mirror of
https://github.com/qpdf/qpdf.git
synced 2024-12-23 03:18:59 +00:00
99 lines
4.2 KiB
ReStructuredText
99 lines
4.2 KiB
ReStructuredText
.. _qdf:
|
|
|
|
QDF Mode
|
|
========
|
|
|
|
.. Help text for --qdf refers to the name of this chapter.
|
|
|
|
In QDF mode, qpdf creates PDF files in what we call *QDF
|
|
form*. A PDF file in QDF form, sometimes called a QDF
|
|
file, is a completely valid PDF file that has ``%QDF-1.0`` as its third
|
|
line (after the pdf header and binary characters) and has certain other
|
|
characteristics. The purpose of QDF form is to make it possible to edit
|
|
PDF files, with some restrictions, in an ordinary text editor. This can
|
|
be very useful for experimenting with different PDF constructs or for
|
|
making one-off edits to PDF files (though there are other reasons why
|
|
this may not always work). Note that QDF mode does not support
|
|
linearized files. If you enable linearization, QDF mode is automatically
|
|
disabled.
|
|
|
|
It is ordinarily very difficult to edit PDF files in a text editor for
|
|
two reasons: most meaningful data in PDF files is compressed, and PDF
|
|
files are full of offset and length information that makes it hard to
|
|
add or remove data. A QDF file is organized in a manner such that, if
|
|
edits are kept within certain constraints, the
|
|
:command:`fix-qdf` program, distributed with qpdf, is
|
|
able to restore edited files to a correct state. The
|
|
:command:`fix-qdf` program takes no command-line
|
|
arguments. It reads a possibly edited QDF file from standard input and
|
|
writes a repaired file to standard output.
|
|
|
|
The following attributes characterize a QDF file:
|
|
|
|
- All objects appear in numerical order in the PDF file, including when
|
|
objects appear in object streams.
|
|
|
|
- Objects are printed in an easy-to-read format, and all line endings
|
|
are normalized to UNIX line endings.
|
|
|
|
- Unless specifically overridden, streams appear uncompressed (when
|
|
qpdf supports the filters and they are compressed with a non-lossy
|
|
compression scheme), and most content streams are normalized (line
|
|
endings are converted to just a UNIX-style linefeeds).
|
|
|
|
- All streams lengths are represented as indirect objects, and the
|
|
stream length object is always the next object after the stream. If
|
|
the stream data does not end with a newline, an extra newline is
|
|
inserted, and a special comment appears after the stream indicating
|
|
that this has been done.
|
|
|
|
- If the PDF file contains object streams, if object stream *n*
|
|
contains *k* objects, those objects are numbered from *n+1* through
|
|
*n+k*, and the object number/offset pairs appear on a separate line
|
|
for each object. Additionally, each object in the object stream is
|
|
preceded by a comment indicating its object number and index. This
|
|
makes it very easy to find objects in object streams.
|
|
|
|
- All beginnings of objects, ``stream`` tokens, ``endstream`` tokens,
|
|
and ``endobj`` tokens appear on lines by themselves. A blank line
|
|
follows every ``endobj`` token.
|
|
|
|
- If there is a cross-reference stream, it is unfiltered.
|
|
|
|
- Page dictionaries and page content streams are marked with special
|
|
comments that make them easy to find.
|
|
|
|
- Comments precede each object indicating the object number of the
|
|
corresponding object in the original file.
|
|
|
|
When editing a QDF file, any edits can be made as long as the above
|
|
constraints are maintained. This means that you can freely edit a page's
|
|
content without worrying about messing up the QDF file. It is also
|
|
possible to add new objects so long as those objects are added after the
|
|
last object in the file or subsequent objects are renumbered. If a QDF
|
|
file has object streams in it, you can always add the new objects before
|
|
the xref stream and then change the number of the xref stream, since
|
|
nothing generally ever references it by number.
|
|
|
|
It is not generally practical to remove objects from QDF files without
|
|
messing up object numbering, but if you remove all references to an
|
|
object, you can run qpdf on the file (after running
|
|
:command:`fix-qdf`), and qpdf will omit the now-orphaned
|
|
object.
|
|
|
|
When :command:`fix-qdf` is run, it goes through the file
|
|
and recomputes the following parts of the file:
|
|
|
|
- the ``/N``, ``/W``, and ``/First`` keys of all object stream
|
|
dictionaries
|
|
|
|
- the pairs of numbers representing object numbers and offsets of
|
|
objects in object streams
|
|
|
|
- all stream lengths
|
|
|
|
- the cross-reference table or cross-reference stream
|
|
|
|
- the offset to the cross-reference table or cross-reference stream
|
|
following the ``startxref`` token
|