OCRmyPDF/docs/cookbook.rst

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

========
Cookbook
========

Basic examples
==============

Help!
-----

ocrmypdf has built-in help.

.. code-block:: bash

    ocrmypdf --help

Add an OCR layer and convert to PDF/A
-------------------------------------

.. code-block:: bash

    ocrmypdf input.pdf output.pdf

Add an OCR layer and output a standard PDF
------------------------------------------

.. code-block:: bash

    ocrmypdf --output-type pdf input.pdf output.pdf

Create a PDF/A with all color and grayscale images converted to JPEG
--------------------------------------------------------------------

.. code-block:: bash

    ocrmypdf --output-type pdfa --pdfa-image-compression jpeg input.pdf output.pdf

Modify a file in place
----------------------

The file will only be overwritten if OCRmyPDF is successful.

.. code-block:: bash

    ocrmypdf myfile.pdf myfile.pdf

Correct page rotation
---------------------

OCR will attempt to automatic correct the rotation of each page. This
can help fix a scanning job that contains a mix of landscape and
portrait pages.

.. code-block:: bash

    ocrmypdf --rotate-pages myfile.pdf myfile.pdf

You can increase (decrease) the parameter ``--rotate-pages-threshold``
to make page rotation more (less) aggressive. The threshold number is the ratio
of how confidence the OCR engine is that the document image should be changed,
compared to kept the same. The default value is quite conservative; on some files
it may not attempt rotations at all unless it is very confident that the current
rotation is wrong. A lower value of ``2.0`` will produce more rotations, and
more false positives. Run with ``-v1`` to see the confidence level for each
page to see if there may be a better value for your files.

If the page is "just a little off horizontal", like a crooked picture,
then you want ``--deskew``. ``--rotate-pages`` is for when the cardinal
angle is wrong.

OCR languages other than English
--------------------------------

OCRmyPDF assumes the document is in English unless told otherwise. OCR
quality may be poor if the wrong language is used.

.. code-block:: bash

    ocrmypdf -l fra LeParisien.pdf LeParisien.pdf
    ocrmypdf -l eng+fra Bilingual-English-French.pdf Bilingual-English-French.pdf

Language packs must be installed for all languages specified. See
:ref:`Installing additional language packs <lang-packs>`.

Unfortunately, the Tesseract OCR engine has no ability to detect the
language when it is unknown.

Produce PDF and text file containing OCR text
---------------------------------------------

This produces a file named "output.pdf" and a companion text file named
"output.txt".

.. code-block:: bash

    ocrmypdf --sidecar output.txt input.pdf output.pdf

.. note::

    The sidecar file contains the **OCR text** found by OCRmyPDF. If the document
    contains pages that already have text, that text will not appear in the
    sidecar. If the option ``--pages`` is used, only those pages on which OCR
    was performed will be included in the sidecar. If certain pages were skipped
    because of options like ``--skip-big`` or ``--tesseract-timeout``, those pages
    will not be in the sidecar.

    If you don't want to generate the output PDF, use ``--output-type=none`` to
    avoid generating one. Set the output filename to ``-`` (i.e. redirect to stdout).

    To extract all text from a PDF, whether generated from OCR or otherwise,
    use a program like Poppler's ``pdftotext`` or ``pdfgrep``.

OCR images, not PDFs
--------------------

Option: use Tesseract
~~~~~~~~~~~~~~~~~~~~~

If you are starting with images, you can just use Tesseract directly to
convert images to PDFs:

.. code-block:: bash

    tesseract my-image.jpg output-prefix pdf

.. code-block:: bash

    # When there are multiple images
    tesseract text-file-containing-list-of-image-filenames.txt output-prefix pdf

Tesseract's PDF output is quite good – OCRmyPDF uses it internally, in
some cases. However, OCRmyPDF has many features not available in
Tesseract like image processing, metadata control, and PDF/A generation.

Option: use img2pdf
~~~~~~~~~~~~~~~~~~~

You can also use a program like
`img2pdf <https://gitlab.mister-muffin.de/josch/img2pdf>`__ to convert
your images to PDFs, and then pipe the results to run ocrmypdf. The
``-`` tells ocrmypdf to read standard input.

