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Next
====
Before Release:
* Make ./performance_check usable by other people by having published
files to use for testing.
* https://opensource.adobe.com/dc-acrobat-sdk-docs/standards/pdfstandards/pdf/PDF32000_2008.pdf
* Evaluate issues tagged with `next`
* Stay on top of https://github.com/pikepdf/pikepdf/pull/315
Next:
* Support json v2 in the C API. At a minimum, write_json,
create_from_json, and update_from_json need to be there and should
take the same kinds of functions as the C API for logger.
Pending changes:
* Consider also exposing a way to set a new logger and to get the
logger from QPDF and QPDFJob in the C API.
* Check about runpath in the linux-bin distribution. I think the
appimage build specifically is setting the runpath, which is
actually desirable in this case. Make sure to understand and
document this. Maybe add a check for it in the build.
Soon: Break ground on "Document-level work"
Fix Multiple Direct Object Owner Issue
======================================
These are some ideas I've had, but I'm parking them until I fully
understand m-holger's proposal to split QPDFObject into QPDFObject and
QPDFValue.
* Add std::weak_ptr<QPDFObject> parent to QPDFObject. When adding a
direct object to an array or dictionary, set its parent. When
removing it, clear the parent pointer.
* When a direct object that already has a parent is added to
something, it is a warning and will become an error in qpdf 12.
There needs to be unsafe add methods used by unsafeShallowCopy.
These will add but not modify the parent pointer.
This allows an object to be moved from one object to another by
removing it, which returns the now orphaned object, and then inserting
it somewhere else. It also doesn't break the pattern of adding a
direct object to something and subsequently mutating it. It just
prevents the same object from being added to more than one thing.
Note that arrays and dictionaries still need to contain
QPDFObjectHandle because of indirect objects. This only pertains to
direct objects, which are always "resolved" in QPDFObjectHandle.
Possible future JSON enhancements
=================================
* Add to JSON output the information available from a few additional
informational options:
* --check: add but maybe not by default?
* --show-linearization: add but maybe not by default? Also figure
out whether warnings reported for some of the PDF specs (1.7) are
qpdf problems. This may not be worth adding in the first
increment.
* --show-xref: add
* Consider having --check, --show-encryption, etc., just select the
right keys when in json mode. I don't think I want check on by
default, so that might be different.
* Consider having warnings be included in the json in a "warnings" key
in json mode.
QPDFJob
=======
Here are some ideas for QPDFJob that didn't make it into 10.6. Not all
of these are necessarily good -- just things to consider.
* How do we chain jobs? The idea would be that the input and/or output
of a QPDFJob could be a QPDF object rather than a file. For input,
it's pretty easy. For output, none of the output-specific options
(encrypt, compress-streams, objects-streams, etc.) would have any
affect, so we would have to treat this like inspect for error
checking. The QPDF object in the state where it's ready to be sent
off to QPDFWriter would be used as the input to the next QPDFJob.
For the job json, I think we can have the output be an identifier
that can be used as the input for another QPDFJob. For a json file,
we could the top level detect if it's an array with the convention
that exactly one has an output, or we could have a subkey with other
job definitions or something. Ideally, any input
(copy-attachments-from, pages, etc.) could use a QPDF object. It
wouldn't surprise me if this exposes bugs in qpdf around foreign
streams as this has been a relatively fragile area before.
Documentation
=============
* Do a full pass through the documentation.
* Make sure `qpdf` is consistent. Use QPDF when just referring to
the package.
* Make sure markup is consistent
* Autogenerate where possible
* Consider which parts might be good candidates for moving to the
wiki.
* Commit 'Manual - enable line wrapping in table cells' from
Mon Jan 17 12:22:35 2022 +0000 enables table cell wrapping. See if
this can be incorporated directly into sphinx_rtd_theme and the
workaround can be removed.
* When possible, update the debian package to include docs again. See
https://bugs.debian.org/1004159 for details.
Document-level work
===================
* Ideas here may by superseded by #593.
