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We need to know whether pushInheritedAttributesToPage or getAllPages have been called when generating JSON output. When reading the JSON back in, we have to call the same methods so that object numbers will line up properly.
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Next
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====
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Before Release:
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* At next release, hide release-qpdf-10.6.3.0cmake* versions at readthedocs
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* Stay on top of https://github.com/pikepdf/pikepdf/pull/315
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* Release qtest with updates to qtest-driver and copy back into qpdf
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Next:
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* JSON v2 fixes
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Pending changes:
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* Consider also exposing a way to set a new logger and to get the
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logger from QPDF and QPDFJob in the C API.
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* Check about runpath in the linux-bin distribution. I think the
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appimage build specifically is setting the runpath, which is
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actually desirable in this case. Make sure to understand and
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document this. Maybe add a check for it in the build.
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* Make job JSON accept a single element and treat as an array of one
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when an array is expected. This allows for making things repeatable
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in the future without breaking compatibility and is needed for the
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remote-attachment fix to be backward-compatible.
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* Decide what to do about #664 (get*Box)
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* Add an option --ignore-encryption to ignore encryption information
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and treat encrypted files as if they weren't encrypted. This should
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make it possible to solve #598 (--show-encryption without a
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password). We'll need to make sure we don't try to filter any
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streams in this mode. Ideally we should be able to combine this with
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--json so we can look at the raw encrypted strings and streams if we
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want to, though be sure to document that the resulting JSON won't be
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convertible back to a valid PDF. Since providing the password may
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reveal additional details, --show-encryption could potentially retry
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with this option if the first time doesn't work. Then, with the file
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open, we can read the encryption dictionary normally.
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* In libtests, separate executables that need the object library
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from those that strictly use public API. Move as many of the test
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drivers from the qpdf directory into the latter category as long
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as doing so isn't too troublesome from a coverage standpoint.
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* Consider adding fuzzer code for JSON
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* Consider generating a non-flat pages tree before creating output to
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better handle files with lots of pages. If there are more than 256
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pages, add a second layer with the second layer nodes having no more
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than 256 nodes and being as evenly sizes as possible. Don't worry
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about the case of more than 65,536 pages. If the top node has more
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than 256 children, we'll live with it.
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Soon: Break ground on "Document-level work"
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JSON v2 fixes
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=============
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* Get rid of separate format for --json and --json-output. Instead,
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--json-output can just require an outfile and change some defaults
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like which keys are present and json-stream-data. This makes it
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easier to support use cases like being able to use information in
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other top-level keys ("pages", "attachments", etc.) to drive
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modifications made to objects without having to run qpdf twice. I
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think --json-output should make the default key be only "qpdf" and
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the default json-stream-data mode be inline, but make it so you can
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use --json-stream-data and --json-stream-prefix with --json and
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--json-keys with --json-output. These would be exactly the same:
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--json-output --json-keys=all -
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--json --json-stream-data=inline
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And these:
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--json-output -
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--json --json-stream-data=inline --json-key=qpdf
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* Change the name of the "qpdf-v2" key to "qpdf". Use that in place of
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"objects" and change its content to a two-element array whose first
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element is metadata required (or useful) for parsing and whose
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second element contains the actual data. Use of an array is the only
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way to ensure that the metadata is guaranteed to be parsed before we
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start parsing the objects. Example:
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{
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"qpdf": [
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{
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"jsonversion": 2,
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"pushedinheritedpageresources": false,
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"calledgetallpages": false,
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"maxobjectid": 10
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},
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{
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"pdfversion": "1.3",
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"objects": {
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...
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}
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}
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]
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}
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This implies a few things:
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* QPDF::writeJSON will have to take an argument indicating whether
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additional keys are being written which determines whether it
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outputs the outer braces or not.
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* This changes the policy about additional extra keys. Have a
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guarantee that qpdf will never add a key whose name is or starts
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with "xdata". We still have to ignore unknown keys for future
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compatibility, but at least this gives people a namespace they can
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know will never conflict with future keys.
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* Change schema validation so that if the schema contains an array
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with more than one element, the output has to have an array with
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the same number of elements whose individual elements are
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validated according to the regular rules.
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* When reading back in, we'll have to call
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pushInheritedAttributesToPage or getAllPages based on the values
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of the metadata.
