qpdf/manual/json.rst

.. cSpell:ignore moddifyannotations
.. cSpell:ignore feff

.. _json:

qpdf JSON
=========

.. _json-overview:

Overview
--------

Beginning with qpdf version 11.0.0, the qpdf library and command-line
program can produce a JSON representation of a PDF file. qpdf version
11 introduces JSON format version 2. Prior to qpdf 11, versions 8.3.0
onward had a more limited JSON representation accessible only from the
command-line. For details on what changed, see :ref:`json-v2-changes`.
The rest of this chapter documents qpdf JSON version 2.

Please note: this chapter discusses *qpdf JSON format*, which
represents the contents of a PDF file. This is distinct from the
*QPDFJob JSON format* which provides a higher-level interface
interacting with qpdf the way the command-line tool does. For
information about that, see :ref:`qpdf-job`.

The qpdf JSON format is specific to qpdf. There are two ways to use
qpdf JSON:

- The :qpdf:ref:`--json` command-line flag causes creation of a JSON
  representation of all the objects in a PDF file, excluding stream
  data. This includes an unambiguous representation of the PDF object
  structure and also provides JSON-formatted summaries of other
  information about the file. This functionality is built into
  ``QPDFJob`` and can be accessed from the ``qpdf`` command-line tool
  or from the ``QPDFJob`` C or C++ API.

- qpdf can create a JSON file that completely represents a PDF file.
  You can think of this as using JSON as an *alternative syntax* for
  representing a PDF file. Using qpdf JSON, it is possible to
  convert a PDF file to JSON, manipulate the structure or contents of
  the objects at a low level, and convert the results back to a PDF
  file. This functionality can be accessed from the command-line with
  the :qpdf:ref:`--json-output`, :qpdf:ref:`--json-input`, and
  :qpdf:ref:`--update-from-json` flags, or from the API using the
  ``QPDF::writeJSON``, ``QPDF::createFromJSON``, and
  ``QPDF::updateFromJSON`` methods.

.. _json-terminology:

JSON Terminology
----------------

Notes about terminology:

- In JavaScript and JSON, that thing that has keys and values is
  typically called an *object*.

- In PDF, that thing that has keys and values is typically called a
  *dictionary*. An *object* is a PDF object such as integer, real,
  boolean, null, string, array, dictionary, or stream.

- Some languages that use JSON call an *object* a *dictionary*, a
  *map*, or a *hash*.

- Sometimes, it's called on *object* if it has fixed keys and a
  *dictionary* if it has variable keys.

This manual is not entirely consistent about its use of *dictionary*
vs. *object* because sometimes one term or another is clearer in
context. Just be aware of the ambiguity when reading the manual. We
frequently use the term *dictionary* to refer to a JSON object because
of the consistency with PDF terminology.

.. _what-qpdf-json-is-not:

What qpdf JSON is not
---------------------

Please note that qpdf JSON offers a convenient syntax for manipulating
PDF files at a low level using JSON syntax. JSON syntax is much easier
to work with than native PDF syntax, and there are good JSON libraries
in virtually every commonly used programming language. Working with
PDF objects in JSON removes the need to worry about stream lengths,
cross reference tables, and PDF-specific representations of Unicode or
binary strings that appear outside of content streams. It does not
eliminate the need to understand the semantic structure of PDF files.
Working with qpdf JSON still requires familiarity with the PDF
specification.

In particular, qpdf JSON *does not* provide any of the following
capabilities:

- Text extraction. While you could use qpdf JSON syntax to navigate to
  a page's content streams and font structures, text within pages is
  still encoded using PDF syntax within content streams, and there is
  no assistance for text extraction.

- Reflowing text, document structure. qpdf JSON does not add any new
  information or insight into the content of PDF files. If you have a
  PDF file that lacks any structural information, qpdf JSON won't help
  you solve any of those problems.

This is what we mean when we say that JSON provides a *alternative
syntax* for working with PDF data. Semantically, it is identical to
native PDF.

.. _qpdf-json:

qpdf JSON Format
----------------

This section describes how qpdf represents PDF objects in JSON format.
It also describes how to work with qpdf JSON to create or
modify PDF files.

