OCRmyPDF/docs/advanced.rst

.. SPDX-FileCopyrightText: 2022 James R. Barlow
.. SPDX-License-Identifier: CC-BY-SA-4.0

=================
Advanced features
=================

Control of unpaper
==================

OCRmyPDF uses ``unpaper`` to provide the implementation of the
``--clean`` and ``--clean-final`` arguments.
`unpaper <https://github.com/Flameeyes/unpaper/blob/main/doc/basic-concepts.md>`__
provides a variety of image processing filters to improve images.

By default, OCRmyPDF uses only ``unpaper`` arguments that were found to
be safe to use on almost all files without having to inspect every page
of the file afterwards. This is particularly true when only ``--clean``
is used, since that instructs OCRmyPDF to only clean the image before
OCR and not the final image.

However, if you wish to use the more aggressive options in ``unpaper``,
you may use ``--unpaper-args '...'`` to override the OCRmyPDF's defaults
and forward other arguments to unpaper. This option will forward
arguments to ``unpaper`` without any knowledge of what that program
considers to be valid arguments. The string of arguments must be quoted
as shown in the examples below. No filename arguments may be included.
OCRmyPDF will assume it can append input and output filename of
intermediate images to the ``--unpaper-args`` string.

In this example, we tell ``unpaper`` to expect two pages of text on a
sheet (image), such as occurs when two facing pages of a book are
scanned. ``unpaper`` uses this information to deskew each independently
and clean up the margins of both.

.. code-block:: bash

    ocrmypdf --clean --clean-final --unpaper-args '--layout double' input.pdf output.pdf
    ocrmypdf --clean --clean-final --unpaper-args '--layout double --no-noisefilter' input.pdf output.pdf

.. warning::

   Some ``unpaper`` features will reposition text within the image.
   ``--clean-final`` is recommended to avoid this issue.

.. warning::

   Some ``unpaper`` features cause multiple input or output files to be
   consumed or produced. OCRmyPDF requires ``unpaper`` to consume one
   file and produce one file; errors will result if this assumption is not
   met.

.. note::

   ``unpaper`` uses uncompressed PBM/PGM/PPM files for its intermediate
   files. For large images or documents, it can take a lot of temporary
   disk space.

Control of OCR options
======================

OCRmyPDF provides many features to control the behavior of the OCR
engine, Tesseract.

When OCR is skipped
-------------------

If a page in a PDF seems to have text, by default OCRmyPDF will exit
without modifying the PDF. This is to ensure that PDFs that were
previously OCRed or were "born digital" rather than scanned are not
processed.

If ``--skip-text`` is issued, then no image processing or OCR will be
performed on pages that already have text. The page will be copied to
the output. This may be useful for documents that contain both "born
digital" and scanned content, or to use OCRmyPDF to normalize and
convert to PDF/A regardless of their contents.

If ``--redo-ocr`` is issued, then a detailed text analysis is performed.
Text is categorized as either visible or invisible. Invisible text (OCR)
is stripped out. Then an image of each page is created with visible text
masked out. The page image is sent for OCR, and any additional text is
inserted as OCR. If a file contains a mix of text and bitmap images that
contain text, OCRmyPDF will locate the additional text in images without
disrupting the existing text. Some PDF OCR solutions render text as
technically printable or visible in some way, perhaps by drawing it and
then painting over it. OCRmyPDF cannot distinguish this type of OCR
text from real text, so it will not be "redone".

If ``--force-ocr`` is issued, then all pages will be rasterized to
images, discarding any hidden OCR text, rasterizing any printable
text, and flattening form fields or interactive objects into their visual
representation. This is useful for redoing OCR, for fixing OCR text
with a damaged character map (text is selectable but not searchable),
and destroying redacted information.

Time and image size limits
--------------------------

By default, OCRmyPDF permits tesseract to run for three minutes (180
seconds) per page. This is usually more than enough time to find all
text on a reasonably sized page with modern hardware.