* QPDFPageCopier -- object for moving pages around within files or
between files and performing various transformations. Reread/rewrite
_page-selection in the manual if needed.
* Handle all the stuff of pages and split-pages
* Do n-up, booklet, collation
* Look through cli and see what else...flatten-*?
* See comments in QPDFPageDocumentHelper.hh for addPage -- search
for "a future version".
* Make it efficient for bulk operations
* Make certain doc-level features selectable
* qpdf.cc should do all its page operations, including
overlay/underlay, splitting, and merging, using this
* There should also be example code
* After doc-level checks are in, call --check on the output files in
the "Copy Annotations" tests.
* Document-level checks. For example, for forms, make sure all form
fields point to an annotation on exactly one page as well as that
all widget annotations are associated with a form field. Hook this
into QPDFPageCopier as well as the doc helpers. Make sure it is
called from --check.
* See also issues tagged with "pages"
* Add flags to CLI to select which document-level options to
preserve or not preserve. We will probably need a pair of mutually
exclusive, repeatable options with a way to specify all, none, only
{x,y}, or all but {x,y}.
* If a page contains a reference a file attachment annotation, when
that page is copied, if the file attachment appears in the top-level
EmbeddedFiles tree, that entry should be preserved in the
destination file. Otherwise, we probably will require the use of
--copy-attachments-from to preserve these. What will the strategy be
for deduplicating in the automatic case?
Text Appearance Streams
=======================
This is a list of known issues with text appearance streams and things
we might do about it.
* For variable text, the spec says to pull any resources from /DR that
are referenced in /DA but if the resource dictionary already has
that resource, just use the one that's there. The current code looks
only for /Tf and adds it if needed. We might want to instead merge
/DR with resources and then remove anything that's unreferenced. We
have all the code required for that in ResourceFinder except
TfFinder also gets the font size, which ResourceFinder doesn't do.
* There are things we are missing because we don't look at font
metrics. The code from TextBuilder (work) has almost everything in
it that is required. Once we have knowledge of character widths, we
can support quadding and multiline text fields (/Ff 4096), and we
can potentially squeeze text to fit into a field. For multiline,
first squeeze vertically down to the font height, then squeeze
horizontally with Tz. For single line, squeeze horizontally with Tz.
If we use Tz, issue a warning.
* When mapping characters to widths, we will need to care about
character encoding. For built-in fonts, we can create a map from
Unicode code point to width and then go from the font's encoding to
unicode to the width. See misc/character-encoding/ (not on github)
and font metric information for the 14 standard fonts in my local
pdf-spec directory.
* Once we know about character widths, we can correctly support
auto-sized variable text fields (0 Tf). If this is fixed, search for
"auto-sized" in cli.rst.
Fuzz Errors
===========
* https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=<N>
* Ignoring these:
* Out of memory in dct: 35001, 32516
External Libraries
==================
Current state (10.0.2):
* qpdf/external-libs repository builds external-libs on a schedule.
It detects and downloads the latest versions of zlib, jpeg, and
openssl and creates source and binary distribution zip files in an
artifact called "distribution".
* Releases in qpdf/external-libs are made manually. They contain
qpdf-external-libs-{bin,src}.zip.
* The qpdf build finds the latest non-prerelease release and downloads
the qpdf-external-libs-*.zip files from the releases in the setup
stage.
* To upgrade to a new version of external-libs, create a new release
of qpdf/external-libs (see README-maintainer in external-libs) from
the distribution artifact of the most recent successful build after
ensuring that it works.
Desired state:
* The qpdf/external-libs repository should create release candidates.
Ideally, every scheduled run would make its zip files available. A
personal access token with actions:read scope for the
qpdf/external-libs repository is required to download the artifact
from an action run, and qpdf/qpdf's secrets.GITHUB_TOKEN doesn't
have this access. We could create a service account for this
purpose. As an alternative, we could have a draft release in
qpdf/external-libs that the qpdf/external-libs build could update
with each candidate. It may also be possible to solve this by
developing a simple GitHub app.
