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 (c) drawing interchange and file formats</pre>
<pre> attrib 10, 20, 30 (text start), 40 (text height), 1 (value, see text
above for handling of ascii control characters), 2 (attribute
tag), 70 (attribute flags; see attdef above), 73 (field
length -optional 0), 50 (text rotation -optional 0), 41 (rel-
ative x scale factor -optional 1), 51 (obliquing angle
-optional 0), 7 (text style name -optional "standard"), 71
(text generation flags -optional 0, see text above), 72 (text
justification type -optional 0, see text above), 11, 21, 31
(alignment point -optional, appears only if 72 group is
present and nonzero).</pre>
<pre> polyline 66 ("vertices follow flag"), 70 (polyline flags), 40 (default
starting width), 41 (default ending width), 71 and 72 (poly-
gon mesh m and n vertex counts -optional 0), 73 and 74
(smooth surface m and n densities -optional 0), 75 (smooth
surface type -optional 0). the default widths apply to any
vertex that doesn't supply widths (see below).</pre>
<pre> the "vertices follow" flag is always 1, indicating that a
series of vertex entities is expected to follow the polyline,
terminated by a sequence end (seqend) entity. the "polyline
flags" group is a bit-coded field with bits defined as fol-
lows:</pre>
<pre> flag bit value meaning
1 this is a closed polyline (or a polygon
mesh closed in the m direction)
2 curve-fit vertices have been added
4 spline-fit vertices have been added
8 this is a 3d polyline
16 this is a 3d polygon mesh. group 75 indi-
cates the smooth surface type, as follows:</pre>
<pre> 0 = no smooth surface fitted
5 = quadratic b-spline surface
6 = cubic b-spline surface
8 = bezier surface</pre>
<pre> 32 the polygon mesh is closed in the n direc-
tion</pre>
<pre> vertex 10, 20, 30 (location), 40 (starting width -optional, see
above), 41 (ending width -optional, see above), 42 (bulge),
70 (vertex flags), 50 (curve fit tangent direction
-optional). the bulge is the tangent of 1/4 the included
angle for an arc segment, made negative if the arc goes
clockwise from the start point to the end point; a bulge of 0
indicates a straight segment, and a bulge of 1 is a semicir-
cle. the meanings of the bit-coded "vertex flags" are shown
in the following table.
</pre>
<pre> 15
autocad reference manual</pre>
<pre> flag bit value meaning
1 extra vertex created by curve fitting
2 curve fit tangent defined for this vertex.
a curve fit tangent direction of 0 may be
omitted from the dxf output, but is signif-
icant if this bit is set.
4 unused (never set in dxf files)
8 spline vertex created by spline fitting
16 spline frame control point
32 3d polyline vertex
64 3d polygon mesh vertex</pre>
<pre> 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).</pre>
<pre> 3dline 10, 20, 30 (start point), 11, 21, 31 (end point).</pre>
<pre> 3dface four points defining the corners of the face: (10, 20, 30),
(11, 21, 31), (12, 22, 32), and (13, 23, 33). 70 (invisible
edge flags -optional 0). if only three points were entered
(forming a triangular face), the third and fourth points will
be the same. the meanings of the bit-coded "invisible edge
flags" are shown in the following table.</pre>
<pre> flag bit value meaning
1 first edge is invisible
2 second edge is invisible
4 third edge is invisible
8 fourth edge is invisible</pre>
<pre> dimension 2 (name of pseudo-block containing the current dimension pic-
ture), 10, 20, 30 (definition point for all dimension types),
11, 21, 31 (middle point of dimension text), 12, 22, 32
(insertion point for clones of a dimension (for baseline and
continue), 70 (dimension type; 0=rotated, horizontal, or ver-
tical; 1=aligned; 2=angular; 3=diameter; 4=radius - the value
128 is added to this field if the dimension text has been
positioned at a user-defined location rather than at the
default location), 1 (dimension text explicitly entered by
the user. if null, the dimension measurement is drawn as the
text. otherwise, this text is drawn (but if it includes the
sequence "<>", the dimension measurement is drawn in place of
the "<>")), 13, 23, 33 (definition point for linear and angu-
lar dimensions), 14, 24, 34 (definition point for linear and
angular dimensions), 15, 25, 35 (definition point for diame-
ter, radius, and angular dimensions), 16, 26, 36 (point
defining dimension arc for angular dimensions), 40 (leader
length for radius and diameter dimensions), 50 (angle of
rotated, horizontal, or vertical linear dimensions).