If a page is skipped, it will be inserted without OCR. If preprocessing
was requested, the preprocessed image layer will be inserted.

If you want to adjust the amount of time spent on OCR, change
``--tesseract-timeout``. You can also automatically skip images that
exceed a certain number of megapixels with ``--skip-big``. (A 300 DPI,
8.5×11" page image is 8.4 megapixels.)

.. code-block:: bash

    # Allow 300 seconds for OCR; skip any page larger than 50 megapixels
    ocrmypdf --tesseract-timeout 300 --skip-big 50 bigfile.pdf output.pdf

OCR for huge images
-------------------

Tesseract has internal limits on the size
of images it will process. If you issue
``--tesseract-downsample-large-images``, OCRmyPDF will downsample images
to fit Tesseract limits. (The limits are usually entered only for scanned
images of oversized media, such as large maps or blueprints exceeding
110 cm or 43 inches in either dimension, and at high DPI.)

``--tesseract-downsample-above Npixels`` adjusts the threshold at which images
will be downsampled. By default, only images that exceed any of Tesseract's
internal limits are downsampled.

You will also need to set ``--tesseract-timeout`` high enough to allow
for processing.

Only the image sent for OCR is downsampled. The original image is
preserved.

.. code-block:: bash

    # Allow 600 seconds for OCR on huge images
    ocrmypdf --tesseract-timeout 600 \
        --tesseract-downsample-large-images \
        bigfile.pdf output.pdf

    # Downsample images above 5000 pixels on the longest dimension to
    # 5000 pixels
    ocrmypdf --tesseract-timeout 120 \
        --tesseract-downsample-large-images \
        --tesseract-downsample-above 5000 \
        bigfile.pdf output_downsampled_ocr.pdf


Overriding default tesseract
----------------------------

OCRmyPDF checks the system ``PATH`` for the ``tesseract`` binary.

Some relevant environment variables that influence Tesseract's behavior
include:

.. envvar:: TESSDATA_PREFIX

   Overrides the path to Tesseract's data files. This can allow
   simultaneous installation of the "best" and "fast" training data
   sets. OCRmyPDF does not manage this environment variable.

.. envvar:: OMP_THREAD_LIMIT

   Controls the number of threads Tesseract will use. OCRmyPDF will
   manage this environment variable if it is not already set.

For example, if you have a development build of Tesseract don't wish to
use the system installation, you can launch OCRmyPDF as follows:

.. code-block:: bash

    env \
        PATH=/home/user/src/tesseract/api:$PATH \
        TESSDATA_PREFIX=/home/user/src/tesseract \
        ocrmypdf input.pdf output.pdf

In this example ``TESSDATA_PREFIX`` is required to redirect Tesseract to
an alternate folder for its "tessdata" files.

Overriding other support programs
---------------------------------

In addition to tesseract, OCRmyPDF uses the following external binaries:

-  ``gs`` (Ghostscript)
-  ``unpaper``
-  ``pngquant``
-  ``jbig2``

In each case OCRmyPDF will search the ``PATH`` environment variable to
locate the binaries. By modifying the ``PATH`` environment variable, you
can override the binaries that OCRmyPDF uses.

Changing Tesseract configuration variables
------------------------------------------

You can override Tesseract's default `control
parameters <https://tesseract-ocr.github.io/tessdoc/tess3/ControlParams.html>`__
with a configuration file.

As an example, this configuration will disable Tesseract's dictionary
for current language. Normally the dictionary is helpful for
interpolating words that are unclear, but it may interfere with OCR if
the document does not contain many words (for example, a list of part
numbers).

Create a file named "no-dict.cfg" with these contents:

::

    load_system_dawg 0
    language_model_penalty_non_dict_word 0
    language_model_penalty_non_freq_dict_word 0

then run ocrmypdf as follows (along with any other desired arguments):

.. code-block:: bash

    ocrmypdf --tesseract-config no-dict.cfg input.pdf output.pdf

.. warning::

   Some combinations of control parameters will break Tesseract or break
   assumptions that OCRmyPDF makes about Tesseract's output.