.. _json.objects:

qpdf JSON Object Representation
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This section describes the representation of PDF objects in qpdf JSON
version 2. PDF objects are represented within the ``"objects"``
dictionary of a qpdf JSON file. This is true both for PDF serialized
to JSON (:qpdf:ref:`--json-output`, ``QPDF::writeJSON``) or objects as
they appear in the output of ``qpdf`` with the :qpdf:ref:`--json`
option.

Each key in the ``"objects"`` dictionary is either ``"trailer"`` or a
string of the form ``"obj:O G R"`` where ``O`` and ``G`` are the
object and generation numbers and ``R`` is the literal string ``R``.
This is the PDF syntax for the indirect object reference prepended by
``obj:``. The value, representing the object itself, is a JSON object
whose structure is described below.

Top-level Stream Objects
  Stream objects are represented as a JSON object with the single key
  ``"stream"``. The stream object has a key called ``"dict"`` whose
  value is the stream dictionary as an object value (described below)
  with the ``"/Length"`` key omitted. Other keys are determined by the
  value for json stream data (:qpdf:ref:`--json-stream-data`, or a
  parameter of type ``qpdf_json_stream_data_e``) as follows:

  - ``none``: stream data is not represented; no other keys are
    present

  - ``inline``: the stream data appears as a base64-encoded string as
    the value of the ``"data"`` key

  - ``file``: the stream data is written to a file, and the path to
    the file is stored in the ``"datafile"`` key. A relative path is
    interpreted as relative to the current directory when qpdf is
    invoked.

  Keys other than ``"dict"``, ``"data"``, and ``"datafile"`` are
  ignored. This is primarily for future compatibility in case a newer
  version of qpdf includes additional information.

  As with the native PDF representation, the stream data must be
  consistent with whatever filters and decode parameters are specified
  in the stream dictionary.

Top-level Non-stream Objects
  Non-stream objects are represented as a dictionary with the single
  key ``"value"``. Other keys are ignored for future compatibility.
  The value's structure is described in "Object Values" below.

  Note: in files that use object streams, the trailer "dictionary" is
  actually a stream, but in the JSON representation, the value of the
  ``"trailer"`` key is always written as a dictionary (with a
  ``"value"`` key like other non-stream objects). There will also be a
  a stream object whose key is the object ID of the cross-reference
  stream, even though this stream will generally be unreferenced. This
  makes it possible to assume ``"trailer"`` points to a dictionary
  without having to consider whether the file uses object streams or
  not. It is also consistent with how ``QPDF::getTrailer`` behaves in
  the C++ API.

Object Values
  Within ``"value"`` or ``"stream"."dict"``, PDF objects are
  represented as follows:

  - Objects of type Boolean or null are represented as JSON objects of
    the same type.

  - Objects that are numeric are represented as numeric in the JSON
    without regard to precision. Internally, qpdf stores numeric
    values as strings, so qpdf will preserve arbitrary precision
    numerical values when reading and writing JSON. It is likely that
    other JSON readers and writers will have implementation-dependent
    ways of handling numerical values that are out of range.

  - Name objects are represented as JSON strings that start with ``/``
    and are followed by the PDF name in canonical form with all PDF
    syntax resolved. For example, the name whose canonical form (per
    the PDF specification) is ``text/plain`` would be represented in
    JSON as ``"/text/plain"`` and in PDF as ``"/text#2fplain"``.

  - Indirect object references are represented as JSON strings that
    look like a PDF indirect object reference and have the form ``"O G
    R"`` where ``O`` and ``G`` are the object and generation numbers
    and ``R`` is the literal string ``R``. For example, ``"3 0 R"``
    would represent a reference to the object with object ID 3 and
    generation 0.

  - PDF strings are represented as JSON strings in one of two ways:

    - ``"u:utf8-encoded-string"``: this format is used when the PDF
      string can be unambiguously represented as a Unicode string and
      contains no unprintable characters. This is the case whether the
      input string is encoded as UTF-16, UTF-8 (as allowed by PDF
      2.0), or PDF doc encoding. Strings are only represented this way
      if they can be encoded without loss of information.

    - ``"b:hex-string"``: this format is used to represent any binary
      string value that can't be represented as a Unicode string.
      ``hex-string`` must have an even number of characters that range
      from ``a`` through ``f``, ``A`` through ``F``, or ``0`` through
      ``9``.

    qpdf writes empty strings as ``"u:"``, but both ``"b:"`` and
    ``"u:"`` are valid representations of the empty string.