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* Support json v2 in the C API. At a minimum, write_json,
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create_from_json, and update_from_json need to be there and should
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take the same kinds of functions as the C API for logger.
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* Address json.rst comment from m-holger: "The discussion of stream
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objects is very wordy. Would a table similar to the style of the PDF
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spec be easier to use?"
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Possible future JSON enhancements
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=================================
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* Add to JSON output the information available from a few additional
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informational options:
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* --check: add but maybe not by default?
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* --show-linearization: add but maybe not by default? Also figure
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out whether warnings reported for some of the PDF specs (1.7) are
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qpdf problems. This may not be worth adding in the first
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increment.
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* --show-xref: add
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* Consider having --check, --show-encryption, etc., just select the
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right keys when in json mode. I don't think I want check on by
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default, so that might be different.
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* Consider having warnings be included in the json in a "warnings" key
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in json mode.
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QPDFJob
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=======
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Here are some ideas for QPDFJob that didn't make it into 10.6. Not all
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of these are necessarily good -- just things to consider.
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* How do we chain jobs? The idea would be that the input and/or output
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of a QPDFJob could be a QPDF object rather than a file. For input,
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it's pretty easy. For output, none of the output-specific options
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(encrypt, compress-streams, objects-streams, etc.) would have any
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affect, so we would have to treat this like inspect for error
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checking. The QPDF object in the state where it's ready to be sent
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off to QPDFWriter would be used as the input to the next QPDFJob.
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For the job json, I think we can have the output be an identifier
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that can be used as the input for another QPDFJob. For a json file,
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we could the top level detect if it's an array with the convention
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that exactly one has an output, or we could have a subkey with other
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job definitions or something. Ideally, any input
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(copy-attachments-from, pages, etc.) could use a QPDF object. It
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wouldn't surprise me if this exposes bugs in qpdf around foreign
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streams as this has been a relatively fragile area before.
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Documentation
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=============
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* Do a full pass through the documentation.
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* Make sure `qpdf` is consistent. Use QPDF when just referring to
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the package.
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* Make sure markup is consistent
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* Autogenerate where possible
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* Consider which parts might be good candidates for moving to the
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wiki.
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* Commit 'Manual - enable line wrapping in table cells' from
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Mon Jan 17 12:22:35 2022 +0000 enables table cell wrapping. See if
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this can be incorporated directly into sphinx_rtd_theme and the
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workaround can be removed.
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* When possible, update the debian package to include docs again. See
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https://bugs.debian.org/1004159 for details.
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Document-level work
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===================
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* Ideas here may by superseded by #593.
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* QPDFPageCopier -- object for moving pages around within files or
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between files and performing various transformations. Reread/rewrite
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_page-selection in the manual if needed.
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* Handle all the stuff of pages and split-pages
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* Do n-up, booklet, collation
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* Look through cli and see what else...flatten-*?
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* See comments in QPDFPageDocumentHelper.hh for addPage -- search
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for "a future version".
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* Make it efficient for bulk operations
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* Make certain doc-level features selectable
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* qpdf.cc should do all its page operations, including
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overlay/underlay, splitting, and merging, using this
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* There should also be example code
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* After doc-level checks are in, call --check on the output files in
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the "Copy Annotations" tests.
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* Document-level checks. For example, for forms, make sure all form
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fields point to an annotation on exactly one page as well as that
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all widget annotations are associated with a form field. Hook this
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into QPDFPageCopier as well as the doc helpers. Make sure it is
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called from --check.
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* See also issues tagged with "pages"
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* Add flags to CLI to select which document-level options to
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preserve or not preserve. We will probably need a pair of mutually
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exclusive, repeatable options with a way to specify all, none, only
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{x,y}, or all but {x,y}.
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* If a page contains a reference a file attachment annotation, when
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that page is copied, if the file attachment appears in the top-level
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EmbeddedFiles tree, that entry should be preserved in the
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destination file. Otherwise, we probably will require the use of
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--copy-attachments-from to preserve these. What will the strategy be
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for deduplicating in the automatic case?
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Text Appearance Streams
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=======================
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This is a list of known issues with text appearance streams and things
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we might do about it.
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* For variable text, the spec says to pull any resources from /DR that
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are referenced in /DA but if the resource dictionary already has
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that resource, just use the one that's there. The current code looks
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only for /Tf and adds it if needed. We might want to instead merge
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/DR with resources and then remove anything that's unreferenced. We
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have all the code required for that in ResourceFinder except
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TfFinder also gets the font size, which ResourceFinder doesn't do.
