the following header variables existed prior to autocad release 11
but now have independent settings for each active viewport. 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 11 or higher), the values in the vport
table entries override the values of these header variables.
table 11-4. revised vport header variables
+===============================================================+
| variable | type | description |
|----------------|--------|-------------------------------------|
| $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,| viewing direction (direction from |
| | 30 | target, in wcs) |
|----------------|--------|-------------------------------------|
| $viewsize | 40 | height of view |
+----------------|--------|-------------------------------------+
the date/time variables ($tdcreate and $tdupdate) are output as real
numbers in the following format:
.
the elapsed time variables ($tdindwg and $tdusrtimer) have a similar
format:
.
the date and time variables are described on page 299.
tables section
--------------
the tables section contains several tables, each of which 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 introduced with a 0 group
with the label table. this is followed by a 2 group identifying the
particular table (vport, ltype, layer, style, view, dimstyle, ucs or
appid) and a 70 group that specifies the maximum number of table
entries that may follow. table names are always output in uppercase
characters.
the tables in a drawing can 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. this group
is provided so that a program which reads dxf files can allocate an
array 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 following table), 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 70 group flag bit values that apply to all table entries are
described in the following chart. additional 70 group values that
apply to layer, style, and view table entries are described in the
appropriate sections below.
table 11-5. group 70 bit codes that apply to all table entries
+===============================================================+
| flag bit | meaning |
| value | |
|----------|----------------------------------------------------|
| 16 | if set, table entry is externally dependent on |
| | an xref |
|----------|----------------------------------------------------|
| 32 | if this bit and bit 16 are both set, the |
| | externally dependent xref has been successfully |
| | resolved |
|----------|----------------------------------------------------|
| 64 | if set, the table entry was referenced by at least |
| | one entity in the drawing the last time the drawing|
| | was edited. (this flag is for the benefit of |
| | autocad commands; 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.
appid 2 (user-supplied application name), 70 (standard flag
values).
these table entries maintain a set of names for all
applications registered with a drawing.dimstyle 2 (dimension style name), 70 (standard flag values), and
the following, described by dimension variable name:
3 (dimpost), 4 (dimapost), 5 (dimblk), 6 (dimblk1),
7 (dimblk2), 40 (dimscale), 41 (dimasz), 42 (dimexo),
43 (dimdli), 44 (dimexe), 45 (dimrnd), 46 (dimdle),
47 (dimtp), 48 (dimtm), 140 (dimtxt), 141 (dimcen),
142 (dimtsz), 143 (dimaltf), 144 (dimlfac), 145 (dimtvp),
146 (dimtfac), 147 (dimgap), 71 (dimtol), 72 (dimlim),
73 (dimtih), 74 (dimtoh), 75 (dimse1), 76 (dimse2),
77 (dimtad), 78 (dimzin), 170 (dimalt), 171 (dimaltd),
172 (dimtofl), 173 (dimsah), 174 (dimtix), 175 (dimsoxd),
176 (dimclrd), 177 (dimclre), 178 (dimclrt).ltype 2 (linetype name), 70 (standard flag values), 3
(descriptive text for linetype), 72 (alignment code; value
is always 65, the ascii code for `a'), 73 (number of dash
length items), 40 (total pattern length), and optionally:
49 (dash length 1), 49 (dash length 2), and so on.layer 2 (layer name), 70 (standard flag values), 62 (color
number, negative if layer is off), 6 (linetype name).
in addition to the standard flags, the 70 group flag is
bit coded as follows: table 11-6. group 70 bit codes for layer table
+=====================================================+
| flag bit | meaning |
| value | |
|----------|------------------------------------------|
| 1 | if set, layer is frozen |
|----------|------------------------------------------|
| 2 | if set, layer is frozen by default in |
| | new viewports |
|----------|------------------------------------------|
| 4 | if set, layer is locked |
+-----------------------------------------------------+
if no value (0) is set, the layer is on and thawed. the
fourth bit (8) and the eighth bit (128) are not used.
xref-dependent layers are output during dxfout. for these
layers, the associated linetype name in the dxf file is
always continuous.style 2 (style name), 70 (standard flag values), 40 (fixed text
height; 0 if not fixed), 41 (width factor), 50 (oblique
angle), 71 (text generation flags), 42 (last height used),
3 (primary font filename), 4 (big-font 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 filename)
is meaningful (all the other groups are output, however).
the text generation flags are a bit-coded field with the
following bit meanings: table 11-7. group 71 bit codes for style table
+=====================================================+
| flag bit | meaning |
| value | |
|----------|------------------------------------------|
| 2 | text is backward (mirrored in x) |
|----------|------------------------------------------|
| 4 | text is upside down (mirrored in y) |
+-----------------------------------------------------+ucs 2 (ucs name), 70 (standard flag values), 10, 20, 30
(origin), 11, 21, 31 (x axis direction), 12, 22, 32
(y axis direction). all in world coordinates.view 2 (name of view), 70 (standard flag values), 40 and
41 (view height and width, in dcs), 10 and 20 (view
center point, in dcs), 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 in appendix a). if the first bit (1) is set in the 70 group flags, this
is a paper space view. (see chapter 2 of the "autolisp programmer's reference"
for information on dcs, the display coordinate system.)vport 2 (viewport name), 70 (standard flag values), 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,
in wcs), 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), 68 (status field),
69 (id), 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
viewport configuration all have the name *active. the
first such entry describes the current viewport.
