the Attribute is invisible (does not display) 2 means the Attribute is constant 4 means that verification is required on input for this Attribute Text Generation Flags --------------------- This is an optional 71 group that appears in TEXT, ATTDEF and ATTRIB entities. It is a bit coded field in which 2 means the text is mirrored in the X direction (eg. regular image) 4 means the text is upside down Text Justifiaction Type ----------------------- This is an optional 72 group that appears in TEXT, ATTDEF and ATTRIB entities. Its value (NOT bit coded) indicates the text justification style used on this entity in which 0 means left justified 1 means baseline centered text 2 means right justified 3 means "aligned" text 4 means "middle" (fully) centered text 5 means "fit" text If this group appears with a nonzero value 11 and 21 groups will also appear in the entity and specify the alignment point of the text (centre, rightmost or second alignment point) Block Type Flags ---------------- This is a 70 group that appears in BLKDEF entities. It is a bit coded field in which 1 means this is an "annonymous block generated by hatching or other internal operations 2 means this block has Attributes WRITING DXF INTERFACE PROGRAMS ****************************** Writing a program that contructs a DXF file can be difficult, because you must maintain consistancy 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 dont' need to make any entries, and in fact the entire TABLES section can be dropped if nothing in it is required. 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 reference layer names even though you haven't defined them in the LAYER table. Such layers will be automatically created with the colour 7 (white) and the CONTINUOUS linetype. The EOF item must be present at the end of the file. LOADING A DXF FILE ****************** A drawing interchange file can be loaded into an AutoCAD drawing by means of the DXFIN command. First enter the Drawing Editor with the "Create new drawing" task from the main menu. Then issue the DXFIN command. Command: DXFIN File name (name) Enter the name of the drawing interchange file to be loaded. In order to load a complete DXF file you must use a newly created drawing, before any entities have been drawn. If any errors are detected during the input the new drawing will be discarded. Otherwise an automatic "ZOOM All" is performed to set the drawing extents.