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* There are things we are missing because we don't look at font
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metrics. The code from TextBuilder (work) has almost everything in
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it that is required. Once we have knowledge of character widths, we
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can support quadding and multiline text fields (/Ff 4096), and we
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can potentially squeeze text to fit into a field. For multiline,
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first squeeze vertically down to the font height, then squeeze
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horizontally with Tz. For single line, squeeze horizontally with Tz.
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If we use Tz, issue a warning.
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* When mapping characters to widths, we will need to care about
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character encoding. For built-in fonts, we can create a map from
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Unicode code point to width and then go from the font's encoding to
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unicode to the width. See misc/character-encoding/ (not on github)
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and font metric information for the 14 standard fonts in my local
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pdf-spec directory.
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* Once we know about character widths, we can correctly support
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auto-sized variable text fields (0 Tf). If this is fixed, search for
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"auto-sized" in cli.rst.
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Fuzz Errors
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===========
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* https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=<N>
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* Ignoring these:
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* Out of memory in dct: 35001, 32516
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External Libraries
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==================
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Current state (10.0.2):
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* qpdf/external-libs repository builds external-libs on a schedule.
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It detects and downloads the latest versions of zlib, jpeg, and
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openssl and creates source and binary distribution zip files in an
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artifact called "distribution".
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* Releases in qpdf/external-libs are made manually. They contain
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qpdf-external-libs-{bin,src}.zip.
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* The qpdf build finds the latest non-prerelease release and downloads
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the qpdf-external-libs-*.zip files from the releases in the setup
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stage.
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* To upgrade to a new version of external-libs, create a new release
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of qpdf/external-libs (see README-maintainer in external-libs) from
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the distribution artifact of the most recent successful build after
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ensuring that it works.
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Desired state:
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* The qpdf/external-libs repository should create release candidates.
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Ideally, every scheduled run would make its zip files available. A
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personal access token with actions:read scope for the
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qpdf/external-libs repository is required to download the artifact
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from an action run, and qpdf/qpdf's secrets.GITHUB_TOKEN doesn't
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have this access. We could create a service account for this
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purpose. As an alternative, we could have a draft release in
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qpdf/external-libs that the qpdf/external-libs build could update
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with each candidate. It may also be possible to solve this by
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developing a simple GitHub app.
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* Scheduled runs of the qpdf build in the qpdf/qpdf repository (not a
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fork or pull request) could download external-libs from the release
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candidate area instead of the latest stable release. Pushes to the
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build branch should still use the latest release so it always
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matches the main branch.
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* Periodically, we would create a release of external-libs from the
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release candidate zip files. This could be done safely because we
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know the latest qpdf works with it. This could be done at least
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before every release of qpdf, but potentially it could be done at
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other times, such as when a new dependency version is available or
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after some period of time.
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Other notes:
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* The external-libs branch in qpdf/qpdf was never documented. We might
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be able to get away with deleting it.
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* See README-maintainer in qpdf/external-libs for information on
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creating a release. This could be at least partially scripted in a
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way that works for the qpdf/qpdf repository as well since they are
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very similar.
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ABI Changes
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===========
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This is a list of changes to make next time there is an ABI change.
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Comments appear in the code prefixed by "ABI". Always Search for ABI
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in source and header files to find items not listed here. Also search
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for "[[deprecated" to find deprecated APIs that can be removed.
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Page splitting/merging
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======================
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* Update page splitting and merging to handle document-level
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constructs with page impact such as interactive forms and article
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threading. Check keys in the document catalog for others, such as
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outlines, page labels, thumbnails, and zones. For threads,
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Subramanyam provided a test file; see ../misc/article-threads.pdf.
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Email Q-Count: 431864 from 2009-11-03.
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* bookmarks (outlines) 12.3.3
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* support bookmarks when merging
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* prune bookmarks that don't point to a surviving page when merging
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or splitting
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* make sure conflicting named destinations work possibly test by
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including the same file by two paths in a merge
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* see also comments in issue 343
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Note: original implementation of bookmark preservation for split
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pages caused a very high performance hit. The problem was
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introduced in 313ba081265f69ac9a0324f9fe87087c72918191 and reverted
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in the commit that adds this paragraph. The revert includes marking
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a few tests cases as $td->EXPECT_FAILURE. When properly coded, the
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test cases will need to be adjusted to only include the parts of
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the outlines that are actually copied. The tests in question are
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"split page with outlines". When implementing properly, ensure that
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the performance is not adversely affected by timing split-pages on
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a large file with complex outlines such as the PDF specification.