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 later, so see 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),
although a block definition can contain an insert entity.
external references are written in the dxf file as any block
definition, except they also include a text string (group code 1) of
the path and filename of the external reference. this is the text
string format:
xref filename
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.
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 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
database, and that as autocad is further enhanced, new groups will
be added to entities to accommodate additional features.
accommodating dxf files from future releases of autocad will be
easier if you write your dxf processing program in a table-driven
way, ignoring any groups not presently defined, and making no
assumptions about the order of groups in an entity.
each entity begins with a 0 group identifying the entity type. the
names used for the entities are given on the following pages. 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 nondefault
values for these properties. when a group is omitted, its default
value upon input (when using dxfin) is indicated in the third
column. if the value of a group is equal to the default, it is
omitted upon output (when using dxfout).
table 11-8. group codes common to all entities
+==================================================================+
| group | meaning | if omitted, |
| code | | defaults to...|
|--------|-----------------------------------------|---------------|
| 6 | linetype name (if not bylayer). the | bylayer |
| | special name byblock indicates a | |
| | floating linetype | |
|--------|-----------------------------------------|---------------|
| 38 | elevation (if nonzero). exists only in | 0 |
| | output from versions prior to r11. | |
| | otherwise, z coordinates are supplied | |
| | as 3x-groups as part of each of the | |
| | entity's defining points | |
|--------|-----------------------------------------|---------------|
| 39 | thickness (if nonzero) | 0 |
|--------|-----------------------------------------|---------------|
| 62 | color number (if not bylayer). zero | bylayer |
| | indicates the byblock (floating) color. | |
| | 256 indicates the bylayer color | |
|--------|-----------------------------------------|---------------|
| 67 | absent or zero indicates entity is in | 0 |
| | model space. one indicates entity is | |
| | in paper space, other values are | |
| | reserved | |
|--------|-----------------------------------------|---------------|
| 210, | these groups are included for each line,| 0,0,1 |
| 220, | point, circle, shape, text, arc, trace, | |
| 230 | solid, block reference, polyline, | |
| | dimension, attribute, and attribute | |
| | definition entity if its extrusion | |
| | direction is not parallel to the world | |
| | z axis. they 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
next. 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 descriptions, 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.
line 10, 20, 30 (start point), 11, 21, 31 (endpoint).
point 10, 20, 30 (point).
point entities have an optional 50 group that determines
the orientation of pdmode images. the group value is the
negative of the entity coordinate systems (ecs) angle of
the ucs x axis in effect when the point was drawn. the x
axis of the ucs in effect when the point was drawn is
always parallel to the xy plane for the point's ecs, and
the angle between the ucs x axis and the ecs x axis is a
single 2d angle. the value in group 50 is the angle from
horizontal (the effective x axis) to the ecs x axis.
entity coordinate systems (ecs) are described later in
this section.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 (oblique angle -optional 0), 7
(text style name -optional standard), 71 (text generation
flags -optional 0), 72 (horizontal justification type -
optional 0), 73 (vertical justification type -optional 0)
11, 21, 31 (alignment point -optional, appears only if 72
or 73 group is present and nonzero). the "text generation flags" are a bit-coded field with
meanings as follows: table 11-9. group 71 bit codes for text entity
+=====================================================+
| flag bit | meaning |
| value | |
|----------|------------------------------------------|
| 2 | text is backward (mirrored in x) |
|----------|------------------------------------------|
| 4 | text is upside down (mirrored in y) |
+-----------------------------------------------------+ the justification-type value (group codes 72 and 73, not
bit-coded) indicates the text-justification style used on
the text, as shown in the following table: table 11-10. group 72 & 73 bit codes for text entity
+======================================================+
| group 73 | group 72 (horizontal alignment) |
|(vertical |-----------------------------------------|
| alignment) |0 |1 |2 |3 |4 |5 |
|------------|-----|-------|------|-------|------|-----|
|3 (top) |tleft|tcenter|tright| | | |
|------------|-----|-------|------|-------|------|-----|
|2 (middle) |mleft|mcenter|mright| | | |
|------------|-----|-------|------|-------|------|-----|
|1 (bottom) |bleft|bcenter|bright| | | |
|------------|-----|-------|------|-------|------|-----|
|0 (baseline)|left |center |right |aligned|middle| fit |
+------------------------------------------------------+ if the justification is anything other than baseline/left
(groups 72 and 73 both 0), group codes 11, 21, and 31
specify the alignment point (or the second alignment point
for align or fit). dxfout handles ascii control characters in text strings by
expanding the character into a ^ (caret) followed by the
appropriate letter. for example, an ascii control-g (bel,
decimal code 7) is output as ^g. if the text itself
contains 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 (oblique angle -optional 0).block 2 (block name), 3 (this is also the block name), 70 (block
type flag), 10, 20, 30 (block base point), and if the
block is an xref block it will also contain group code 1
(xref pathname). block entities appear only in the blocks
section, not in the entities section. the "block type
flag" (group 70) is bit-coded, with the following bit
meanings: table 11-11. group 70 bit codes for block table
+=======================================================+
| flag bit | meaning |
| value | |
|----------|--------------------------------------------|
| 1 | this is an anonymous block generated by |
| | hatching, associative dimensioning, other |
| | internal operations, or an application |
|----------|--------------------------------------------|
| 2 | this block has attributes |
|----------|--------------------------------------------|
| 4 | this block is an external reference (xref) |
