qpdf/TODO

General
=======

 * 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.

 * 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 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.

 * 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.

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.