.DXF File Format – Classes, tables and blocks

CLASSES section

Each class record describes an application-defined class whose instances appear in BLOCKS, ENTITIES or OBJECTS. Every field is required, and a class definition is assumed to be fixed in the class hierarchy.

Proxy capability flags (code 90)

  0
SECTION
  2
CLASSES
  0
CLASS
  1
<class dxf record>
  2
<class name>
  3
<app name>
 90
<flag>
280
<flag>
281
<flag>
  0
ENDSEC

Default classes

Classes AutoCAD registers. A file may hold others, depending on the applications in use. Read from the 2014 reference and checked against the 2012 edition, whose table extracts in a clearer order; the two agree.

TABLES section

The section holds the symbol tables APPID, BLOCK_RECORD, DIMSTYLE, LAYER, LTYPE, STYLE, UCS, VIEW and VPORT. Their order may change, but LTYPE always comes before LAYER. Each table is 0 TABLE, 2 the table name in upper case, its handle, the AcDbSymbolTable subclass marker and a 70 group giving the maximum number of entries; then the entries; then 0 ENDTAB.

Deleted entries are not written, so fewer entries may follow than the 70 group says. The count is meant only for sizing an array; do not use it to drive reading. Each entry is a 0 group with the entry type (the same as the table name), a 2 group with the entry’s name, a 70 group of flags and the entry’s other groups. Both tables and entries are database objects with a handle after their 2 groups. DIMSTYLE is the one exception: its handle is group 105, because it used group 5 for something else in earlier releases.

Codes common to all tables

Codes common to all table entries

Standard flags (code 70)

Every table entry’s 70 group uses these bits, plus the table’s own bits listed in its section:

APPID

Registered application names, used by extended data.

BLOCK_RECORD

DIMSTYLE

Each code holds the dimension variable of the same name; see the header variables for their values.

DIMFRAC (276) has no header variable in the reference’s list, so its values are not given there.

LAYER

Xref-dependent layers are written by SAVEAS with the linetype name CONTINUOUS.

LTYPE

Codes 74, 75, 340, 46, 50, 44, 45 and 9 are not returned by AutoLISP’s tblsearch or tblnext; tblobjname gives them. Note that rotation here is in radians, unlike angles elsewhere in DXF.

STYLE

A STYLE entry also records a shape file loaded with LOAD. Then bit 1 of the 70 group is set and only the 3 group (the shape file name) means anything, though the other groups are still written.

UCS

Each 71 and 13/23/33 pair sets the origin used for that orthographic view when UCSBASE is this UCS. With 71 5, 13 1.0, 23 2.0, 33 3.0, choosing LEFT moves the UCS origin to (1, 2, 3); without the pair, the origin would be this UCS’s own.

VIEW

When 72 is 1 these follow, defining the UCS made current whenever the view is restored:

Render mode (code 281, VIEW and VPORT)

Every mode except 0 uses the 3D graphics pipeline. The values match the SHADEMODE command and the AcDbAbstractViewTableRecord::RenderMode enumeration.

VPORT

VPORT may hold several entries with the same name, making up a multiple-viewport configuration. The entries of the active configuration are all named *ACTIVE; the first of them is the current viewport.

  0
TABLE
  2
STYLE
  5
1C
 70
3
1001
APP_X
1040
42.0
  0
STYLE
  5
3A
  2
ENTRY_1
 70
64
 40
.4
 41
1.0
 50
0.0
 71
0
 42
0.4
  3
BUFONTS.TXT
  0
STYLE
  5
C2
  2
ENTRY_2
…

BLOCKS section

The BLOCKS section holds every block definition, including the anonymous blocks made by HATCH and associative dimensioning. A definition is a BLOCK entity, the entities of the block in the same format as in ENTITIES, then an ENDBLK entity. BLOCK and ENDBLK appear only here, and definitions never nest, though a block may contain INSERT entities. External references are written as block definitions with a 1 group giving the path and file name.

The UCS in effect when a block is defined becomes the WCS for its entities, with the origin moved to the block’s base point.

Three empty definitions always appear: *Model_Space, *Paper_Space and *Paper_Space0. The first paper space layout is *Paper_Space, the second *Paper_Space0, the third *Paper_Space1 and so on. In ENTITIES, all paper space entities are written first and then the model space ones; group 67 tells them apart.

BLOCK

Block type flags (code 70)

The block table entry’s handle, with any xdata and persistent reactors, follows each BLOCK record, which carries everything the block table record stores.

ENDBLK

Not given by the reference

The reference names but does not list the values of: BLOCK_RECORD 70 (insertion units), 280 (explodability) and 281 (scalability); VPORT 70 (bit flags and perspective mode), 74 (UCSICON) and 170 (shade plot); VIEW and VPORT 71 (VIEWMODE); STYLE 1071 (how the TrueType bits are packed); and DIMSTYLE 276 (DIMFRAC).

Sources

• AutoCAD 2014 DXF Reference – Autodesk, March 2013, chapters 3 (CLASSES), 4 (TABLES) and 5 (BLOCKS), and the symbol table example in chapter 9
• AutoCAD 2012 DXF Reference – Autodesk, February 2011, used only to check the order of the default class table