|----------|--------------------------------------------|
| 8 | not used |
|----------|--------------------------------------------|
| 16 | this block is externally dependent |
|----------|--------------------------------------------|
| 32 | this is a resolved external reference, or |
| | dependent of an external reference |
|----------|--------------------------------------------|
| 64 | this definition is referenced |
+-------------------------------------------------------+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" on page 260 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 (oblique angle -optional 0), 7 (text
style name -optional standard), 71 (text generation flags
-optional 0, see "text" on page 260), 72 (horizontal text
justification type - optional 0, see "text" on page 260),
74 (vertical text justification type -optional 0 see group
73 in "text" on page 260), 11, 21, 31 (alignment point
-optional, appears only if 72 or 74 group is present and
nonzero). the "attribute flags" (group code 70) are a bit-coded
field in which the bits have the following meanings: table 11-12. group 70 bit codes for attdef entity
+=======================================================+
| flag bit | meaning |
| value | |
|----------|--------------------------------------------|
| 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) |
+-------------------------------------------------------+attrib 10, 20, 30 (text start), 40 (text height), 1 (value, see
"text" on page 260 for handling ascii control characters),
2 (attribute tag), 70 (attribute flags; see attdef), 73
(field length -optional 0), 50 (text rotation -optional
0), 41 (relative x scale factor -optional 1), 51 (oblique
angle -optional 0), 7 (text style name -optional
standard), 71 (text generation flags -optional 0, see
"text" on page 260), 72 (horizontal text justification
type -optional 0, see "text" on page 260), 74 (vertical
text justification type -optional 0, see group 73 in
"text" on page 260), 11, 21, 31 (alignment point
-optional, appears only if 72 or 74 group is present
and nonzero).polyline 66 (vertices-follow flag), 10, 20, 30 (polyline elevation
- 30 supplies elevation, 10 and 20 are always set to
zero), 70 (polyline flag -optional 0), 40 (default
starting width - optional 0), 41 (default ending width
-optional 0), 71 and 72 (polygon mesh m and n vertex counts
-optional 0), 73 and 74 (smooth surface m and n densities
-optional 0), 75 (curves and smooth surface type -optional
0). the default widths apply to any vertex that doesn't
supply widths (see later). 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 flag (group code 70) is a bit-coded field
with bits defined as follows: table 11-13. group 70 bit codes for polyline entity
+======================================================+
| flag bit | meaning |
| value | |
|----------|-------------------------------------------|
| 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 indicates 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 |
| | direction |
|----------|-------------------------------------------|
| 64 | this polyline is a polyface mesh |
|----------|-------------------------------------------|
| 128 | the linetype pattern is generated |
| | continuously around the vertices of |
| | this polyline |
+------------------------------------------------------+ a polyface mesh is represented in dxf as a variant of a
polyline entity. the polyline header is identified as
introducing a polyface mesh by the presence of the 64 bit
in the polyline flags (70) group. the 71 group specifies
the number of vertices in the mesh, and the 72 group, the
number of faces. while these counts are correct for all
meshes created with the pface command, applications are
not required to place correct values in these fields, and
autocad actually never relies upon their accuracy. following the polyline header is a sequence of vertex
entities that specify the vertex coordinates and faces
that compose the mesh. vertices such as these are
described in the following subsection on vertex. applications might want to represent polygons with an
arbitrarily large number of sides in polyface meshes.
however, the autocad entity structure imposes a limit on
the number of vertices that a given face entity can
specify. you can represent more complex polygons by
decomposing them into triangular wedges. their edges
should be made invisible to prevent visible artifacts of
this subdivision from being drawn. the pface command
performs this subdivision automatically, but when
applications generate polyface meshes directly, the
applications must do this themselves. the number of vertices per face is the key parameter in
this subdivision process. the pfacevmax system variable
provides an application with the number of vertices per
face entity. this value is read-only, and is set to 4. polyface meshes created with the pface command are always
generated with all the vertex coordinate entities first,
followed by the face definition entities. the code within
autocad that processes polyface meshes does not, at
present, require this ordering; it works even with
interleaved vertex coordinates and face definitions as
long as no face specifies a vertex with an index that
appears after it in the database. programs that read
polyface meshes from dxf would be wise to be as tolerant
of odd vertex and face ordering as autocad is.vertex 10, 20, 30 (location), 40 (starting width -optional, see
earlier), 41 (ending width -optional, see above), 42
(bulge -optional 0), 70 (vertex flags -optional 0), 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 endpoint; a bulge of 0 indicates a straight segment,
and a bulge of 1 is a semicircle. the meaning of the bit-
coded vertex flag (group code 70) is shown in the
following table table 11-14. group 70 bit codes for vertex entity
+========================================================+
| flag bit | meaning |
| value | |
|----------|---------------------------------------------|
| 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 |
| | significant 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 |
|----------|---------------------------------------------|
| 128 | polyface mesh vertex |
+--------------------------------------------------------+ every vertex that is part of a polyface mesh has the 128
bit set in its vertex flags (70) group. if the entity
specifies the coordinates of a vertex of the mesh, the 64
bit is set as well and the 10, 20, and 30 groups give the
vertex coordinates. the vertex indexes are determined by
the order in which the vertex entities appear within the
polyline, with the first numbered 1. if the vertex defines a face of the mesh, its vertex flags
(70) group has the 128 bit set but not the 64 bit. the 10,
20, and 30 (location) groups of the face entity are
irrelevant and are always written as zero in a dxf file.