Changing the PDF renderer
=========================

rasterizing
  Converting a PDF to an image for display.

rendering
  Creating a new PDF from other data (such as an existing PDF).

OCRmyPDF has these PDF renderers: ``sandwich`` and ``hocr``. The
renderer may be selected using ``--pdf-renderer``. The default is
``auto`` which lets OCRmyPDF select the renderer to use. Currently,
``auto`` always selects ``sandwich``.

The ``sandwich`` renderer
-------------------------

The ``sandwich`` renderer uses Tesseract's new text-only PDF feature,
which produces a PDF page that lays out the OCR in invisible text. This
page is then "sandwiched" onto the original PDF page, allowing lossless
application of OCR even to PDF pages that contain other vector objects.

Currently this is the best renderer for most uses, however it is
implemented in Tesseract so OCRmyPDF cannot influence it. Currently some
problematic PDF viewers like Mozilla PDF.js and macOS Preview have
problems with segmenting its text output, and
mightrunseveralwordstogether.

When image preprocessing features like ``--deskew`` are used, the
original PDF will be rendered as a full page and the OCR layer will be
placed on top.

The ``hocr`` renderer
---------------------

The ``hocr`` renderer works with older versions of Tesseract. The image
layer is copied from the original PDF page if possible, avoiding
potentially lossy transcoding or loss of other PDF information. If
preprocessing is specified, then the image layer is a new PDF. (You may
need to disable PDF/A conversion nad optimization to eliminate all
lossy transformations.)

Unlike ``sandwich`` this renderer is implemented within OCRmyPDF; anyone
looking to customize how OCR is presented should look here. A major
disadvantage of this renderer is it not capable of correctly handling
text outside the Latin alphabet (specifically, it supports the ISO 8859-1
character set). Pull requests to improve the situation are welcome.

Currently, this renderer has the best compatibility with Mozilla's
PDF.js viewer.

This works in all versions of Tesseract.

Rendering and rasterizing options
=================================

.. versionadded:: 14.3.0

The ``--continue-on-soft-render-error`` option allows OCRmyPDF to
proceed if a page cannot be rasterized/rendered. This is useful if you are
trying to get the best possible OCR from a PDF that is not well-formed,
and you are willing to accept some pages that may not visually match the
input, and that may not OCR well.

Color conversion strategy
=========================

.. versionadded:: 15.0.0

OCRmyPDF uses Ghostscript to convert PDF to PDF/A. In some cases, this
conversion requires color conversion. The default strategy is to convert
using the ``LeaveColorUnchanged`` strategy, which preserves the original
color space wherever possible (some rare color spaces might still be
converted).

Usually document scanners produce PDFs in the sRGB color space, and do
not need to be converted, so the default strategy is appropriate.

Suppose that you have a document that was prepared for professional
printing in a Separation or CMYK color space, and text was converted to
curves. In this case, you may want to use a different color conversion
strategy. The ``--color-conversion-strategy`` option allows you to select a
different strategy, such as ``RGB``.

Return code policy
==================

OCRmyPDF writes all messages to ``stderr``. ``stdout`` is reserved for
piping output files. ``stdin`` is reserved for piping input files.

The return codes generated by the OCRmyPDF are considered part of the
stable user interface. They may be imported from
``ocrmypdf.exceptions``.

