Advanced features

Control of unpaper

OCRmyPDF uses unpaper to provide the implementation of the --clean and --clean-final arguments. unpaper 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.

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. An deviation from that condition will result in errors.

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 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.

If --force-ocr is issued, then all pages will be rasterized to images, discarding any hidden OCR text, and rasterizing any printable text. 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. Any forms and vector graphics will be rasterized as well.

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 is 8.4 megapixels.)

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

Overriding default tesseract

OCRmyPDF checks the system PATH for the tesseract binary.

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

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.

OMP_THREAD_LIMIT

Controls the number of threads Tesseract will use. OCRmyPDF will manage this environment if it is not already set. (Currently, it will set it to 1 because this gives the best results in testing.)

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

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

  • qpdf

In each case OCRmyPDF will search the PATH environment variable to locate the binaries.

Changing tesseract configuration variables

You can override tesseract’s default control parameters 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):

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.

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. 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.

The tesseract renderer

The tesseract renderer was removed. OCRmyPDF’s new approach to text layer grafting makes it functionally equivalent to sandwich.

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.

Return codes

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.

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 -k argument is issued on the command line, OCRmyPDF will keep the temporary folder and print the location, whether it succeeded or failed (provided the Python interpreter did not crash). An example message is:

Temporary working files retained at:
/tmp/com.github.ocrmypdf.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