qpdf/TODO

Next
====

*** ABI changes have been made.  build.mk has been updated.

 * 64-bit windows build, remaining steps

    - new external-libs have been built and copied into
      ~/Q/storage/releases/qpdf/external-libs.  Release is done in
      git.  Just need to upload when ready.  Remember to document that
      this version is needed for > 2.3.1.

    - update README-windows.txt docs to indicate that MSVC 2010 is the
      supported version and to update the information about mingw,
      including the need for the _FILE_OFFSET_BITS workaround on the
      32-bit version.

 * Document that your compiler has to support long long.

 * Figure out why we have to specify a stream's length in advance when
   providing stream data, and remove this restriction if possible.

 * Add a way to create new QPDFObjectHandles with a string
   representation of them, such as
   QPDFObjectHandle::parse("<< /a 1 /b 2 >>");


Soon
====

 * Provide an option to copy encryption parameters from another file.
   This would make it possible to decrypt a file, manually work with
   it, and then re-encrypt it using the original encryption parameters
   including a possibly unknown owner password.

 * See if I can support the new encryption formats mentioned in the
   open bug on sourceforge.  Check other sourceforge bugs.

 * Splitting/merging concepts

   newPDF() could create a PDF with just a trailer, no pages, and a
   minimal info.  Then the page routines could be used to add pages to
   it.

   Starting with any pdf, you should be able to copy objects from
   another pdf.  The copy should be smart about never traversing into
   a /Page or /Pages.

   We could provide a method of copying objects from one PDF into
   another.  This would do whatever optimization is necessary (maybe
   just optimizePagesTree) and then traverse the set of objects
   specified to find all objects referenced by the set.  Each of those
   would be copied over with a table mapping old ID to new ID.  This
   would be done from bottom up most likely disallowing cycles or
   handling them sanely.

   Command line could be something like

   --pages [ --new ] { file [password] numeric-range ... } ... --

   The first file referenced would be the one whose other data would
   be preserved (like trailer, info, encryption, outlines, etc.).
   --new as first file would just use an empty file as the starting
   point.

   Example: to grab pages 1-5 from file1 and 11-15 from file2

   --pages file1.pdf 1-5 file2.pdf 11-15 --

   To implement this, we would remove all pages from file1 except
   pages 1 through 5.  Then we would take pages 11 through 15 from
   file2 and add them to a set for transfer.  This would end up
   generating a list of indirect objects.  We would copy those objects
   shallowly to the new PDF keeping track of the mapping and replacing
   any indirect object keys as appropriate, much like QPDFWriter does.

   When all the objects are registered, we would add those pages to
   the result.

   This approach could work for both splitting and merging.  It's
   possible it could be implemented now without any new APIs, but most
   of the work should be doable by the library with only a small set
   of additions.

   newPDF()
   QPDFObjectCopier c(qpdf1, qpdf2)
   QPDFObjectHandle obj = c.copyObject(<object from qpdf1>)
     Without traversing pages, copies all indirect objects referenced
     by <object from qpdf1> preserving referential integrity and
     returns an object handle in qpdf2 of the same object.  If called
     multiple times on the same object, retraverses in case there were
     changes.

   QPDFObjectHandle obj = c.getMapping(<object from qpdf1>)
     find the object in qpdf2 corresponding to the object from qpdf1.
     Return the null object if none.

General
=======

 * Look for %PDF header somewhere within the first 1024 bytes of the
   file.  Also accept headers of the form "%!PS−Adobe−N.n PDF−M.m".
   See Implementation notes 13 and 14 in appendix H of the PDF 1.7
   specification.  This is bug 3267974.

 * Update qpdf docs about non-ascii passwords.  See thread from
   2010-12-07,08 for details.

 * Look at page splitting.  Subramanyam provided a test file; see
   ../misc/article-threads.pdf.  Email Q-Count: 431864 from
   2009-11-03.  See also "Splitting by Pages" below.