.. list-table:: Return codes
    :widths: 5 35 60
    :header-rows: 1

    *	- Code
        - Name
        - Interpretation
    *	- 0
        - ``ExitCode.ok``
        - Everything worked as expected.
    *	- 1
        - ``ExitCode.bad_args``
        - Invalid arguments, exited with an error.
    *	- 2
        - ``ExitCode.input_file``
        - The input file does not seem to be a valid PDF.
    *	- 3
        - ``ExitCode.missing_dependency``
        - An external program required by OCRmyPDF is missing.
    *	- 4
        - ``ExitCode.invalid_output_pdf``
        - An output file was created, but it does not seem to be a valid PDF. The file will be available.
    *	- 5
        - ``ExitCode.file_access_error``
        - The user running OCRmyPDF does not have sufficient permissions to read the input file and write the output file.
    *	- 6
        - ``ExitCode.already_done_ocr``
        - The file already appears to contain text so it may not need OCR. See output message.
    *	- 7
        - ``ExitCode.child_process_error``
        - An error occurred in an external program (child process) and OCRmyPDF cannot continue.
    *	- 8
        - ``ExitCode.encrypted_pdf``
        - The input PDF is encrypted. OCRmyPDF does not read encrypted PDFs. Use another program such as ``qpdf`` to remove encryption.
    *	- 9
        - ``ExitCode.invalid_config``
        - A custom configuration file was forwarded to Tesseract using ``--tesseract-config``, and Tesseract rejected this file.
    *   - 10
        - ``ExitCode.pdfa_conversion_failed``
        - A valid PDF was created, PDF/A conversion failed. The file will be available.
    *	- 15
        - ``ExitCode.other_error``
        - Some other error occurred.
    *	- 130
        - ``ExitCode.ctrl_c``
        - The program was interrupted by pressing Ctrl+C.


.. _tmpdir:

Changing temporary storage location
===================================

OCRmyPDF generates many temporary files during processing.

To change where temporary files are stored, change the ``TMPDIR``
environment variable for ocrmypdf's environment. (Python's
``tempfile.gettempdir()`` returns the root directory in which temporary
files will be stored.) For example, one could redirect ``TMPDIR`` to a
large RAM disk to avoid wear on HDD/SSD and potentially improve
performance.

On Windows, the ``TEMP`` environment variable is used instead.

Debugging the intermediate files
================================

OCRmyPDF normally saves its intermediate results to a temporary folder
and deletes this folder when it exits, whether it succeeded or failed.

If the ``--keep-temporary-files`` (``-k```) argument is issued on the
command line, OCRmyPDF will keep the temporary folder and print the location,
whether it succeeded or failed. An example message is:

.. code-block:: none

    Temporary working files retained at:
    /tmp/ocrmypdf.io.u20wpz07

The organization of this folder is an implementation detail and subject
to change between releases. However the general organization is that
working files on a per page basis have the page number as a prefix
(starting with page 1), an infix indicates the processing stage, and a
suffix indicates the file type. Some important files include:

-  ``_rasterize.png`` - what the input page looks like
-  ``_ocr.png`` - the file that is sent to Tesseract for OCR; depending
   on arguments this may differ from the presentation image
-  ``_pp_deskew.png`` - the image, after deskewing
-  ``_pp_clean.png`` - the image, after cleaning with unpaper
-  ``_ocr_tess.pdf`` - the OCR file; appears as a blank page with invisible
   text embedded
-  ``_ocr_tess.txt`` - the OCR text (not necessarily all text on the page,
   if the page is mixed format)
-  ``fix_docinfo.pdf`` - a temporary file created to fix the PDF DocumentInfo
   data structure
-  ``graft_layers.pdf`` - the rendered PDF with OCR layers grafted on
-  ``pdfa.pdf`` - ``graft_layers.pdf`` after conversion to PDF/A
-  ``pdfa.ps`` - a PostScript file used by Ghostscript for PDF/A conversion
-  ``optimize.pdf`` - the PDF generated before optimization
-  ``optimize.out.pdf`` - the PDF generated by optimization
-  ``origin`` - the input file
-  ``origin.pdf`` - the input file or the input image converted to PDF
-  ``images/*`` - images extracted during the optimization process; here
   the prefix indicates a PDF object ID not a page number