* Scheduled runs of the qpdf build in the qpdf/qpdf repository (not a
fork or pull request) could download external-libs from the release
candidate area instead of the latest stable release. Pushes to the
build branch should still use the latest release so it always
matches the main branch.
* Periodically, we would create a release of external-libs from the
release candidate zip files. This could be done safely because we
know the latest qpdf works with it. This could be done at least
before every release of qpdf, but potentially it could be done at
other times, such as when a new dependency version is available or
after some period of time.
Other notes:
* The external-libs branch in qpdf/qpdf was never documented. We might
be able to get away with deleting it.
* See README-maintainer in qpdf/external-libs for information on
creating a release. This could be at least partially scripted in a
way that works for the qpdf/qpdf repository as well since they are
very similar.
ABI Changes
===========
This is a list of changes to make next time there is an ABI change.
Comments appear in the code prefixed by "ABI". Always Search for ABI
in source and header files to find items not listed here. Also search
for "[[deprecated" to find deprecated APIs that can be removed.
Page splitting/merging
======================
* Update page splitting and merging to handle document-level
constructs with page impact such as interactive forms and article
threading. Check keys in the document catalog for others, such as
outlines, page labels, thumbnails, and zones. For threads,
Subramanyam provided a test file; see ../misc/article-threads.pdf.
Email Q-Count: 431864 from 2009-11-03.
* bookmarks (outlines) 12.3.3
* support bookmarks when merging
* prune bookmarks that don't point to a surviving page when merging
or splitting
* make sure conflicting named destinations work possibly test by
including the same file by two paths in a merge
* see also comments in issue 343
Note: original implementation of bookmark preservation for split
pages caused a very high performance hit. The problem was
introduced in 313ba081265f69ac9a0324f9fe87087c72918191 and reverted
in the commit that adds this paragraph. The revert includes marking
a few tests cases as $td->EXPECT_FAILURE. When properly coded, the
test cases will need to be adjusted to only include the parts of
the outlines that are actually copied. The tests in question are
"split page with outlines". When implementing properly, ensure that
the performance is not adversely affected by timing split-pages on
a large file with complex outlines such as the PDF specification.
When pruning outlines, keep all outlines in the hierarchy that are
above an outline for a page we care about. If one of the ancestor
outlines points to a non-existent page, clear its dest. If an
outline does not have any children that point to pages in the
document, just omit it.
Possible strategy:
* resolve all named destinations to explicit destinations
* concatenate top-level outlines
* prune outlines whose dests don't point to a valid page
* recompute all /Count fields
Test files
* page-labels-and-outlines.pdf: old file with both page labels and
outlines. All destinations are explicit destinations. Each page
has Potato and a number. All titles are feline names.
* outlines-with-actions.pdf: mixture of explicit destinations,
named destinations, goto actions with explicit destinations, and
goto actions with named destinations; uses /Dests key in names
dictionary. Each page has Salad and a number. All titles are
silly words. One destination is an indirect object.
* outlines-with-old-root-dests.pdf: like outlines-with-actions
except it uses the PDF-1.1 /Dests dictionary for named
destinations, and each page has Soup and a number. Also pages are
numbered with upper-case Roman numerals starting with 0. All
titles are silly words preceded by a bullet.
If outline handling is significantly improved, see
../misc/bad-outlines/bad-outlines.pdf and email:
https://mail.google.com/mail/u/0/#search/rfc822msgid%3A02aa01d3d013%249f766990%24de633cb0%24%40mono.hr)
* Form fields: should be similar to outlines.
Analytics
=========
Consider features that make it easier to detect certain patterns in
PDF files. The information below could be computed using an external
program that reads the existing json, but if it's useful enough, we
could add it directly to the json output.
* Add to "pages" in the json:
* "inheritsresources": bool; whether there are any inherited
attributes from ancestor page tree nodes
* "sharedresources": a list of indirect objects that are
"/Resources" dictionaries or "XObject" resource dictionary subkeys
of either the page itself or of any form XObject referenced by the
page.