    There is full support for UTF-16 surrogate pairs. Binary strings
    encoded with ``"b:..."`` are the internal PDF representations.
    As such, the following are equivalent:

    - ``"u:\ud83e\udd54"`` -- representation of U+1F954 as a surrogate
      pair in JSON syntax

    - ``"b:FEFFD83EDD54"`` -- representation of U+1F954 as the bytes
      of a UTF-16 string in PDF syntax with the leading ``FEFF``
      indicating UTF-16

    - ``"b:efbbbff09fa594"`` -- representation of U+1F954 as the
      bytes of a UTF-8 string in PDF syntax (as allowed by PDF 2.0)
      with the leading ``EF``, ``BB``, ``BF`` sequence (which is just
      UTF-8 encoding of ``FEFF``).

    - A JSON string whose contents are ``u:`` followed by the UTF-8
      representation of U+1F954. This is the potato emoji.
      Unfortunately, I am not able to render it in the PDF version
      of this manual.

  - PDF arrays are represented as JSON arrays of objects as described
    above

  - PDF dictionaries are represented as JSON objects whose keys are
    the string representations of names and whose values are
    representations of PDF objects.

.. _json.output:

qpdf JSON Output
~~~~~~~~~~~~~~~~

The format of the JSON written by qpdf's :qpdf:ref:`--json-output`
flag or the ``QPDF::writeJSON`` API call is a JSON object consisting
of a single key: ``"qpdf-v2"``. Any other top-level keys are ignored.
While unknown keys in other places are ignored for future
compatibility, in this case, ignoring other top-level keys is an
explicit decision to allow users to include other keys for their own
use. No new top-level keys will be added in JSON version 2.

The ``"qpdf-v2"`` key points to a JSON object with the following keys:

- ``"pdfversion"`` -- a string containing PDF version as indicated in
  the PDF header (e.g. ``"1.7"``, ``"2.0"``)

- ``"maxobjectid"`` -- a number indicating the object ID of the
  highest numbered object in the file. This is provided to make it
  easier for software that wants to add new objects to the file as you
  can safely start with one above that number when creating new
  objects. Note that the value of ``"maxobjectid"`` may be higher than
  the actual maximum object that appears in the input PDF since it
  takes into consideration any dangling indirect object references
  from the original file. This prevents you from unwittingly creating
  an object that doesn't exist but that is referenced, which may have
  unintended side effects. (The PDF specification explicitly allows
  dangling references and says to treat them as nulls. This can happen
  if objects are removed from a PDF file.)

- ``"objects"`` -- the actual PDF objects as described in
  :ref:`json.objects`.

Note that writing JSON output is done by ``QPDF``, not ``QPDFWriter``.
As such, none of the things ``QPDFWriter`` does apply. This includes
recompression of streams, renumbering of objects, anything to do with
object streams (which are not represented by qpdf JSON at all since
they are PDF syntax, not semantics), encryption, decryption,
linearization, QDF mode, etc. See :ref:`rewriting` for a more in-depth
discussion.

.. _json.example:

qpdf JSON Example
~~~~~~~~~~~~~~~~~

The JSON below shows an example of a simple PDF file represented in
qpdf JSON format.

.. code-block:: json

   {
     "qpdf-v2": {
       "pdfversion": "1.3",
       "maxobjectid": 5,
       "objects": {
         "obj:1 0 R": {
           "value": {
             "/Pages": "2 0 R",
             "/Type": "/Catalog"
           }
         },
         "obj:2 0 R": {
           "value": {
             "/Count": 1,
             "/Kids": [ "3 0 R" ],
             "/Type": "/Pages"
           }
         },
         "obj:3 0 R": {
           "value": {
             "/Contents": "4 0 R",
             "/MediaBox": [ 0, 0, 612, 792 ],
             "/Parent": "2 0 R",
             "/Resources": {
               "/Font": {
                 "/F1": "5 0 R"
               }
             },
             "/Type": "/Page"
           }
         },
         "obj:4 0 R": {
           "stream": {
             "data": "eJxzCuFSUNB3M1QwMlEISQOyzY2AyEAhJAXI1gjIL0ksyddUCMnicg3hAgDLAQnI",
             "dict": {
               "/Filter": "/FlateDecode"
             }
           }
         },
         "obj:5 0 R": {
           "value": {
             "/BaseFont": "/Helvetica",
             "/Encoding": "/WinAnsiEncoding",
             "/Subtype": "/Type1",
             "/Type": "/Font"
           }
         },
         "trailer": {
           "value": {
             "/ID": [
               "b:98b5a26966fba4d3a769b715b2558da6",
               "b:98b5a26966fba4d3a769b715b2558da6"
             ],
             "/Root": "1 0 R",
             "/Size": 6
           }
         }
       }
     }
   }

