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