drawing interchange and file formats(AutoCAD DXF 10.0版文件格式说明)
a dxf file is composed of a multiplicity of groups, each of which occupies
two lines in the dxf file. the first line of a group is a group code,
which is a positive nonzero integer output in fortran "i3" format (that is,
right justified and blank filled in a three character field). the second
line of the group is the group value, in a format which depends on the type
of the group as specified by the group code.
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autocad reference manual the specific assignment of group codes depends upon the item being
described in the file. however, the type of the value this group supplies
is derived from the group code in the following way: group code range following value
0 - 9 string
10 - 59 floating-point
60 - 79 integer
210 - 239 floating-point
999 comment (string)thus a program can easily read the value following a group code without
knowing the particular use of this group in an item in the file. the
appearance of values in the dxf file is not affected by the setting of the
units command: coordinates are always represented as decimal (or possibly
scientific notation if very large) numbers, and angles are always repre-
sented in decimal degrees with zero degrees to the east of origin. variables, table entries, and entities are described by a group that intro-
duces the item, giving its type and/or name, followed by multiple groups
that supply the values associated with the item. in addition, special
groups are used for file separators such as markers for the beginning and
end of sections, tables, and the file itself. entities, table entries, and file separators are always introduced with a 0
group code that is followed by a name describing the item.
c.1.3.2 group codes group codes are used both to indicate the type of the value of the group,
as explained above, and to indicate the general use of the group. the spe-
cific function of the group code depends on the actual variable, table
item, or entity description. this section indicates the general use of
groups, noting as "(fixed)" any that always have the same function. group code value type
0 identifies the start of an entity, table entry, or file
separator. the text value that follows indicates which.
1 the primary text value for an entity.
2 a name; attribute tag, block name, etc.
3-4 other textual or name values.
5 entity handle expressed as a hexadecimal string.
6 line type name (fixed).
7 text style name (fixed).
8 layer name (fixed).
9 variable name identifier (used only in header section of
the dxf file).
10 primary x coordinate (start point of a line or text
entity, center of a circle, etc.).
11-18 other x coordinates.
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