.. _json.input:

qpdf JSON Input
~~~~~~~~~~~~~~~

Output in the JSON output format described in :ref:`json.output` can
be used in two different ways:

- By using the :qpdf:ref:`--json-input` flag or calling
  ``QPDF::createFromJSON`` in place of ``QPDF::processFile``, a qpdf
  JSON file can be used in place of a PDF file as the input to qpdf.

- By using the :qpdf:ref:`--update-from-json` flag or calling
  ``QPDF::updateFromJSON`` on an initialized ``QPDF`` object, a qpdf
  JSON file can be used to apply changes to an existing ``QPDF``
  object. That ``QPDF`` object can have come from any source including
  a PDF file, a qpdf JSON file, or the result of any other process
  that results in a valid, initialized ``QPDF`` object.

Here are some important things to know about qpdf JSON input.

- When a qpdf JSON file is used as the primary input file, it must be
  complete. This means

  - A PDF version number must be specified with the ``"pdfversion"``
    key

  - Stream data must be present for all streams

  - The trailer dictionary must be present, though only the
    ``"/Root"`` key is required.

- Certain fields from the input are ignored whether creating or
  updating from a JSON file:

  - ``"maxobjectid"`` is ignored, so it is not necessary to update it
    when adding new objects.

  - ``"/Length"`` is ignored in all stream dictionaries. qpdf doesn't
    put it there when it creates JSON output, and it is not necessary
    to add it.

  - ``"/Size"`` is ignored if it appears in a trailer dictionary as
    that is always recomputed by ``QPDFWriter``.

  - Unknown keys at the to top level of the file, within ``objects``,
    at the top level of each individual object (inside the object that
    has the ``"value"`` or ``"stream"`` key) and directly within
    ``"stream"`` are ignored for future compatibility. You should
    avoid putting your own values in those places if you wish to avoid
    risking that your JSON files will not work in future versions of
    qpdf. The exception to this advice is at the top level of the
    overall file where it is explicitly supported for you to add your
    own keys. For example, you could add your own metadata at the top
    level, and qpdf will ignore it. Note that extra top-level keys are
    not preserved when qpdf reads your JSON file.

- When qpdf reads a PDF file, the internal object numbers are always
  preserved. However, when qpdf writes a file using ``QPDFWriter``,
  ``QPDFWriter`` does its own numbering and, in general, does not
  preserve input object numbers. That means that a qpdf JSON file that
  is used to update an existing PDF must have object numbers that
  match the input file it is modifying. In practical terms, this means
  that you can't use a JSON file created from one PDF file to modify
  the *output of running qpdf on that file*.

  To put this more concretely, the following is valid:

  ::

    qpdf --json-output in.pdf pdf.json
    # edit pdf.json
    qpdf in.pdf out.pdf --update-from-json=pdf.json

  The following will not produce predictable results because
  ``out.pdf`` won't have the same object numbers as ``pdf.json`` and
  ``in.pdf``.

  ::

    qpdf --json-output in.pdf pdf.json
    # edit pdf.json
    qpdf in.pdf out.pdf --update-from-json=pdf.json
    # edit pdf.json again
    # Don't do this
    qpdf out.pdf out2.pdf --update-from-json=pdf.json

- When updating from a JSON file (:qpdf:ref:`--update-from-json`,
  ``QPDF::updateFromJSON``), existing objects are updated in place.
  This has the following implications:

  - You may omit both ``"data"`` and ``"datafile"`` if the object you
    are updating is already a stream. In that case the original stream
    data is preserved. You must always provide a stream dictionary,
    but it may be empty. Note that an empty stream dictionary will
    clear the old dictionary. There is no way to indicate that an old
    stream dictionary should be left alone, so if your intention is to
    replace the stream data and preserve the dictionary, the
    original dictionary must appear in the JSON file.

  - You can change one object type to another object type including
    replacing a stream with a non-stream or a non-stream with a
    stream. If you replace a non-stream with a stream, you must
    provide data for the stream.

  - Objects that you do not wish to modify can be omitted from the
    JSON. That includes the trailer. That means you can use the output
    of a qpdf JSON file that was written using
    :qpdf:ref:`--json-object` to have it include only the objects you
    intend to modify.

  - You can omit the ``"pdfversion"`` key. The input PDF version will
    be preserved.

.. _json.workflow-cli:

qpdf JSON Workflow: CLI
~~~~~~~~~~~~~~~~~~~~~~~