the vertex indexes that define the mesh are given by 71,
72, 73, and 74 groups, the values of which are integers
specifying one of the previously defined vertices by
index. if the index is negative, the edge that begins with
that vertex is invisible. the first zero vertex marks the
end of the vertices of the face. since the 71 through 74
groups are optional fields with default values of zero,
they are present in dxf only if nonzero.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).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 are 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: table 11-15. group 70 bit codes for 3d face entity
+===========================================+
| flag bit | meaning |
| value | |
|----------|--------------------------------|
| 1 | first edge is invisible |
|----------|--------------------------------|
| 2 | second edge is invisible |
|----------|--------------------------------|
| 4 | third edge is invisible |
|----------|--------------------------------|
| 8 | fourth edge is invisible |
+-------------------------------------------+viewport 10,20,30 (center point of entity in paper space
coordinates), 40 (width in paper space units), 41 (height
in paper space units), 68 (viewport status field), 69
(viewport id, permanent during editing sessions, but
mutable between sessions; the paper space viewport entity
always has an id of 1). the value of the viewport status field (68) is interpreted
as follows: -1 on, but is fully off-screen or is one of
the viewports not active because the
$maxactvp count is currently being
exceeded. 0 off.
on, active and the value indicates the
order of "stacking" for the viewports,
with 1 applying to the active viewport,
which is also the highest, 2 applying
to the next viewport in the stack,
and so on.
in addition, the extended entity data groups in the
following table apply to viewports. note: in contrast to normal entity data, the same extended
entity group code can appear multiple times, and order is
important. table 11-16. extended entity group codes for viewports
+========================================================+
| group | description |
|---------|----------------------------------------------|
| 1001 | application name. this field will always be |
| | the string "acad" |
|---------|----------------------------------------------|
| 1000 | begin viewport data. this field will always |
| | be the string "mview". other data groups may |
| | appear in the future |
|---------|----------------------------------------------|
| 1002 | begin window descriptor data. this field will|
| | always be the string "{" |
|---------|----------------------------------------------|
| 1070 | extended entity data version number. for |
| | releases 11 and 12, this field will always be|
| | the integer 16 |
|---------|----------------------------------------------|
| 1010 | view target point x value |
|---------|----------------------------------------------|
| 1020 | view target point y value |
|---------|----------------------------------------------|
| 1030 | view target point z value |
|---------|----------------------------------------------|
| 1010 | view direction vector x value |
|---------|----------------------------------------------|
| 1020 | view direction vector y value |
|---------|----------------------------------------------|
| 1030 | view direction vector z value |
|---------|----------------------------------------------|
| 1040 | view twist angle |
|---------|----------------------------------------------|
| 1040 | view height |
|---------|----------------------------------------------|
| 1040 | view center point x value |
|---------|----------------------------------------------|
| 1040 | view center point y value |
|---------|----------------------------------------------|
| 1040 | perspective lens length |
|---------|----------------------------------------------|
| 1040 | front clip plane z value |
|---------|----------------------------------------------|
| 1040 | back clip plane z value |
|---------|----------------------------------------------|
| 1070 | view mode |
|---------|----------------------------------------------|
| 1070 | circle zoom |
|---------|----------------------------------------------|
| 1070 | fast zoom setting |
|---------|----------------------------------------------|
| 1070 | ucsicon setting |
|---------|----------------------------------------------|
| 1070 | snap on/off |
|---------|----------------------------------------------|
| 1070 | grid on/off |
|---------|----------------------------------------------|
| 1070 | snap style |
|---------|----------------------------------------------|
| 1070 | snap isopair |
|---------|----------------------------------------------|
| 1040 | snap angle |
|---------|----------------------------------------------|
| 1040 | snap base point ucs x coordinate |
|---------|----------------------------------------------|
| 1040 | snap base point ucs y coordinate |
|---------|----------------------------------------------|
| 1040 | snap x spacing |
|---------|----------------------------------------------|
| 1040 | snap y spacing |
|---------|----------------------------------------------|
| 1040 | grid x spacing |
|---------|----------------------------------------------|
| 1040 | grid y spacing |
|---------|----------------------------------------------|
| 1070 | hidden in plot flag |
|---------|----------------------------------------------|
| 1002 | begin frozen layer list (possibly empty). |
| | this field will always be the string "{" |
|---------|----------------------------------------------|
| 1003... | the names of layers frozen in this viewport. |
| | this list may include xref-dependent layers. |
| | any number of 1003 groups may appear here |
|---------|----------------------------------------------|
| 1002 | end frozen layer list. this field will always|
| | be the string "}" |
|---------|----------------------------------------------|
| 1002 | end viewport data. this field will always be |
| | the string "}" |
+--------------------------------------------------------+dimension 2 (name of pseudo-block containing the current dimension
entity geometry), 3 (dimension style name), 10, 20, 30
(definition point for all dimension types), 11, 21, 31
(middle point of dimension text), 12, 22, 32 (dimension
block translation vector), 70 (dimension type), 1
(dimension text explicitly entered by the user. if null or
"<>", the dimension measurement is drawn as the text, if
