OCRmyPDF/docs/advanced.md

% 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.

```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.

### OCR processing mode

:::{versionadded} 17.0.0
The `--mode` (`-m`) argument consolidates OCR processing options.
:::

OCRmyPDF provides a unified `--mode` argument to control how pages with
existing text are handled:

| Mode | Behavior | Legacy equivalent |
|------|----------|-------------------|
| `default` | Error if text is found | (no flag) |
| `force` | Rasterize all content and run OCR | `--force-ocr` |
| `skip` | Skip pages with existing text | `--skip-text` |
| `redo` | Re-OCR pages, stripping old OCR layer | `--redo-ocr` |

```bash
# Skip pages that already have text
ocrmypdf --mode skip input.pdf output.pdf
# or equivalently:
ocrmypdf -m skip input.pdf output.pdf

# Force OCR on all pages (rasterizes everything)
ocrmypdf --mode force input.pdf output.pdf

# Re-do OCR, replacing old invisible text
ocrmypdf --mode redo input.pdf output.pdf
```

The legacy flags (`--force-ocr`, `--skip-text`, `--redo-ocr`) remain as
silent aliases for backward compatibility.

### 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 `--mode skip` (or `--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 `--mode redo` (or `--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 `--mode force` (or `--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.)

```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. By default,
`--tesseract-downsample-large-images` is enabled, and OCRmyPDF will
downsample images to fit Tesseract limits. (The limits are usually encountered
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.) This feature can disabled
using `--no-tesseract-downsample-large-images`.

`--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 (32767 pixels on either dimension).

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.

```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:

```{eval-rst}
.. 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.
```

```{eval-rst}
.. 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:

```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):

```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 page segmentation mode

The directive `--tesseract-pagesegmode Nmode` forwards the desired page segmentation
mode to Tesseract OCR. The default is 3.

Page segmentation can improve OCR results when you know that a PDF ought to be
analyzed a particular way, such as PDFs whose pages contain only a single line of
text. For the vast majority of users, changing the page segmentation mode will only
make things worse.

As of June 2024, the Tesseract page segmentation modes are:

| ID  | Description                                                                                   |
| --- | --------------------------------------------------------------------------------------------- |
| 0   | Orientation and script detection (OSD) only.                                                  |
| 1   | Automatic page segmentation with OSD.                                                         |
| 2   | Automatic page segmentation, but no OSD, or OCR. (not implemented)                            |
| 3   | Fully automatic page segmentation, but no OSD. (Default)                                      |
| 4   | Assume a single column of text of variable sizes.                                             |
| 5   | Assume a single uniform block of vertically aligned text.                                     |
| 6   | Assume a single uniform block of text.                                                        |
| 7   | Treat the image as a single text line.                                                        |
| 8   | Treat the image as a single word.                                                             |
| 9   | Treat the image as a single word in a circle.                                                 |
| 10  | Treat the image as a single character.                                                        |
| 11  | Sparse text. Find as much text as possible in no particular order.                            |
| 12  | Sparse text with OSD.                                                                         |
| 13  | Raw line. Treat the image as a single text line, bypassing hacks that are Tesseract-specific. |

Modes 0, 1, 2, and 12 (all of those that enable orientation and script detection)
are not compatible with OCRmyPDF, which performs OSD in a separate step from OCR.
Their use may interfere with `--rotate-pages` and other features.

It is currently not possible to use advanced Tesseract OCR features, such as creating
OCR information, when using Tesseract through OCRmyPDF.

## Choosing a PDF rasterizer

:::{versionadded} 17.0.0
:::

rasterizing

: Converting a PDF page to an image for OCR processing.

OCRmyPDF supports two PDF rasterizers:

| Rasterizer | Package | Advantages | Disadvantages |
|------------|---------|------------|---------------|
| pypdfium2 | Python package | Faster, fewer version issues | Requires pypdfium2 package |
| Ghostscript | System binary | More widely packaged | Version consistency issues, restrictive AGPLv3 |

The `--rasterizer` argument controls which rasterizer is used:

```bash
# Automatic selection (default) - prefers pypdfium when available
ocrmypdf --rasterizer auto input.pdf output.pdf

# Force pypdfium2
ocrmypdf --rasterizer pypdfium input.pdf output.pdf

# Force Ghostscript
ocrmypdf --rasterizer ghostscript input.pdf output.pdf
```

pypdfium2 is a Python binding for pdfium, the PDF rendering library used
by Google Chrome and Chromium. It generally produces output identical to
Ghostscript but with better performance.

:::{note}
If pypdfium2 is not installed and `--rasterizer pypdfium` is requested,
OCRmyPDF will exit with an error. Install it with: `pip install pypdfium2`
:::

## Changing the PDF renderer

rendering

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

:::{versionchanged} 17.0.0
The fpdf2 renderer is now the default, replacing the legacy hOCR renderer.
:::

OCRmyPDF uses PDF renderers to create the invisible text layer. The
renderer may be selected using `--pdf-renderer`. The default is
`auto` which selects `fpdf2`.