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When pruning outlines, keep all outlines in the hierarchy that are
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above an outline for a page we care about. If one of the ancestor
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outlines points to a non-existent page, clear its dest. If an
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outline does not have any children that point to pages in the
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document, just omit it.
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Possible strategy:
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* resolve all named destinations to explicit destinations
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* concatenate top-level outlines
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* prune outlines whose dests don't point to a valid page
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* recompute all /Count fields
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Test files
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* page-labels-and-outlines.pdf: old file with both page labels and
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outlines. All destinations are explicit destinations. Each page
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has Potato and a number. All titles are feline names.
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* outlines-with-actions.pdf: mixture of explicit destinations,
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named destinations, goto actions with explicit destinations, and
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goto actions with named destinations; uses /Dests key in names
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dictionary. Each page has Salad and a number. All titles are
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silly words. One destination is an indirect object.
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* outlines-with-old-root-dests.pdf: like outlines-with-actions
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except it uses the PDF-1.1 /Dests dictionary for named
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destinations, and each page has Soup and a number. Also pages are
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numbered with upper-case Roman numerals starting with 0. All
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titles are silly words preceded by a bullet.
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If outline handling is significantly improved, see
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../misc/bad-outlines/bad-outlines.pdf and email:
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https://mail.google.com/mail/u/0/#search/rfc822msgid%3A02aa01d3d013%249f766990%24de633cb0%24%40mono.hr)
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* Form fields: should be similar to outlines.
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Analytics
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=========
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Consider features that make it easier to detect certain patterns in
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PDF files. The information below could be computed using an external
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program that reads the existing json, but if it's useful enough, we
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could add it directly to the json output.
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* Add to "pages" in the json:
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* "inheritsresources": bool; whether there are any inherited
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attributes from ancestor page tree nodes
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* "sharedresources": a list of indirect objects that are
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"/Resources" dictionaries or "XObject" resource dictionary subkeys
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of either the page itself or of any form XObject referenced by the
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page.
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* Add to "objectinfo" in json: "directpagerefcount": the number of
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pages that directly reference this object (i.e., you can find an
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indirect reference to the object in the page dictionary without
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traversing over any indirect objects)
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General
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=======
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NOTE: Some items in this list refer to files in my personal home
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directory or that are otherwise not publicly accessible. This includes
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things sent to me by email that are specifically not public. Even so,
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I find it useful to make reference to them in this list.
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* Look at https://bestpractices.coreinfrastructure.org/en
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* Rework tests so that nothing is written into the source directory.
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Ideally then the entire build could be done with a read-only
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source tree.
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* Large file tests fail with linux32 before and after cmake. This was
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first noticed after 10.6.3. I don't think it's worth fixing.
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* Consider updating the fuzzer with code that exercises
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copyAnnotations, file attachments, and name and number trees. Check
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fuzzer coverage.
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* Add code for creation of a file attachment annotation. It should
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also be possible to create a widget annotation and a form field.
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Update the pdf-attach-file.cc example with new APIs when ready.
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* Flattening of form XObjects seems like something that would be
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useful in the library. We are seeing more cases of completely valid
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PDF files with form XObjects that cause problems in other software.
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Flattening of form XObjects could be a useful way to work around
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those issues or to prepare files for additional processing, making
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it possible for users of the qpdf library to not be concerned about
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form XObjects. This could be done recursively; i.e., we could have a
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method to embed a form XObject into whatever contains it, whether
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that is a form XObject or a page. This would require more
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significant interpretation of the content stream. We would need a
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test file in which the placement of the form XObject has to be in
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the right place, e.g., the form XObject partially obscures earlier
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code and is partially obscured by later code. Keys in the resource
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dictionary may need to be changed -- create test cases with lots of
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duplicated/overlapping keys.
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* Part of closed_file_input_source.cc is disabled on Windows because
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of odd failures. It might be worth investigating so we can fully
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exercise this in the test suite. That said, ClosedFileInputSource
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is exercised elsewhere in qpdf's test suite, so this is not that
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pressing.
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* 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.
|