" " [one blank space], the text is suppressed. anything
else is drawn as the text). 13, 23, 33 (definition point
for linear and angular dimensions), 14, 24, 34 (definition
point for linear and angular dimensions), 15, 25, 35
(definition point for diameter, 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). the dimension type (group code 70) is an integer-coded
field with the following values: table 11-17. group 70 integer codes for dimension entity
+========================================================+
| group | description |
|---------|----------------------------------------------|
| 0 | rotated, horizontal, or vertical |
|---------|----------------------------------------------|
| 1 | aligned |
|---------|----------------------------------------------|
| 2 | angular |
|---------|----------------------------------------------|
| 3 | diameter |
|---------|----------------------------------------------|
| 4 | radius |
|---------|----------------------------------------------|
| 5 | angular 3-point |
|---------|----------------------------------------------|
| 6 | ordinate |
|---------|----------------------------------------------|
| 64 | ordinate type. this is a bit value (bit 7) |
| | used only with integer value 6. if set, |
| | ordinate is x-type; if not set, ordinate is |
| | y-type |
|---------|----------------------------------------------|
| 128 | this is a bit value (bit 8) added to the |
| | other group 70 values if the dimension text |
| | has been positioned at a user-defined |
| | location rather than at the default location |
+--------------------------------------------------------+ in addition, all dimension types have an optional group
(code 51) that indicates the horizontal direction for the
dimension 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 entity coordinate systems (ecs)
angle of the ucs x axis in effect when the dimension was
drawn. 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. linear dimension types with an oblique angle have an
optional group (code 52). when added to the rotation angle
of the linear dimension (group code 50) this gives the
angle of the extension lines. the optional group code 53
is the rotation angle of the dimension text away from its
default orientation (the direction of the dimension line). for all dimension types, the following groups represent 3d
wcs points: 10, 20, 30
13, 23, 33
14, 24, 34
15, 25, 35 for all dimension types, the following groups represent 3d
ecs points: 11, 21, 31
12, 22, 32
16, 26, 36linear (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. refer to figure 11-1 on page 267 of the "autocad
customization manual"
angular (13,23,33) and (14,24,34) the endpoints of the first
extension line.
(10,20,30) and (15,25,35) the endpoints of the second
extension line.
(16,26,36) the point used to specify the
dimension line arc. refer to figure 11-2 on page 267 of the "autocad
customization manual"angular (15,25,35) the vertex of the angle.
(3-point) (13,23,33) the endpoints of the first extension line.
(13,23,33) the endpoints of the first extension line.
(14,24,34) the endpoints of the second extension line.
(10,20,30) the point used to specify the dimension line
arc. refer to figure 11-3 on page 267 of the "autocad
customization manual"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. refer to figure 11-4 on page 268 of the "autocad
customization manual"radius (15,25,35) the point used to pick the circle/arc to
dimension.
(10,20,30) the center of that circle. refer to figure 11-5 on page 268 of the "autocad
customization manual"ordinate (13,23,33) the point used to select the feature.
(14,24,34) the point used to locate the leader end point. refer to figure 11-6 on page 268 of the "autocad
customization 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 the
entity's own entity coordinate 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 the entity's 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 are 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 following properties:
o its origin coincides with the wcs origin.
o the orientation of the x and y axes within the xy plane are
calculated in an arbitrary, but consistent manner. autocad
performs this calculation using the arbitrary axis algorithm
(described later).
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.
table 11-18. coordinate systems associated with an entity type
+==================================================================+
| entities | notes |
|----------------------------|-------------------------------------|
| line, point, 3dface, | these entities do not lie in a |
| 3d polyline, 3d vertex, | particular plane. all points are |
| 3d mesh, 3d mesh vertex | 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, | nature. all points are expressed |
| shape, insert, 2d polyline,| in entity coordinates. all of these |
| 2d vertex | 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 |
|----------------------------|-------------------------------------|
| viewport | expressed in world coordinates |
|----------------------------|-------------------------------------|
| 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:
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.
these are a few ramifications of this process:
o you cannot reliably find out what ucs was in effect when an entity
was acquired.
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
coordinates 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 coordinate 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 fraction 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).
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 (abs (nx) < 1/64) and (abs (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.
extended entity data
====================
extended entity data is created by applications such as the advanced
modeling extension (ame), or by routines written with autolisp or
ads. extended entity data is also produced by creating postscript
output with psout. if an entity contains extended data, it follows
the entity's normal definition data.
the group codes 1000 through 1071 describe extended entity data. the
following is an example of an entity containing extended entity data
in dxf format.
___
0 |
insert |
8 |
0 |
5 |
5 |
f11 |
15 |-- normal entity definition data.
2 |
block_a |
10 |
0.0 |
20 |
0.0 |
30 |
0.0 ___|
1001 |
ame_sol |
1002 |
{ |
1070 |
0 |
1071 |
1.95059e+06 |
1070 |
519 |
1010 |
2.54717 |
1020 |
2.122642 |-- extended entity data.
