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Next
====
* Find PDF header in the first 1024 bytes of the file. Treat the
location of the PDF header as offset 0 for purposes of resolving
explicit file locations as this is what other implementations
appear to do.
General
=======
* See if I can support the encryption format used with /R 5 /V 5
(AESV3), even though a qpdf-announce subscriber with an adobe.com
email address mentioned that this is deprecated. There is also a
new encryption format coming in a future release (PDF 2.0), which
may be better to support. As of the qpdf 3.0 release, the
specification was not publicly available yet.
AESV3 encryption is supported with PDF 1.7 extension level 3 and is
being deprecated, but there are plenty of files out there. The
encryption format is decribed in adobe_supplement_iso32000.pdf.
Such a file must specify that it uses these extensions in its
document catalog:
<<
/Type /Catalog
/Extensions <<
/ADBE <<
/BaseVersion /1.7
/ExtensionLevel 3
>>
>>
>>
Possible sha256 implementations: http://sol-biotech.com/code/sha2/,
http://hashlib2plus.sourceforge.net/
* Consider the possibility of doing something locale-aware to support
non-ASCII passwords. Update documentation if this is done.
* 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.
* Consider impact of article threads on page splitting/merging.
Subramanyam provided a test file; see ../misc/article-threads.pdf.
Email Q-Count: 431864 from 2009-11-03. Other things to consider:
outlines, page labels, thumbnails, zones. There are probably
others.
* 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 recognize
all embedded files in a document, 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.
A sourceforge user asks if qpdf can handle extracting and embedded
resources and references these tools, which may be useful as a
reference.
http://multivalent.sourceforge.net/Tools/pdf/Extract.html
http://multivalent.sourceforge.net/Tools/pdf/Embed.html
* 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, consider having an option to
re-encode inline images with an ASCII encoding.
* From github issue 2, provide more in-depth output for examining
hint stream contents.
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