* Add to "objectinfo" in json: "directpagerefcount": the number of
pages that directly reference this object (i.e., you can find an
indirect reference to the object in the page dictionary without
traversing over any indirect objects)
General
=======
NOTE: Some items in this list refer to files in my personal home
directory or that are otherwise not publicly accessible. This includes
things sent to me by email that are specifically not public. Even so,
I find it useful to make reference to them in this list.
* Add an option --ignore-encryption to ignore encryption information
and treat encrypted files as if they weren't encrypted. This should
make it possible to solve #598 (--show-encryption without a
password). We'll need to make sure we don't try to filter any
streams in this mode. Ideally we should be able to combine this with
--json so we can look at the raw encrypted strings and streams if we
want to, though be sure to document that the resulting JSON won't be
convertible back to a valid PDF. Since providing the password may
reveal additional details, --show-encryption could potentially retry
with this option if the first time doesn't work. Then, with the file
open, we can read the encryption dictionary normally.
* In libtests, separate executables that need the object library
from those that strictly use public API. Move as many of the test
drivers from the qpdf directory into the latter category as long
as doing so isn't too troublesome from a coverage standpoint.
* Consider generating a non-flat pages tree before creating output to
better handle files with lots of pages. If there are more than 256
pages, add a second layer with the second layer nodes having no more
than 256 nodes and being as evenly sizes as possible. Don't worry
about the case of more than 65,536 pages. If the top node has more
than 256 children, we'll live with it. This is only safe if all
intermediate page nodes have only /Kids, /Parent, /Type, and /Count.
* Look at https://bestpractices.coreinfrastructure.org/en
* Consider adding fuzzer code for JSON
* Rework tests so that nothing is written into the source directory.
Ideally then the entire build could be done with a read-only
source tree.
* Large file tests fail with linux32 before and after cmake. This was
first noticed after 10.6.3. I don't think it's worth fixing.
* Consider updating the fuzzer with code that exercises
copyAnnotations, file attachments, and name and number trees. Check
fuzzer coverage.
* Add code for creation of a file attachment annotation. It should
also be possible to create a widget annotation and a form field.
Update the pdf-attach-file.cc example with new APIs when ready.
* Flattening of form XObjects seems like something that would be
useful in the library. We are seeing more cases of completely valid
PDF files with form XObjects that cause problems in other software.
Flattening of form XObjects could be a useful way to work around
those issues or to prepare files for additional processing, making
it possible for users of the qpdf library to not be concerned about
form XObjects. This could be done recursively; i.e., we could have a
method to embed a form XObject into whatever contains it, whether
that is a form XObject or a page. This would require more
significant interpretation of the content stream. We would need a
test file in which the placement of the form XObject has to be in
the right place, e.g., the form XObject partially obscures earlier
code and is partially obscured by later code. Keys in the resource
dictionary may need to be changed -- create test cases with lots of
duplicated/overlapping keys.
* Part of closed_file_input_source.cc is disabled on Windows because
of odd failures. It might be worth investigating so we can fully
exercise this in the test suite. That said, ClosedFileInputSource
is exercised elsewhere in qpdf's test suite, so this is not that
pressing.
* If possible, consider adding CCITT3, CCITT4, or any other easy
filters. For some reference code that we probably can't use but may
be handy anyway, see
http://partners.adobe.com/public/developer/ps/sdk/index_archive.html
* If possible, support the following types of broken files:
- Files that have no whitespace token after "endobj" such that
endobj collides with the start of the next object
- See ../misc/broken-files
- See ../misc/bad-files-issue-476. This directory contains a
snapshot of the google doc and linked PDF files from issue #476.
Please see the issue for details.
* Additional form features
* set value from CLI? Specify title, and provide way to
disambiguate, probably by giving objgen of field
* Pl_TIFFPredictor is pretty slow.
* Support for handling file names with Unicode characters in Windows
is incomplete. qpdf seems to support them okay from a functionality
standpoint, and the right thing happens if you pass in UTF-8
encoded filenames to QPDF library routines in Windows (they are
converted internally to wchar_t*), but file names are encoded in
UTF-8 on output, which doesn't produce nice error messages or
output on Windows in some cases.