1030 |
2.049201 |
1005 |
ecd |
1005 |
ee9 |
1005 |
0 |
1040 |
0.0 |
1040 |
1.0 |
1000 |
mild_steel ___|
figure 11-7. example of extended entity data
organization of extended entity data
====================================
as you can see in the above example, group code 1001 indicates the
beginning of extended entity data. this is followed by one or more
1000 group codes. application names are string values (in the
example, the application name is ame_sol). in contrast to normal
entity data, the same group code can appear multiple times, and
order is important.
extended entity data are grouped by registered application name, and
each registered application's group begins with a 1001 group code
with the registered application name as the string value. registered
application names correspond to appid symbol table entries, which
are essentially placeholders for registered application names.
an application can use as many appid names as needed, although one
will often suffice. appid names are permanent, although they can be
purged if they aren't currently used in the drawing.
each appid name can have no more than one data group attached to
each entity. within an application's group, the sequence of extended
entity data groups and their meaning is defined by the application.
note: postscript images and postscript fill requests for polylines
are stored in the autocad database as extended entity data belonging
to the autocad_postscript_figure application.
as the example in the previous figure shows, the group codes for
extended entity data begin at 1000 and currently extend to 1071. the
following list of extended entity data group codes are supported by
autocad, which maintains and manipulates their values as described:
table 11-19. extended entity data group codes and descriptions
+==================================================================+
| entity name | group code | description |
|----------------|------------|------------------------------------|
| string | 1000 | strings in extended entity data |
| | | can be up to 255 bytes long (with |
| | | the 256th byte reserved for the |
| | | null character) |
|----------------|------------|------------------------------------|
| application | 1001 | application names can be up to 31 |
| name | also a | bytes long (the 32d byte is |
| | string | reserved for the null character). |
| | value | use of application names is |
| | | described in more detail later in |
| | | this section |
| | | caution: do not add a 1001 group |
| | | into your extended entity data, as |
| | | autocad will assume it is the |
| | | beginning of a new application |
| | | extended entity data group |
|----------------|------------|------------------------------------|
| control string | 1002 | an extended data control string can|
| | | be either "{"or "}": these braces |
| | | enable applications to organize |
| | | their data by subdividing the data |
| | | into lists. the left brace begins a|
| | | list, and a right brace terminates |
| | | the most recent list; lists can be |
| | | nested |
| | | when autocad reads the extended |
| | | entity data for a particular |
| | | application, it checks to ensure |
| | | that braces are balanced correctly |
|----------------|------------|------------------------------------|
| layer name | 1003 | name of the layer associated with |
| | | the extended entity data |
|----------------|------------|------------------------------------|
| binary data | 1004 | binary data is organized into |
| | | variable-length chunks.the maximum |
| | | length of each chunk is 127 bytes. |
| | | binary data is represented as a |
| | | string of hexadecimal digits, two |
| | | per binary byte, in ascii dxf files|
|----------------|------------|------------------------------------|
| database | 1005 | handles of entities in the |
| handle | | drawing database |
| | | note: when a drawing with handles |
| | | and extended entity data handles |
| | | is imported into another drawing |
| | | using insert, insert *, xref bind, |
| | | xbind, or partial dxfin, the |
| | | extended entity data handles are |
| | | translated in the same manner as |
| | | their corresponding entity handles,|
| | | thus maintaining their binding. |
| | | this is also done in the explode |
| | | block operation, or for any other |
| | | autocad operation. when audit |
| | | detects an extended entity data |
| | | handle that doesn't match the |
| | | handle of an entity in the drawing |
| | | file, it is considered an error. |
| | | if audit is fixing entities, it |
| | | sets the handle to 0. |
|----------------|------------|------------------------------------|
| 3 reals | 1010, | three real values, in the order |
| | 1020, | x, y, z. they can be used as |
| | 1030 | a point or vector record. autocad |
| | | never alters their value |
|----------------|------------|------------------------------------|
| world space | 1011, | unlike a simple 3d point,the world |
| position | 1021, | space coordinates are moved, |
| | 1031 | scaled, rotated, and mirrored |
| | | along with the parent entity to |
| | | which the extended data belongs. |
| | | the world space position is also |
| | | stretched when the stretch command |
| | | is applied to the parent entity |
| | | and this point lies within the |
| | | select window |
|----------------|------------|------------------------------------|
| world space | 1012, | also a 3d point that is scaled, |
| displacement | 1022, | rotated, and mirrored along with |
| | 1032 | the parent (but not moved or |
| | | stretched) |
|----------------|------------|------------------------------------|
| world | 1013, | also a 3d point that is rotated |
| direction | 1023, | and mirrored along with the parent |
| | 1033 | (but not moved, scaled, or |
| | | stretched). |
|----------------|------------|------------------------------------|
| real | 1040 | a real value |
|----------------|------------|------------------------------------|
| distance | 1041 | a real value that is scaled along |
| | | with the parent entity |
|----------------|------------|------------------------------------|
| scale factor | 1042 | also a real value that is scaled |
| | | along with the parent. the |
| | | difference between a distance and |
| | | a scale factor is application- |
| | | defined |
|----------------|------------|------------------------------------|
| integer | 1070 | a 16-bit integer (signed or |
| | | unsigned) |
|----------------|------------|------------------------------------|
| long | 1071 | a 32-bit signed (long) integer |
+------------------------------------------------------------------+for more information on extended entity data and the appid table,
refer to the "autocad development system programmer's reference" and
the "autolisp programmer's reference."