This section includes a few examples of using qpdf JSON.

- Convert a PDF file to JSON format, edit the JSON, and convert back
  to PDF. This is an alternative to using QDF mode (see :ref:`qdf`) to
  modify PDF files in a text editor. Each method has its own
  advantages and disadvantages.

  ::

     qpdf --json-output in.pdf pdf.json
     # edit pdf.json
     qpdf --json-input pdf.json out.pdf

- Extract only a specific object into a JSON file, modify the object
  in JSON, and use the modified object to update the original PDF. In
  this case, we're editing object 4, whatever that may happen to be.
  You would have to know through some other means which object you
  wanted to edit, such as by looking at other JSON output or using a
  tool (possibly but not necessarily qpdf) to identify the object.

  ::

     qpdf --json-output in.pdf pdf.json --json-object=4,0
     # edit pdf.json
     qpdf in.pdf --update-from-json=pdf.json out.pdf

  Rather than using :qpdf:ref:`--json-object` as in the above example,
  you could edit the JSON file to remove the objects you didn't need.
  You could also just leave them there, though the update process
  would be slower.

  You could also add new objects to a file by adding them to
  ``pdf.json``. Just be sure the object number doesn't conflict with
  an existing object. The ``"maxobjectid"`` field in the original
  output can help with this. You don't have to update it if you add
  objects as it is ignored when the file is read back in.

- Use :qpdf:ref:`--json-input` and :qpdf:ref:`--json-output` together
  to demonstrate preservation of object numbers. In this example,
  ``a.json`` and ``b.json`` will have the same objects and object
  numbers. The files may not be identical since strings may be
  normalized, fields may appear in a different order, etc. However
  ``b.json`` and ``c.json`` are probably identical.

  ::

     qpdf --json-output in.pdf a.json
     qpdf --json-input --json-output a.json b.json
     qpdf --json-input --json-output b.json c.json


.. _json.workflow-api:

qpdf JSON Workflow: API
~~~~~~~~~~~~~~~~~~~~~~~

Everything that can be done using the qpdf CLI can be done using the
C++ API. See comments in :file:`QPDF.hh` for ``writeJSON``,
``createFromJSON``, and ``updateFromJSON`` for details.

.. _json-guarantees:

JSON Compatibility Guarantees
-----------------------------

The qpdf JSON representation includes a JSON serialization of the raw
objects in the PDF file as well as some computed information in a more
easily extracted format. QPDF provides some guarantees about its JSON
format. These guarantees are designed to simplify the experience of a
developer working with the JSON format.

Compatibility
   The top-level JSON object is a dictionary (JSON "object"). The JSON
   output contains various nested dictionaries and arrays. With the
   exception of dictionaries that are populated by the fields of
   PDF objects from the file, all instances of a dictionary are
   guaranteed to have exactly the same keys.

   The top-level JSON structure contains a ``"version"`` key whose
   value is simple integer. The value of the ``version`` key will be
   incremented if a non-compatible change is made. A non-compatible
   change would be any change that involves removal of a key, a change
   to the format of data pointed to by a key, or a semantic change
   that requires a different interpretation of a previously existing
   key.

   With a specific qpdf JSON version, future versions of qpdf are free
   to add additional keys but not to remove keys or change the type of
   object that a key points to.

Documentation
   The :command:`qpdf` command can be invoked with the
   :qpdf:ref:`--json-help` option. This will output a JSON
   structure that has the same structure as the JSON output that qpdf
   generates, except that each field in the help output is a description
   of the corresponding field in the JSON output. The specific
   guarantees are as follows:

   - A dictionary in the help output means that the corresponding
     location in the actual JSON output is also a dictionary with
     exactly the same keys; that is, no keys present in help are
     absent in the real output, and no keys will be present in the
     real output that are not in help. It is possible for a key to be
     present and have a value that is explicitly ``null``. As a
     special case, if the dictionary has a single key whose name
     starts with ``<`` and ends with ``>``, it means that the JSON
     output is a dictionary that can have any value as a key. This is
     used for cases in which the keys of the dictionary are things
     like object IDs.

   - A string in the help output is a description of the item that
     appears in the corresponding location of the actual output. The
     corresponding output can have any value including ``null``.

   - An array in the help output always contains a single element. It
     indicates that the corresponding location in the actual output is
     an array of any length, and that each element of the array has
     whatever format is implied by the single element of the help
     output's array.

   For example, the help output indicates includes a ``"pagelabels"``
   key whose value is an array of one element. That element is a
   dictionary with keys ``"index"`` and ``"label"``. In addition to
   describing the meaning of those keys, this tells you that the actual
   JSON output will contain a ``pagelabels`` array, each of whose
   elements is a dictionary that contains an ``index`` key, a ``label``
   key, and no other keys.

Directness and Simplicity
   The JSON output contains the value of every object in the file, but
   it also contains some summary data. This is analogous to how qpdf's
   library interface works. The summary data is similar to the helper
   functions in that it allows you to look at certain aspects of the
   PDF file without having to understand all the nuances of the PDF
   specification, while the raw objects allow you to mine the PDF for
   anything that the higher-level interfaces are lacking.

.. _json.considerations:

JSON: Special Considerations
----------------------------

For the most part, the built-in JSON help tells you everything you need
to know about the JSON format, but there are a few non-obvious things to
be aware of:

- If a PDF file has certain types of errors in its pages tree (such as
  page objects that are direct or multiple pages sharing the same
  object ID), qpdf will automatically repair the pages tree. If you
  specify ``"objects"`` (and, with qpdf JSON version 1, also
  ``"objectinfo"``) without any other keys, you will see the original
  pages tree without any corrections. If you specify any of keys that
  require page tree traversal (for example, ``"pages"``,
  ``"outlines"``, or ``"pagelabel"``), then ``"objects"`` (and
  ``"objectinfo"``) will show the repaired page tree so that object
  references will be consistent throughout the file. This is not an
  issue with :qpdf:ref:`--json-output`, which doesn't repair the pages
  tree.

- While qpdf guarantees that keys present in the help will be present
  in the output, those fields may be null or empty if the information
  is not known or absent in the file. Also, if you specify
  :qpdf:ref:`--json-key`, the keys that are not listed
  will be excluded entirely except for those that
  :qpdf:ref:`--json-help` says are always present.

- In a few places, there are keys with names containing
  ``pageposfrom1``. The values of these keys are null or an integer. If
  an integer, they point to a page index within the file numbering from
  1. Note that JSON indexes from 0, and you would also use 0-based
  indexing using the API. However, 1-based indexing is easier in this
  case because the command-line syntax for specifying page ranges is
  1-based. If you were going to write a program that looked through
  the JSON for information about specific pages and then use the
  command-line to extract those pages, 1-based indexing is easier.
