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870 lines
37 KiB
Plaintext
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Next
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====
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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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In order:
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* json v2
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Other (do in any order):
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* See if I can change all output and error messages issued by the
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library, when context is available, to have a pipeline rather than a
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FILE* or std::ostream. This makes it possible for people to capture
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output more flexibly.
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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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* 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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* Find all places in the code that write to std::cout, std::err,
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stdout, or stderr to make sure they obey default output stream
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settings for QPDF and QPDFJob. This probably includes adding a
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Pl_Ostream pipeline.
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* Nice to have:
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* Split qpdf.test into multiple tests
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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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* 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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Soon: Break ground on "Document-level work"
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Output JSON v2
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==============
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General things to remember:
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* Make sure all the information from --check and other informational
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options (--show-linearization, --show-encryption, --show-xref,
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--list-attachments, --show-npages) is available in the json output.
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* Consider changing the contract to allow fields to be absent even
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when present in the schema. It's reasonable for people to check for
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presence of a key. Most languages make this easy to do.
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* The choices for json_key (job.yml) will be different for v1 and v2.
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That information is already duplicated in multiple places.
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* Test stream with invalid data
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* When we get to full serialization, add json serialization
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performance test.
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* Add json to the large file tests.
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* We could consider arguments like --replace-object that would take a
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JSON representation of the object and could include indirect
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references, etc. We could also add --delete object.
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* Object representation tests
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* "b:cf80", "b:CF80", "u:π", "u:\u03c0"
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* "b:d83edd54", "u:🥔", "u:\ud83e\udd54"
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When reading a JSON string, any string that doesn't follow the above rules
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is an error. Just use newUnicodeString on "u:" strings. For "b:"
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strings, decode the bytes with hex_decode and use newString.
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Serialized PDF:
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The JSON output will have a "qpdf" key containing
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* jsonVersion
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* pdfVersion
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* objects
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The "qpdf" key replaces "objects" and "objectinfo" in v1 JSON.
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Within .qpdf.objects, the key is "obj:o g R" or "obj:trailer", and the
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value is a dictionary with exactly one of "value" or "stream" as its
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single key.
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Rationale of "obj:o g R" is that indirect object references are just
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"o g R", and so code that wants to resolve one can do so easily by
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just prepending "obj:" and not having to parse or split the string.
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Having a prefix rather than making the key just "o g R" makes it much
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easier to search in the JSON for the definition of an object.
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For non-streams:
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{
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"obj:o g R": {
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"value": ...
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}
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}
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For streams:
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"obj:o g R": {
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"stream": {
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"dict": { ... stream dictionary ... },
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"data": "base64-encoded data",
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"dataFile": "path to base64-encoded data"
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}
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}
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}
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At most one of "data" or "dataFile" will be present. When serializing,
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stream decode parameters will be obeyed, and the stream dictionary
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will reflect the result. There will be the option to omit stream data.
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In the stream dictionary, "/Length" is always removed.
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Streams are filtered or not based on the --decode-level parameter. If
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a stream is filtered, "/Filter" and "/DecodeParms" are removed from
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the stream dictionary. This makes the stream data and dictionary match
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for when the file is read back in.
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CLI:
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* Add new flags
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* --from-json=input.json -- signals reading from a JSON and counts
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as an input file.
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* --json-streams-omit -- stream data is omitted, the default
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* --json-streams-inline -- stream data is included in the "data"
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key as base64-encoded
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* --json-streams-file-prefix=prefix -- stream is written to $prefix-$obj
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where $obj is the object number. The path to the file is stored
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in the "dataFile" key. A relative path is recommended and will be
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interpreted as relative to the current directory. If a relative
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prefix is given, a relative path will stored in "dataFile".
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Example:
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mkdir in-streams
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qpdf in.pdf --json-streams-file-prefix=in-streams/ > out.json
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* --to-json -- changes default to --json-streams-inline implies
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--json-key=qpdf
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Example workflow:
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* qpdf in.pdf --to-json > pdf.json
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* edit pdf.json
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* qpdf --from-json=pdf.json out.pdf
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JSON to PDF:
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For going back from JSON to PDF, we can have
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QPDF::fromJSON(std::shared_ptr<InputSource> which will have logic
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similar to copyForeignObject. Note that this InputSource is not going
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to be this->file. We have to keep it separately.
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The backing input source is this memory block:
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```
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%PDF-1.3
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xref
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0 1
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0000000000 65535 f
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trailer << /Size 1 >>
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startxref
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9
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%%EOF
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```
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* Ignore all keys except .qpdf.
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* Verify that .qpdf.jsonVersion is 2
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* Set this->m->pdf_version based on the .qpdf.pdfVersion key
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* For each object in .qpdf.objects:
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* Walk through the object detecting any indirect objects. For each
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one that is not already known, reserve the object. We can also
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validate but we should try to do the best we can with invalid JSON
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so people can get good error messages.
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* Construct a QPDFObjectHandle from the JSON
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* If the object is the trailer, update the trailer
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* Else if the object doesn't exist, reserve it
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* If the object is reserved, call replaceReserved()
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* Else the object already exists; this is an error.
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For streams, have a stream data provider that, for inline streams,
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does a base64 from the file offsets and for file-based streams, reads
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the file. For the inline case, we have to keep the json InputSource
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around. Otherwise, we don't. It is an error if there is no stream data.
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Documentation:
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Update --json option in cli.rst to mention v2 and update json.rst.
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Other documentation fodder:
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You can't create a PDF from v1 json because
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* The PDF version header is not recorded
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* Strings cannot be unambiguously encoded/decoded
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* Can't tell string from name from indirect object
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* Strings are treated as PDF doc encoding and output as UTF-8, which
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doesn't work since multiple PDF doc code points are undefined
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* There is no representation of stream data
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* You can't tell a stream from a dictionary except by looking in both
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"object" and "objectinfo". Fix this, and then remove "objectinfo".
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Additionally, using "n n R" as a key in "objects" and "objectinfo"
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messes up searching for things.
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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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* replace mode: --replace-object, --replace-stream-raw,
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--replace-stream-filtered
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* update first paragraph of QPDF JSON in the manual to mention this
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* object numbers are not preserved by write, so object ID lookup
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has to be done separately for each invocation
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* you don't have to specify length for streams
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* you only have to specify filtering for streams if providing raw data
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* Allow users to supply a custom progress reporter for QPDFJob
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* Better interoperability with json output:
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* Make sure all the things that print stuff to stdout have json
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equivalents (check, showLinearizationData, etc.)
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* There should be a way to get json output other than having it
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print to stdout. It should be multi-language friendly and allow
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for large amounts of data, such as providing a callback that qpdf
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can write to (like a pipeline)
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* See also JSON v2
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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
|
|
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.
|
|
|
|
* Look at https://bestpractices.coreinfrastructure.org/en
|
|
|
|
* Get rid of remaining assert() calls from non-test code.
|
|
|
|
* 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.
|
|
|
|
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.
|