### The `fpdf2` renderer (default)

:::{versionadded} 17.0.0
:::

The fpdf2 renderer creates text layers using the fpdf2 library. It provides:

- Full multilingual support including RTL languages (Arabic, Hebrew, Persian)
- Accurate text positioning aligned with OCR bounding boxes
- Improved "Occulta" glyphless font handling:
  - Zero-width markers are properly handled
  - Double-width CJK characters are properly sized
- Direct OcrElement tree input (no hOCR intermediate format required)

The fpdf2 renderer is the recommended choice for all installations.

:::{note}
The fpdf2 renderer may be slightly slower than the legacy hocrtransform
renderer for some workloads. This is an area of ongoing optimization.
:::

In both renderers, a text-only layer is rendered and sandwiched (overlaid)
on to either the original PDF page, or newly rasterized version of the
original PDF page (when `--mode force` is used). In this way, loss
of PDF information is generally avoided. (You may need to disable PDF/A
conversion and optimization to eliminate all lossy transformations.)

### The `sandwich` renderer

The `sandwich` renderer uses Tesseract's text-only PDF feature,
which produces a PDF page that lays out the OCR in invisible text.

Currently some problematic PDF viewers like Mozilla PDF.js and macOS
Preview have problems with segmenting its text output, and
mightrunseveralwordstogether. It also does not implement right to left
fonts (Arabic, Hebrew, Persian). The output of this renderer cannot
be edited. The sandwich renderer is retained for testing.

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.

### Legacy renderer options

The `hocr` and `hocrdebug` renderer options are deprecated and
automatically redirect to `fpdf2`. They will be removed in a future version.

## 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`.

## PDF/A output modes

:::{versionchanged} 17.0.0
The default `--output-type` is now `auto` instead of `pdfa`.
:::

OCRmyPDF can produce PDF/A compliant output for long-term archival. The
`--output-type` argument controls PDF/A conversion:

| Output type | Behavior |
|-------------|----------|
| `auto` | Best-effort PDF/A without requiring Ghostscript (default) |
| `pdfa` | PDF/A-2b via Ghostscript |
| `pdfa-1` | PDF/A-1b via Ghostscript |
| `pdfa-2` | PDF/A-2b via Ghostscript (same as `pdfa`) |
| `pdfa-3` | PDF/A-3b via Ghostscript |
| `pdf` | Standard PDF, no PDF/A conversion |
| `none` | No output file (useful with `--sidecar`) |

### Speculative PDF/A conversion

:::{versionadded} 17.0.0
:::

When `--output-type auto` is used (the default), OCRmyPDF attempts a
fast "speculative" PDF/A conversion that avoids Ghostscript when possible:

1. OCRmyPDF adds an sRGB ICC profile and PDF/A XMP metadata using pikepdf
2. If verapdf is available, it validates the result
3. If validation passes, Ghostscript is skipped entirely
4. If validation fails or verapdf is unavailable, falls back to Ghostscript

This approach is faster and avoids some Ghostscript limitations (such as
image transcoding), but only works for PDFs that are already "mostly"
PDF/A compliant.

### PDF/A conversion flow

The following diagram illustrates the PDF/A conversion decision tree:

```{mermaid}
flowchart TD
    A[Start] --> B{--output-type?}
    B -->|pdf| C[Output standard PDF]
    B -->|pdfa/pdfa-N| D[Use Ghostscript]
    B -->|auto| E[Attempt speculative conversion]

    E --> F["Add sRGB ICC + XMP metadata (pikepdf)"]
    F --> G{verapdf available?}

    G -->|No| H{Ghostscript available?}
    G -->|Yes| I[Validate with verapdf]

    I --> J{Validation passed?}
    J -->|Yes| K[Output PDF/A - Ghostscript skipped]
    J -->|No| H

    H -->|Yes| D
    H -->|No| L[Output standard PDF + WARNING]

    D --> M[Ghostscript PDF/A conversion]
    M --> N[Output PDF/A]

    style K fill:#90EE90
    style N fill:#90EE90
    style L fill:#FFB6C1
```

:::{warning}
**Breaking change:** If neither Ghostscript nor verapdf is installed,
`--output-type auto` will produce a standard PDF instead of PDF/A.
This is a change from previous versions where Ghostscript was required
and PDF/A was always produced.
:::

## 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`.

```{eval-rst}
.. 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:

```none
Temporary working files retained at:
/tmp/ocrmypdf.io.u20wpz07
```

When OCRmyPDF is launched as a snap, this corresponds to the snap filesystem, for instance:

> /tmp/snap-private-tmp/snap.ocrmypdf/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_hocr.pdf` - the OCR file; appears as a blank page with invisible
  text embedded
- `_ocr_hocr.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