</pre>
<pre>16
 (c) drawing interchange and file formats</pre>
<pre> in addition, all dimension types have an optional group (code
51) that indicates the "horizontal" direction for the dimen-
sion entity. this determines the orientation of dimension
text and dimension lines for horizontal, vertical and rotated
linear dimensions. the group value is the negative of the
ecs angle of the ucs x axis in effect when the dimension was
drawn. in other words, the x axis of the ucs in effect when
the dimension was drawn is always parallel to the xy plane
for the dimension's ecs, and the angle between the ucs x axis
and the ecs x axis is a single 2d angle. the value in group
51 is the angle from "horizontal" (the effective x axis) to
the ecs x axis. entity coordinate systems (ecs) are
described later in this section.</pre>
<pre> for all dimension types, the following groups represent 3d
wcs points, regardless of the flatland setting.</pre>
<pre> 10, 20, 30
13, 23, 33
14, 24, 34
15, 25, 35</pre>
<pre> for all dimension types, the following groups represent ecs
points, and are 2d or 3d depending on the flatland setting.</pre>
<pre> 11, 21(, 31)
12, 22(, 32)
16, 26(, 36)</pre>
<pre> linear (13,23,33) the point used to specify the first extension line.
(14,24,34) the point used to specify the second extension line.
(10,20,30) the point used to specify the dimension line.</pre>
<pre> angular (13,23,33) and (14,24,34) the endpoints of the first line
(10,20,30) and (15,25,35) the endpoints of the second line
(16,26,36) the point used to specify the dimen-
sion line arc</pre>
<pre> 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.</pre>
<pre> radius (15,25,35) the point used to pick the circle/arc to dimension
(10,20,30) the center of that circle.
</pre>
<pre> 17
autocad reference manual</pre>
<pre>entity coordinate systems (ecs)</pre>
<pre>to save space in the drawing database (and in the dxf file), the points
associated with each entity are expressed in terms of its own entity coor-
dinate system (ecs). the entity coordinate system allows autocad to use a
much more compact means of representation for entities. with ecs, the only
additional information needed to describe its position in 3d space is the
3d vector describing the z axis of the ecs, and the elevation value.</pre>
<pre>for a given z axis (or extrusion) direction, there is an infinite number of
coordinate systems, defined by translating the origin in 3d space and by
rotating the x and y axes around the z axis. however, for the same z axis
direction, there is only one entity coordinate system. it has the follow-
ing properties:</pre>
<pre> o its origin coincides with the wcs origin.
o the orientation of the x and y axes within the xy plane are calcu-
lated in an arbitrary, but consistent manner. autocad performs
this calculation using the "arbitrary axis" algorithm described
below.</pre>
<pre>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.</pre>
<pre> entities notes
line, point, 3dface, 3d these entities do not lie in
polyline, 3d vertex, 3d a particular plane. all
mesh, 3d mesh vertex points are expressed in
world coordinates. of these
entities, only lines and
points can be extruded;
their extrusion direction can
differ from the world z axis.</pre>
<pre> circle, arc, solid, trace, these entities are planar in
text, attrib, attdef, shape, nature. all points are
insert, 2d polyline, 2d expressed in entity coordi-
vertex nates. all these entities
can be extruded; their
extrusion direction can
differ from the world z axis.</pre>
<pre> dimension some of a dimension's points are
expressed in wcs, and some in ecs.</pre>
<pre> others the remaining entities have
no point data and their
coordinate systems are
therefore irrelevant.</pre>
<pre>once autocad has established the ecs for a given entity, here's how it
works:</pre>
<pre>18
 (c) drawing interchange and file formats</pre>
<pre> 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.</pre>
<pre>a few ramifications of this process are:</pre>
<pre> o you can not reliably find out what ucs was in effect when an
entity was acquired. you can only find out where the entity is in
the current ucs if the current ucs has the same z axis direction
as the original ucs (i.e., they both reduce to the same ecs).
o when you enter the xy coordinates of an entity in a given ucs and
then do a dxfout, you probably won't recognize those xy coordi-
nates in the dxf file. you'll have to know the method by which
autocad calculates the x and y axes in order to work with these
values.
o the elevation value stored with an entity and output in dxf files
will be a sum of the z coordinate difference between the ucs xy
plane and the ecs xy plane, and the elevation value that the user
specified at the time the entity was drawn.
</pre>
<pre>arbitrary axis algorithm</pre>
<pre>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.</pre>
<pre>given a unit-length vector to be used as the z axis of a coordinate system,
the arbitrary axis algorithm generates a corresponding x axis for the coor-
dinate system. the y axis follows by application of the right hand rule.</pre>
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