OCRmyPDF/docs/cookbook.md

% SPDX-FileCopyrightText: 2025 James R. Barlow
% SPDX-License-Identifier: CC-BY-SA-4.0

# Cookbook

## Basic examples

### Help!

ocrmypdf has built-in help.

```bash
ocrmypdf --help
```

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

```bash
ocrmypdf input.pdf output.pdf
```

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

```bash
ocrmypdf --output-type pdf input.pdf output.pdf
```

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

```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.

```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.

```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.

```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
`Installing additional language packs <lang-packs>`{.interpreted-text
role="ref"}.

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\".

```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:

```bash
tesseract my-image.jpg output-prefix pdf
```

```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.

```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).

```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)=

## 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 that 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:

```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).

```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 (by causing OCR to time out). This
works if all you want to is to apply image processing or PDF/A
conversion.

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

:::{versionchanged} v14.1.0

Prior to this version, `--tesseract-timeout 0` would prevent other uses
of Tesseract, such as deskewing, from working. This is no longer the
case. Use `--tesseract-non-ocr-timeout` to control the timeout for
non-OCR operations, if needed.
:::

### Remove all text or OCR from my PDF

This is getting ridiculous, but OCRmyPDF can complete strip all textual
information from a PDF and reconstruct it as a \"bag of images\" PDF.

```bash
ocrmypdf --tesseract-timeout 0 --force-ocr input.pdf output.pdf
```

Why would you want to do this? Perhaps you have a PDF where OCR fails to
produce useful results, and just want to get rid of all OCR information.
This command also removes OCR generated by third party tools.

### Optimize images without performing OCR

You can also optimize all images without performing any OCR:

```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.

```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
overlapping 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. If your list of pages is out of numerical order, OCRmyPDF
will sort it for you.

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:

```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 for files they previously OCRed with an
earlier version of Tesseract and OCRmyPDF.

```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. `-O1` is the default.

For further details, see the section on [PDF optimization](optimizer).

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

Some users may consider enabling lossy JBIG2. See:
`jbig2-lossy`{.interpreted-text role="ref"}.

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

Digitally signed PDFs
---------------------

OCRmyPDF cannot preserve digital signatures in PDFs and also add OCR to
them. By default, it will refuse to modify a signed PDF regardless of
other settings. You can override this behavior with
`--invalidate-digital-signatures`; as the name suggests, any digital
signatures will be invalidated.

OCRmyPDF cannot open documents that are encrypted with a digital
certificate.

Versions of OCRmyPDF prior to 14.4.0 would invalidate existing digital
signatures without warning.