* If we ever wanted to do anything more with character encoding, see
../misc/character-encoding/, which includes machine-readable dump
of table D.2 in the ISO-32000 PDF spec. This shows the mapping
between Unicode, StandardEncoding, WinAnsiEncoding,
MacRomanEncoding, and PDFDocEncoding.
* Some test cases on bad files fail because qpdf is unable to find
the root dictionary when it fails to read the trailer. Recovery
could find the root dictionary and even the info dictionary in
other ways. In particular, issue-202.pdf can be opened by evince,
and there's no real reason that qpdf couldn't be made to be able to
recover that file as well.
* Audit every place where qpdf allocates memory to see whether there
are cases where malicious inputs could cause qpdf to attempt to
grab very large amounts of memory. Certainly there are cases like
this, such as if a very highly compressed, very large image stream
is requested in a buffer. Hopefully normal input to output
filtering doesn't ever try to do this. QPDFWriter should be checked
carefully too. See also bugs/private/from-email-663916/
* Interactive form modification:
https://github.com/qpdf/qpdf/issues/213 contains a good discussion
of some ideas for adding methods to modify annotations and form
fields if we want to make it easier to support modifications to
interactive forms. Some of the ideas have been implemented, and
some of the probably never will be implemented, but it's worth a
read if there is an intention to work on this. In the issue, search
for "Regarding write functionality", and read that comment and the
responses to it.
* Look at ~/Q/pdf-collection/forms-from-appian/
* When decrypting files with /R=6, hash_V5 is called more than once
with the same inputs. Caching the results or refactoring to reduce
the number of identical calls could improve performance for
workloads that involve processing large numbers of small files.
* Consider adding a method to balance the pages tree. It would call
pushInheritedAttributesToPage, construct a pages tree from scratch,
and replace the /Pages key of the root dictionary with the new
tree.
* Study what's required to support savable forms that can be saved by
Adobe Reader. Does this require actually signing the document with
an Adobe private key? Search for "Digital signatures" in the PDF
spec, and look at ~/Q/pdf-collection/form-with-full-save.pdf, which
came from Adobe's example site. See also
../misc/digital-sign-from-trueroad/. If digital signatures are
implemented, update the docs on crypto providers, which mention
that this may happen in the future.
* Qpdf does not honor /EFF when adding new file attachments. When it
encrypts, it never generates streams with explicit crypt filters.
Prior to 10.2, there was an incorrect attempt to treat /EFF as a
default value for decrypting file attachment streams, but it is not
supposed to mean that. Instead, it is intended for conforming
writers to obey this when adding new attachments. Qpdf is not a
conforming writer in that respect.
* 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.
* 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. Consider adding on option to provide a
human-readable dump of linearization hint tables. This should
include improving the 'overflow reading bit stream' message as
reported in issue #2. There are multiple calls to stopOnError in
the linearization checking code. Ideally, these should not
terminate checking. It would require re-acquiring an understanding
of all that code to make the checks more robust. In particular,
it's hard to look at the code and quickly determine what is a true
logic error and what could happen because of malformed user input.
See also ../misc/linearization-errors.
* If I ever decide to make appearance stream-generation aware of
fonts or font metrics, see email from Tobias with Message-ID
<5C3C9C6C.8000102@thax.hardliners.org> dated 2019-01-14.
* Look at places in the code where object traversal is being done and,
where possible, try to avoid it entirely or at least avoid ever
traversing the same objects multiple times.
----------------------------------------------------------------------
HISTORICAL NOTES
Performance
===========
As described in https://github.com/qpdf/qpdf/issues/401, there was
great performance degradation between qpdf 7.1.1 and 9.1.1. Doing a
bisect between dac65a21fb4fa5f871e31c314280b75adde89a6c and
release-qpdf-7.1.1, I found several commits that damaged performance.
