SECTION 8.6
711
Interactive Forms
FDF is based on PDF; it uses the same syntax (see Section 3.1, “Lexical Conven-
same file structure (Section 3.4, “File Structure”). However, it differs from PDF in
the following ways:
•
The cross-reference table (Section 3.4.3, “Cross-Reference Table”) is optional.
•
FDF files cannot be updated (see Section 3.4.5, “Incremental Updates”). Ob-
jects can only be of generation 0, and no two objects can have the same object
number.
•
The document structure is much simpler than PDF, since the body of an FDF
document consists of only one required object.
•
The length of a stream may not be specified by an indirect object.
FDF uses the MIME content type
application / vnd . fdf
. On the Windows and
UNIX platforms, FDF files have the extension
. fdf
; on Mac OS, they have file type
' FDF '
.
FDF File Structure
An FDF file is structured in essentially the same way as a PDF file but contains
only those elements required for the export and import of interactive form and
annotation data. It consists of three required elements and one optional element
(see Figure 8.10):
•
A one-line
header
identifying the version number of the PDF specification to
which the file conforms
•
A
body
containing the objects that make up the content of the file
•
An optional
cross-reference table
containing information about the objects in
the file
•
A
trailer
giving the location of various objects within the body of the file