 * Consider writing a PDF merge utility.  With 2.2, it would be
   possible to have a StreamDataProvider that would allow stream data
   to be directly copied from one PDF file to another.  One possible
   strategy would be to have a program that adds all the pages of one
   file to the end of another file.  The basic
   strategy would be to create a table that adds new streams to the
   original file, mapping the new streams' obj/gen to a stream in the
   file whose pages are being appended.  The StreamDataProvider, when
   asked, could simply pipe the streams of the file being appended to
   the provided pipeline and could copy the filter and decode
   parameters from the original file.  Being able to do this requires
   a lot of the same logic as being able to do splitting, so a general
   split/merge program would be a great addition.

 * See whether it's possible to remove the call to
   flattenScalarReferences.  I can't easily figure out why I do it,
   but removing it causes strange test failures in linearization.  I
   would have to study the optimization and linearization code to
   figure out why I added this to begin with and what in the code
   assumes it's the case.  For enqueueObject and unparseChild in
   QPDFWriter, simply removing the checks for indirect scalars seems
   sufficient.  Looking back at the branch in the apex epub
   repository, before flattening scalar references, there was special
   case code in QPDFWriter to avoid writing out indirect nulls.  It's
   still not obvious to me why I did it though.

   To pursue this, remove the call to flattenScalarReferences in
   QPDFWriter.cc and disable the logic_error exceptions for indirect
   scalars.  Just search for flattenScalarReferences in QPDFWriter.cc
   since the logic errors have comments that mention
   flattenScalarReferences.  Then run the test suite.  Several files
   that explicitly test flattening of scalar references fail, but the
   indirect scalars are properly preserved and written.  But then
   there are some linearized files that have a bunch of unreferenced
   objects that contain scalars.  Need to figure out what these are
   and why they're there.  Maybe they're objects that used to be
   stream lengths.  Probably we just need to make sure don't traverse
   through a stream's /Length stream when enqueueing stream
   dictionaries.  This could potentially happen with any object that
   QPDFWriter replaces when writing out files.  Such objects would be
   orphaned in the newly written file.  This could be fixed, but it
   may not be worth fixing.

   If flattenScalarReferences is removed, a new method will be needed
   for checking PDF files.

 * See if we can avoid preserving unreferenced objects in object
   streams even when preserving the object streams.

 * For debugging linearization bugs, consider adding an option to save
   pass 1 of linearization.  This code is sufficient.  Change the
   interface to allow specification of a pass1 file, which would
   change the behavior as in this patch.

------------------------------
Index: QPDFWriter.cc
===================================================================
--- QPDFWriter.cc	(revision 932)
+++ QPDFWriter.cc	(working copy)
@@ -1965,11 +1965,15 @@
 
     // Write file in two passes.  Part numbers refer to PDF spec 1.4.
 
+    FILE* XXX = 0;
     for (int pass = 1; pass <= 2; ++pass)
     {
 	if (pass == 1)
 	{
-	    pushDiscardFilter();
+//	    pushDiscardFilter();
+	    XXX = fopen("/tmp/pass1.pdf", "w");
+	    pushPipeline(new Pl_StdioFile("pass1", XXX));
+	    activatePipelineStack();
 	}
 
 	// Part 1: header
@@ -2204,6 +2208,8 @@
 
 	    // Restore hint offset
 	    this->xref[hint_id] = QPDFXRefEntry(1, hint_offset, 0);
+	    fclose(XXX);
+	    XXX = 0;
 	}
     }
 }
------------------------------

 * Handle embedded files.  PDF Reference 1.7 section 3.10, "File
   Specifications", discusses this.  Once we can definitely recongize
   all embedded files in a docucment, we can update the encryption
   code to handle it properly.  In QPDF_encryption.cc, search for
   cf_file.  Remove exception thrown if cf_file is different from
   cf_stream, and write code in the stream decryption section to use
   cf_file instead of cf_stream.  In general, add interfaces to get
   the list of embedded files and to extract them.  To handle general
   embedded files associated with the whole document, follow root ->
   /Names -> /EmbeddedFiles -> /Names to get to the file specification
   dictionaries.  Then, in each file specification dictionary, follow
   /EF -> /F to the actual stream.  There may be other places file
   specification dictionaries may appear, and there are also /RF keys
   with related files, so reread section 3.10 carefully.