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 intentionally constructed to
make it easy to ignore information you don't need while easily
reading the information you do need. just remember to handle the
groups in any order and ignore any group you don't care about.
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 computer, 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
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;")":return
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
2115 if g% >= 1000 then line input #1, t$ : return
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 1writing a program that constructs a dxf file is more difficult,
because you must maintain consistency within the drawing in order
for autocad to find the file acceptable. autocad lets you 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 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, the blocks
section 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 are 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 program doesn't create the
drawing header, the drawing limits, extents, and current view will
be invalid after performing a dxfin on the drawing generated 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"
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). this program takes advantage of
this flexibility in input format, and does not try 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 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 errors such as omission of required groups.
binary drawing interchange files
********************************
the ascii dxf file format described in the preceding sections of
this chapter 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.
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 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 was 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
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.
o an eight-byte ieee double precision floating-point number stored
with the least-significant byte first and the most-significant
byte last.
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.
extended entity data group codes are represented in binary dxf as a
single byte with the value 255, followed by a 2-byte integer value
containing the actual group code, followed by the actual value.
extended entity data long (group code 1071) values occupy 4 bytes of
data. extended entity data binary chunks (group code 1004) are
represented as a single-byte, unsigned integer length, followed by
the specified number of bytes of chunk data. for example, to
transfer an extended entity data long group, the following values
would appear, occupying 1, 2, and 4 bytes respectively:
255 escape group code.
1071 true group code.
999999 value for the 1071 group code.
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 the file. 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.
binary drawing interchange (dxb) files
**************************************
the dxf file formats described earlier in this chapter are complete
representations of an autocad drawing that can be written and read
by autocad and other programs. however, autoshade and programs
executed via the external commands facility (chapter 3) often 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.
dxbin command
=============
to load a dxb file produced by a program such as autoshade, enter
the dxbin command:
command: dxbin
when autocad prompts you, respond with the name of the file you want
to load. you don't need to include a file type; .dxb is assumed.
dxb file format
===============
important: 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 identifying the record
type, followed by data items. the data items have various forms of
representation and encoding. in the descriptions following, each
data item is prefixed with a letter and a hyphen. the meaning of the
letter codes is as follows:
w- 16-bit integer, byte reversed in the standard 80x86 style
(least- significant byte first, most-significant byte second).f- ieee 64-bit floating-point value stored with lsb first, msb
last (as stored by an 80x87).l- 32-bit integer with the bytes reversed 80x86 style.
n- number which may be either a 16-bit integer or a floating-point
number depending on the most recent setting of the number mode
data item. the number mode defaults to 0, signifying integers.
if set to 1, all n- items will be read as floating-point.u- item which is either a 32-bit integer or a floating-point
number depending on the most recent number mode setting. if a
32-bit integer, the value is scaled by multiplying it by 65536
(2^16). if a floating-point value, no scaling is applied.a- item representing an angle. if number mode is integer, this is
a 32-bit integer representing an angle in units of millionths
of a degree (range 0 to 360,000,000). if a floating-point
number, represents degrees.in the following table, the lengths include the item-type byte and
assume the number mode is set to zero (integer mode). if number mode
is floating-point, add 6 bytes to the length for each n- item
present and 4 bytes for each a-, or u- item present.
table 11-20. byte length for item types
+==================================================================+
| item type | code | data items | length |
| |(decimal) | |(bytes) |
|---------------|----------|----------------------------|----------|
| line | 1 | n-fromx n-fromy | 13 |
| | | n-tox n-toy | |
| | | n-fromx n-fromy n-fromz | |
| | | n-tox n-toy n-toz | |
|---------------|----------|----------------------------|----------|
| point | 2 | n-x n-y | 5 |
|---------------|----------|----------------------------|----------|
| circle | 3 | n-ctrx n-ctry n-rad | 7 |
|---------------|----------|----------------------------|----------|
| arc | 8 | n-ctrx n-ctry n-rad | 19 |
| | | a-starta a-enda | |
|---------------|----------|----------------------------|----------|
| trace | 9 | n-x1 n-y1 n-x2 n-y2 | 17 |
| | | n-x3 n-y3 n-x4 n-y4 | |
|---------------|----------|----------------------------|----------|
| solid | 11 | n-x1 n-y1 n-x2 n-y2 | 17 |
| | | n-x3 n-y3 n-x4 n-y4 | |
|---------------|----------|----------------------------|----------|
| seqend | 17 | (none) | 1 |
|---------------|----------|----------------------------|----------|
| polyline | 19 | w-closureflag | 3 |
|---------------|----------|----------------------------|----------|
| vertex | 20 | n-x n-y | 5 |
|---------------|----------|----------------------------|----------|
| 3dface | 22 | n-x1 n-y1 n-z1 | 25 |
| | | n-x2 n-y2 n-z2 | |
| | | n-x3 n-y3 n-z3 | |
| | | n-x4 n-y4 n-z4 | |
|---------------|----------|----------------------------|----------|
| scale factor | 128 | f-scalefac | 9 |
|---------------|----------|----------------------------|----------|
| new layer | 129 | "layername" nul |layername |
| | | |length + 2|
|---------------|----------|----------------------------|----------|
| line | 130 | n-tox n-toy | 5 |
| extension | | | |
|---------------|----------|----------------------------|----------|
| trace | 131 | n-x3 n-y3 n-x4 n-y4 | 9 |
| extension | | | |
|---------------|----------|----------------------------|----------|
| block base | 132 | n-bx n-by | 5 |
|---------------|----------|----------------------------|----------|
| bulge | 133 | u-2h/d | 5 |
|---------------|----------|----------------------------|----------|
| width | 134 | n-startw n-endw | 5 |
|---------------|----------|----------------------------|----------|
| number mode | 135 | w-mode | 3 |
|---------------|----------|----------------------------|----------|
| new color | 136 | w-colornum | 3 |
|---------------|----------|----------------------------|----------|
| 3dline | 137 | n-tox n-toy n-toz | 7 |
| extension | | | |
+------------------------------------------------------------------+
the line extension item extends the last line or line extension from
its to point to a new to point:. the trace extension item similarly
extends the last trace solid, or trace extension from its x3,y3-
x4,y4 ending line to a new x3,y3--x4,y4 line.