I fixed some of them to improve performance by about 70% (as measured
by saying that old times were 170% of new times). The remaining
commits that broke performance either can't be correct because they
would re-introduce an old bug or aren't worth correcting because of
the high value they offer relative to a relatively low penalty. For
historical reference, here are the commits. The numbers are the time
in seconds on the machine I happened to be using of splitting the
first 100 pages of PDF32000_2008.pdf 20 times and taking an average
duration.
Commits that broke performance:
* d0e99f195a987c483bbb6c5449cf39bee34e08a1 -- object description and
context: 0.39 -> 0.45
* a01359189b32c60c2d55b039f7aefd6c3ce0ebde (minus 313ba08) -- fix
dangling references: 0.55 -> 0.6
* e5f504b6c5dc34337cc0b316b4a7b1fca7e614b1 -- sparse array: 0.6 -> 0.62
Other intermediate steps that were previously fixed:
* 313ba081265f69ac9a0324f9fe87087c72918191 -- copy outlines into
split: 0.55 -> 4.0
* a01359189b32c60c2d55b039f7aefd6c3ce0ebde -- fix dangling references:
4.0 -> 9.0
This commit fixed the awful problem introduced in 313ba081:
* a5a016cdd26a8e5c99e5f019bc30d1bdf6c050a2 -- revert outline
preservation: 9.0 -> 0.6
Note that the fix dangling references commit had a much worse impact
prior to removing the outline preservation, so I also measured its
impact in isolation.
A few important lessons (in README-maintainer)
* Indirection through PointerHolder<Members> is expensive, and should
not be used for things that are created and destroyed frequently
such as QPDFObjectHandle and QPDFObject.
* Traversal of objects is expensive and should be avoided where
possible.
Also, it turns out that PointerHolder is more performant than
std::shared_ptr. (This was true at the time but subsequent
implementations of std::shared_ptr became much more efficient.)
QPDFPagesTree
=============
On a few occasions, I have considered implementing a QPDFPagesTree
object that would allow the document's original page tree structure to
be preserved. See comments at the top QPDF_pages.cc for why this was
abandoned.
Partial work is in refs/attic/QPDFPagesTree. QPDFPageTree is mostly
implemented and mostly tested. There are not enough cases of different
kinds of operations (pclm, linearize, json, etc.) with non-flat pages
trees. Insertion is not implemented. Insertion is potentially complex
because of the issue of inherited objects. We will have to call
pushInheritedAttributesToPage before adding any pages to the pages
tree. The test suite is failing on that branch.
Some parts of page tree repair are silent (no warnings). All page tree
repair should warn. The reason is that page tree repair will change
object numbers, and knowing that is important when working with JSON
output.
If we were to do this, we would still need keep a pages cache for
efficient insertion. There's no reason we can't keep a vector of page
objects up to date and just do a traversal the first time we do
getAllPages just like we do now. The difference is that we would not
flatten the pages tree. It would be useful to go through QPDF_pages
and reimplement everything without calling flattenPagesTree. Then we
can remove flattenPagesTree, which is private. That said, with the
addition of creating non-flat pages trees, there is really no reason
not to flatten the pages tree for internal use.
In its current state, QPDFPagesTree does not proactively fix /Type or
correct page objects that are used multiple times. You have to
traverse the pages tree to trigger this operation. It would be nice if
we would do that somewhere but not do it more often than necessary so
isPagesObject and isPageObject are reliable and can be made more
reliable. Maybe add a validate or repair function? It should also make
sure /Count and /Parent are correct.
Rejected Ideas
==============
* Investigate whether there is a way to automate the memory checker
tests for Windows.
* Provide support in QPDFWriter for writing incremental updates.
Provide support in qpdf for preserving incremental updates. The
goal should be that QDF mode should be fully functional for files
with incremental updates including fix_qdf.
Note that there's nothing that says an indirect object in one
update can't refer to an object that doesn't appear until a later
update. This means that QPDF has to treat indirect null objects
differently from how it does now. QPDF drops indirect null objects
that appear as members of arrays or dictionaries. For arrays, it's
handled in QPDFWriter where we make indirect nulls direct. This is
in a single if block, and nothing else in the code cares about it.