 * The description of Crypt filters is unclear with respect to how to
   use them to override /StmF for specific streams.  I'm not sure
   whether qpdf will do the right thing for any specific individual
   streams that might have crypt filters.  The specification seems to
   imply that only embedded file streams and metadata streams can have
   crypt filters, and there are already special cases in the code to
   handle those.  Most likely, it won't be a problem, but someday
   someone may find a file that qpdf doesn't work on because of crypt
   filters.  There is an example in the spec of using a crypt filter
   on a metadata stream.

   For now, we notice /Crypt filters and decode parameters consistent
   with the example in the PDF specification, and the right thing
   happens for metadata filters that happen to be uncompressed or
   otherwise compressed in a way we can filter.  This should handle
   all normal cases, but it's more or less just a guess since I don't
   have any test files that actually use stream-specific crypt filters
   in them.

 * The second xref stream for linearized files has to be padded only
   because we need file_size as computed in pass 1 to be accurate.  If
   we were not allowing writing to a pipe, we could seek back to the
   beginning and fill in the value of /L in the linearization
   dictionary as an optimization to alleviate the need for this
   padding.  Doing so would require us to pad the /L value
   individually and also to save the file descriptor and determine
   whether it's seekable.  This is probably not worth bothering with.

 * The whole xref handling code in the QPDF object allows the same
   object with more than one generation to coexist, but a lot of logic
   assumes this isn't the case.  Anything that creates mappings only
   with the object number and not the generation is this way,
   including most of the interaction between QPDFWriter and QPDF.  If
   we wanted to allow the same object with more than one generation to
   coexist, which I'm not sure is allowed, we could fix this by
   changing xref_table.  Alternatively, we could detect and disallow
   that case.  In fact, it appears that Adobe reader and other PDF
   viewing software silently ignores objects of this type, so this is
   probably not a big deal.

 * Pl_PNGFilter is only partially implemented.  If we ever decoded
   images, we'd have to finish implementing it along with the other
   filter decode parameters and types.  For just handling xref
   streams, there's really no need as it wouldn't make sense to use
   any kind of predictor other than 12 (PNG UP filter).

 * If we ever want to have check mode check the integrity of the free
   list, this can be done by looking at the code from prior to the
   object stream support of 4/5/2008.  It's in an if (0) block and
   there's a comment about it.  There's also something about it in
   qpdf.test -- search for "free table".  On the other hand, the value
   of doing this seems very low since no viewer seems to care, so it's
   probably not worth it.

 * QPDFObjectHandle::getPageImages() doesn't notice images in
   inherited resource dictionaries.  See comments in that function.

 * Based on an idea suggested by user "Atom Smasher", consider
   providing some mechanism to recover earlier versions of a file
   embedded prior to appended sections.

 * From a suggestion in bug 3152169, consisder having an option to
   re-encode inline images with an ASCII encoding.


Splitting by Pages
==================

Although qpdf does not currently support splitting a file into pages,
the work done for linearization covers almost all the work.  To do
page splitting.  If this functionality is needed, study
obj_user_to_objects and object_to_obj_users created in
QPDF_optimization for ideas.  It's quite possible that the information
computed by calculateLinearizationData is actually sufficient to do
page splitting in many circumstances.  That code knows which objects
are used by which pages, though it doesn't do anything page-specific
with outlines, thumbnails, page labels, or anything else.

Another approach would be to traverse only pages that are being output
taking care not to traverse into the pages tree, and then to fabricate
a new pages tree.

Either way, care must be taken to handle other things such as
outlines, page labels, thumbnails, threads, zones, etc. in a sensible
way.  This may include simply omitting information other than page
content.