the scale factor is a floating-point value by which all integer
coordinates are multiplied to obtain the floating-point coordinates
used by the actual entities. the initial scale factor when a file is
read is 1.0. the new layer item creates a layer if none exists,
giving the new layer the same defaults as the layer new command, and
sets that layer as the current layer for subsequent entities. at the
end of the dxb file load, the layer in effect before the command is
restored.
the block base item specifies the base (origin) point of a created
block. the block base must be defined before the first entity record
is encountered. if dxb is not defining a block, this specification
will be ignored.
a polyline consists of straight segments of fixed width connecting
the vertices, except as overridden by the bulge and width items
described below. the closure flag should be 0 or 1; if it is 1, then
there is an implicit segment from the last vertex (immediately
before the seqend) to the first vertex.
a bulge item, encountered between two vertex items (or after the
last vertex of a closed polyline), indicates that the two vertices
are connected by an arc rather than a straight segment. if the line
segment connecting the vertices would have length d, and the
perpendicular distance from the midpoint of that segment to the arc
is h, then the magnitude of the bulge is (2 * h / d). the sign is
negative if the arc from the first vertex to the second is
clockwise. a semicircle thus has a bulge of 1 (or -1). if the number
mode is 0 (integer), bulge items are scaled by 2 16. if the number
mode has been set to floating-point, then the floating-point value
supplied is just 2*h/d (not scaled).
the width item indicates the starting and ending widths of the
segment (straight or curved) connecting two vertices. this width
stays in effect until the next width item or the seqend. if there is
a width item between the polyline item and the first vertex, it is
stored as a default width for the polyline; this saves considerable
database space if the polyline has several segments of this width.
the number mode item controls the mode of items with types given in
the table above as n-, a-, or u-. if the value supplied is zero,
these values will be integers, otherwise floating-point. the storage
and implicit scaling conventions for these values in both modes are
described earlier.
lines share the same cells to remember the last to-point, so you
shouldn't mix extension groups for the two entities without an
initial group before the extension. there is no extension group for
3dfaces, as there's no obvious edge to extend from.
the new color group specifies the color for subsequent entities in
the dxb file. the w-colornum word argument is in the range from 0 to
256. 0 means color by block, 1-255 are the standard autocad colors,
and 256 means color by layer. a color outside the range from 0 to
256 sets the color back to the current entity color (you can do this
deliberately, and it can be quite handy). the initial entity color
of material added by dxbin is the current entity color.
all points specified in the dxb file are interpreted in terms of the
current ucs at the time the dxbin command is executed.
writing dxb files
=================
there is no direct autocad command to write a dxb file, but the
special adi plotter driver can write such a file. if you want to
create a dxb file from an autocad drawing, configure the adi plotter
and select its dxb file output option.
initial graphics exchange specification (iges) files
****************************************************
using the commands described in this section, you can instruct
autocad to read and write iges-format interchange files.
note: the format of iges files and the mapping performed to
translate between autocad drawing information and iges are described
in the separate autocad/iges interface specifications document.
igesout command
===============
you can generate an initial graphics exchange specification (iges)
interchange file from an existing autocad drawing by means of the
igesout command:
command: igesout
when autocad prompts you, respond with a filename or press 5 to
accept the default.
the default name for the output file is the same as that of the
current drawing, but with a file type of .igs. if you specify an
explicit filename without including a file type, .igs is assumed. if
a file with the same name already exists, it is deleted. if filedia
is on, and a file with the same name already exists, autocad tells
you; allowing you to ok or cancel the deletion.
igesin command
==============
an iges interchange file can be converted into an autocad drawing by
means of the igesin command:
command: igesin
when autocad prompts you, respond with the name of the iges file to
be loaded.
to load a complete iges file, you must use igesin 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.
note: if the drawing you are using as a prototype is not empty, you
might find it helpful to open a new drawing using the no
prototype... button of the create new drawing dialogue box, as
described in chapter 4 of the autocad reference manual. you should
also be aware that some third-party applications include an acad.lsp
or .mnl file that modifies your drawing upon startup.
if a serious error is encountered, the input process stops and an
error message is displayed reporting where the error was found. the
partial drawing is not discarded.
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