.. code-block:: bash

    img2pdf my-images*.jpg | ocrmypdf - myfile.pdf

``img2pdf`` is recommended because it does an excellent job at
generating PDFs without transcoding images.

Option: use OCRmyPDF (single images only)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

For convenience, OCRmyPDF can also convert single images to PDFs on its
own. If the resolution (dots per inch, DPI) of an image is not set or is
incorrect, it can be overridden with ``--image-dpi``. (As 1 inch is 2.54
cm, 1 dpi = 0.39 dpcm).

.. code-block:: bash

    ocrmypdf --image-dpi 300 image.png myfile.pdf

If you have multiple images, you must use ``img2pdf`` to convert the
images to PDF.

Not recommended
~~~~~~~~~~~~~~~

We caution against using ImageMagick or Ghostscript to convert images to
PDF, since they may transcode images or produce downsampled images,
sometimes without warning.

Image processing
================

OCRmyPDF perform some image processing on each page of a PDF, if
desired. The same processing is applied to each page. It is suggested
that the user review files after image processing as these commands
might remove desirable content, especially from poor quality scans.

-  ``--rotate-pages`` attempts to determine the correct orientation for
   each page and rotates the page if necessary.
-  ``--remove-background`` attempts to detect and remove a noisy
   background from grayscale or color images. Monochrome images are
   ignored. This should not be used on documents that contain color
   photos as it may remove them.
-  ``--deskew`` will correct pages were scanned at a skewed angle by
   rotating them back into place.
-  ``--clean`` uses
   `unpaper <https://www.flameeyes.eu/projects/unpaper>`__ to clean up
   pages before OCR, but does not alter the final output. This makes it
   less likely that OCR will try to find text in background noise.
-  ``--clean-final`` uses unpaper to clean up pages before OCR and
   inserts the page into the final output. You will want to review each
   page to ensure that unpaper did not remove something important.

.. note::

   In many cases image processing will rasterize PDF pages as images,
   potentially losing quality.

.. warning::

   ``--clean-final`` and ``--remove-background`` may leave undesirable
   visual artifacts in some images where their algorithms have
   shortcomings. Files should be visually reviewed after using these
   options.

Example: OCR and correct document skew (crooked scan)
-----------------------------------------------------

Deskew:

.. code-block:: bash

    ocrmypdf --deskew input.pdf output.pdf

Image processing commands can be combined. The order in which options
are given does not matter. OCRmyPDF always applies the steps of the
image processing pipeline in the same order (rotate, remove background,
deskew, clean).

.. code-block:: bash

    ocrmypdf --deskew --clean --rotate-pages input.pdf output.pdf

Don't actually OCR my PDF
=========================

If you set ``--tesseract-timeout 0`` OCRmyPDF will apply its image
processing without performing OCR, if all you want to is to apply image
processing or PDF/A conversion.

.. code-block:: bash

    ocrmypdf --tesseract-timeout=0 --remove-background input.pdf output.pdf

Optimize images without performing OCR
--------------------------------------

You can also optimize all images without performing any OCR:

.. code-block:: bash

    ocrmypdf --tesseract-timeout=0 --optimize 3 --skip-text input.pdf output.pdf

Process only certain pages
--------------------------

You can ask OCRmyPDF to only apply `image processing <#image-processing>`__
and OCR to certain pages.

.. code-block:: bash

    ocrmypdf --pages 2,3,13-17 input.pdf output.pdf

Hyphens denote a range of pages and commas separate page numbers. If you prefer
to use spaces, quote all of the page numbers: ``--pages '2, 3, 5, 7'``.

OCRmyPDF will warn if your list of page numbers contains duplicates or
overlap pages. OCRmyPDF does not currently account for document page numbers,
such as an introduction section of a book that uses Roman numerals. It simply
counts the number of virtual pieces of paper since the start.