  Besides, it's more convenient to subtract 1 in a real programming
  language than it is to add 1 in shell code.

- The image information included in the ``page`` section of the JSON
  output includes the key ``"filterable"``. Note that the value of
  this field may depend on the :qpdf:ref:`--decode-level` that you
  invoke qpdf with. The JSON output includes a top-level key
  ``"parameters"`` that indicates the decode level that was used for
  computing whether a stream was filterable. For example, jpeg images
  will be shown as not filterable by default, but they will be shown
  as filterable if you run :command:`qpdf --json
  --decode-level=all`.

- The ``encrypt`` key's values will be populated for non-encrypted
  files. Some values will be null, and others will have values that
  apply to unencrypted files.

- The qpdf library itself never loads an entire PDF into memory. This
  remains true for PDF files represented in JSON format. In general,
  qpdf will hold the entire object structure in memory once a file has
  been fully read (objects are loaded into memory lazily but stay
  there once loaded), but it will never have more than two copies of a
  stream in memory at once. That said, if you ask qpdf to write JSON
  to memory, it will do so, so be careful about this if you are
  working with very large PDF files. There is nothing in the qpdf
  library itself that prevents working with PDF files much larger than
  available system memory. qpdf can both read and write such files in
  JSON format. If you need to work with a PDF file's json
  representation in memory, it is recommended that you use either
  ``none`` or ``file`` as the argument to
  :qpdf:ref:`--json-stream-data`, or if using the API, use
  ``qpdf_sj_none`` or ``pdf_sj_file`` as the json stream data value.
  If using ``none``, you can use other means to obtain the stream
  data.

.. _json-v2-changes:

Changes from JSON v1 to v2
--------------------------

The following changes were made to qpdf's JSON output format for
version 2.

- The representation of objects has changed. For details, see
  :ref:`json.objects`.

  - The representation of strings is now unambiguous for all strings.
    Strings a prefixed with either ``u:`` for Unicode strings or
    ``b:`` for byte strings.

  - Names are shown in qpdf's canonical form rather than in PDF
    syntax. (Example: the PDF-syntax name ``/text#2fplain`` appeared
    as ``"/text#2fplain"`` in v1 but appears as ``"/text/plain"`` in
    v2.

  - The top-level representation of an object in ``"objects"`` is a
    dictionary containing either a ``"value"`` key or a ``"stream"``
    key, making it possible to distinguish streams from other objects.

- The ``"objectinfo"`` key has been removed in favor of a
  representation in ``"objects"`` that differentiates between a stream
  and other kinds of objects. In v1, it was not possible to tell a
  stream from a dictionary within ``"objects"``.

- Within the ``"objects"`` dictionary, keys are now ``"obj:O G R"``
  where ``O`` and ``G`` are the object and generation number.
  ``"trailer"`` remains the key for the trailer dictionary. In v1, the
  ``obj:`` prefix was not present. The rationale for this change is as
  follows:

  - Having a unique prefix (``obj:``) makes it much easier to search
    in the JSON file for the definition of an object

  - Having the key still contain ``O G R`` makes it much easier to
    construct the key from an indirect reference. You just have to
    prepend ``obj:``. There is no need to parse the indirect object
    reference.

- In the ``"encrypt"`` object, the ``"modifyannotations"`` was
  misspelled as ``"moddifyannotations"`` in v1. This has been
  corrected.

Motivation for qpdf JSON version 2
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

qpdf JSON version 2 was created to make it possible to manipulate PDF
files using JSON syntax instead of native PDF syntax. This makes it
possible to make low-level updates to PDF files from just about any
programming language or even to do so from the command-line using
tools like ``jq`` or any editor that's capable of working with JSON
files. There were several limitations of JSON format version 1 that
made this impossible:

- Strings, names, and indirect object references in the original PDF
  file were all converted to strings in the JSON representation. For
  casual human inspection, this was fine, but in the general case,
  there was no way to tell the difference between a string that looked
  like a name or indirect object reference from an actual name or
  indirect object reference.

- PDF strings were not unambiguously represented in the JSON format.
  The way qpdf JSON v1 represented a string was to try to convert the
  string to UTF-8. This was done by assuming a string that was not
  explicitly marked as Unicode was encoded in PDF doc encoding. The
  problem is that there is not a perfect bidirectional mapping between
  Unicode and PDF doc encoding, so if a binary string happened to
  contain characters that couldn't be bidirectionally mapped, there
  would be no way to get back to the original PDF string. Even when
  possible, trying to map from the JSON representation of a binary
  string back to the original string required knowledge of the mapping
  between PDF doc encoding and Unicode.

- There was no representation of stream data. If you wanted to extract
  stream data, you could use :qpdf:ref:`--show-object`, so this wasn't
  that important for inspection, but it was a blocker for being able
  to go from JSON back to PDF. qpdf JSON version 2 allows stream data
  to be included inline as base64-encoded data. There is also an
  option to write all stream data to external files, which makes it
  possible to work with very large PDF files in JSON format even with
  tools that try to read the entire JSON structure into memory.

- The PDF version from PDF header was not represented in qpdf JSON v1.