We could just remove that if block and not break anything except a
few test cases that exercise the current behavior. For
dictionaries, it's more complicated. In this case,
QPDF_Dictionary::getKeys() ignores all keys with null values, and
hasKey() returns false for keys that have null values. We would
probably want to make QPDF_Dictionary able to handle the special
case of keys that are indirect nulls and basically never have it
drop any keys that are indirect objects.
If we make a change to have qpdf preserve indirect references to
null objects, we have to note this in ChangeLog and in the release
notes since this will change output files. We did this before when
we stopped flattening scalar references, so this is probably not a
big deal. We also have to make sure that the testing for this
handles non-trivial cases of the targets of indirect nulls being
replaced by real objects in an update. I'm not sure how this plays
with linearization, if at all. For cases where incremental updates
are not being preserved as incremental updates and where the data
is being folded in (as is always the case with qpdf now), none of
this should make any difference in the actual semantics of the
files.
* 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.
* 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.
* Consider creating a sanitizer to make it easier for people to send
broken files. Now that we have json mode, this is probably no
longer worth doing. Here is the previous idea, possibly implemented
by making it possible to run the lexer (tokenizer) over a whole
file. Make it possible to replace all strings in a file lexically
even on badly broken files. Ideally this should work files that are
lacking xref, have broken links, duplicated dictionary keys, syntax
errors, etc., and ideally it should work with encrypted files if
possible. This should go through the streams and strings and
replace them with fixed or random characters, preferably, but not
necessarily, in a manner that works with fonts. One possibility
would be to detect whether a string contains characters with normal
encoding, and if so, use 0x41. If the string uses character maps,
use 0x01. The output should otherwise be unrelated to the input.
This could be built after the filtering and tokenizer rewrite and
should be done in a manner that takes advantage of the other
lexical features. This sanitizer should also clear metadata and
replace images. If I ever do this, the file from issue #494 would
be a great one to look at.
* Here are some notes about having stream data providers modify
stream dictionaries. I had wanted to add this functionality to make
it more efficient to create stream data providers that may
dynamically decide what kind of filters to use and that may end up
modifying the dictionary conditionally depending on the original
stream data. Ultimately I decided not to implement this feature.
This paragraph describes why.
* When writing, the way objects are placed into the queue for
writing strongly precludes creation of any new indirect objects,
or even changing which indirect objects are referenced from which
other objects, because we sometimes write as we are traversing
and enqueuing objects. For non-linearized files, there is a risk
that an indirect object that used to be referenced would no
longer be referenced, and whether it was already written to the
output file would be based on an accident of where it was
encountered when traversing the object structure. For linearized
files, the situation is considerably worse. We decide which
section of the file to write an object to based on a mapping of
which objects are used by which other objects. Changing this
mapping could cause an object to appear in the wrong section, to
be written even though it is unreferenced, or to be entirely
omitted since, during linearization, we don't enqueue new objects
as we traverse for writing.
* There are several places in QPDFWriter that query a stream's
dictionary in order to prepare for writing or to make decisions
about certain aspects of the writing process. If the stream data
provider has the chance to modify the dictionary, every piece of
code that gets stream data would have to be aware of this. This
would potentially include end user code. For example, any code
that called getDict() on a stream before installing a stream data
provider and expected that dictionary to be valid would
potentially be broken. As implemented right now, you must perform
any modifications on the dictionary in advance and provided
/Filter and /DecodeParms at the time you installed the stream
data provider. This means that some computations would have to be
done more than once, but for linearized files, stream data
providers are already called more than once. If the work done by
a stream data provider is especially expensive, it can implement
its own cache.
The example examples/pdf-custom-filter.cc demonstrates the use of
custom stream filters. This includes a custom pipeline, a custom
stream filter, as well as modification of a stream's dictionary to
include creation of a new stream that is referenced from
/DecodeParms.
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