Regardless of the argument to ``--pages``, OCRmyPDF will optimize all pages/images
in the file and convert it to PDF/A, unless you disable those options. Both of these
steps are "whole file" operations. In this example, we want to OCR only the title
and otherwise change the PDF as little as possible:

.. code-block:: bash

    ocrmypdf --pages 1 --output-type pdf --optimize 0 input.pdf output.pdf

Redo existing OCR
=================

To redo OCR on a file OCRed with other OCR software or a previous
version of OCRmyPDF and/or Tesseract, you may use the ``--redo-ocr``
argument. (Normally, OCRmyPDF will exit with an error if asked to modify
a file with OCR.)

This may be helpful for users who want to take advantage of accuracy
improvements in Tesseract 4.0 for files they previously OCRed with an
earlier version of Tesseract and OCRmyPDF.

.. code-block:: bash

    ocrmypdf --redo-ocr input.pdf output.pdf

This method will replace OCR without rasterizing, reducing quality or
removing vector content. If a file contains a mix of pure digital text
and OCR, digital text will be ignored and OCR will be replaced. As such
this mode is incompatible with image processing options, since they
alter the appearance of the file.

In some cases, existing OCR cannot be detected or replaced. Files
produced by OCRmyPDF v2.2 or earlier, for example, are internally
represented as having visible text with an opaque image drawn on top.
This situation cannot be detected.

If ``--redo-ocr`` does not work, you can use ``--force-ocr``, which will
force rasterization of all pages, potentially reducing quality or losing
vector content.

Improving OCR quality
=====================

The `Image processing <#image-processing>`__ features can improve OCR
quality.

Rotating pages and deskewing helps to ensure that the page orientation
is correct before OCR begins. Removing the background and/or cleaning
the page can also improve results. The ``--oversample DPI`` argument can
be specified to resample images to higher resolution before attempting
OCR; this can improve results as well.

OCR quality will suffer if the resolution of input images is not correct
(since the range of pixel sizes that will be checked for possible fonts
will also be incorrect).

PDF optimization
================

By default OCRmyPDF will attempt to perform lossless optimizations on
the images inside PDFs after OCR is complete. Optimization is performed
even if no OCR text is found.

The ``--optimize N`` (short form ``-O``) argument controls optimization,
where ``N`` ranges from 0 to 3 inclusive, analogous to the optimization
levels in the GCC compiler.

.. list-table::
    :widths: auto
    :header-rows: 1

    *   - Level
        - Comments
    *   - ``--optimize 0``
        - Disables optimization.
    *   - ``--optimize 1``
        - Enables lossless optimizations, such as transcoding images to more
          efficient formats. Also compress other uncompressed objects in the
          PDF and enables the more efficient "object streams" within the PDF.
          (If ``--jbig2-lossy`` is issued, then lossy JBIG2 optimization is used.
          The decision to use lossy JBIG2 is separate from standard optimization
          settings.)
    *   - ``--optimize 2``
        - All of the above, and enables lossy optimizations and color quantization.
    *   - ``--optimize 3``
        - All of the above, and enables more aggressive optimizations and targets lower image quality.

Optimization is improved when a JBIG2 encoder is available and when
``pngquant`` is installed. If either of these components are missing,
then some types of images cannot be optimized.

The types of optimization available may expand over time. By default,
OCRmyPDF compresses data streams inside PDFs, and will change
inefficient compression modes to more modern versions. A program like
``qpdf`` can be used to change encodings, e.g. to inspect the internals
fo a PDF.

.. code-block:: bash

    ocrmypdf --optimize 3 in.pdf out.pdf  # Make it small

Some users may consider enabling lossy JBIG2. See: :ref:`jbig2-lossy`.

.. note::

    Image processing and PDF/A conversion can also introduce lossy transformations
    to your PDF images, even when ``--optimize 1`` is in use.