Paperless preprocess pipeline

Preprocessing pipeline for cleaning, enhancing, and compressing scanned documents before they're imported into paperless-ng. Uses opencv and unpaper.

Features

• Smoothes and de-noises raw scans using the edge-preserving Bilateral filtering algorithm.
• Automatically crops and rotates the document within the scanned image.
• Automatically detects and removes blank pages. Very useful for scanners that scan both sides of a document.
• Thresholds the image to black and white using Otsu's algorithm to improve OCR accuracy and reduce storage requirements.
• Does all of the above before sending the image to unpaper for further noise and artifact reduction!
• Supports batch processing of documents for scanners that have an automatic document feed. Merges cleaned images into a single compressed file.
• Able to process raw scanned image in many formats, including pnm, ppm, pbm, and tiff.

Motivation

The paperless-ng project is a great tool for digitizing and organizing paper documents. It has excellent document post-processing features including machine-learning powered tagging, document metadata autofill, search indexing, and automatic OCR. It presents all your documents in an intuitive, modern UI.

While paperless-ng's post-processing tools are top-notch, in the case of scanned documents it relies entirely on the user to provide quality, pre-processed images for it to consume. Every feature listed above fills a gap in paperless-ng's functionality.

Setup

These instructions have been tested on Ubuntu 22.04.

Scanner setup

Setup your scanner using the SANE (scanner access not easy) software:

# Make sure universe repository is enabled
sudo add-apt-repository universe

# Install SANE (scanner access now easy) tools
sudo apt install libsane sane-utils

Find which backend supports your scanner by going to the SANE backends page and searching for your model of scanner. Be sure to click on the link to the backend-specific manpage to see if there's any further setup necessary.

If you're using a physically connected scanner, add yourself to the scanner group so you can access the device:

sudo usermod -aG scanner $USER

Log out and back in for the group change to take effect. Better yet, reboot.

Connect and turn on your scanner if you haven't already, and check that it's detected by running scanimage -L. You should see output similar to device `epjitsu:libusb:002:014' is a FUJITSU ScanSnap S1300i scanner. If you don't, make sure scanner is awake and ready to scan, and try some of the troubleshooting methods described in the sane Troubleshooting guide. You can also set environment variables to enable various levels of debug output, e.g. SANE_DEBUG_DLL=5 SANE_DEBUG_SANEI_USB=4 scanimage -L. See the manpages for sane-dll, sane-usb, and your specific sane backend for more info on debug levels.

Once successful, find out what scan options are available for your scanner device by running scanimage --help. Run a test scan with your desired options and check that the output image is as expected.

Paperless-ng setup

Run through paperless-ng's setup guide to install and start paperless-ng. I recommend using the docker-compose method. You can configure and run paperless-ng directly from a clone of this repo if you want - the config files and runtime directories will be ignored by git.

For configuration, the only recommended setting to use this pipeline is PAPERLESS_OCR_CLEAN=none. This disables paperless-ng's use of unpaper, which is desirable since it's already used by the preprocessing pipeline.

As a convenience, you can bootstrap paperless-ng's config using this script:

# set paperless secret key to a random string
export PAPERLESS_SECRET_KEY=$(cat /dev/urandom | tr -dc 'a-zA-Z0-9' | fold -w 64 | head -n 1)
# set uid and gid to match current user
export USERMAP_UID=$(id -u)
export USERMAP_GID=$(id -g)
# set timezone to match host timezone
export PAPERLESS_TIME_ZONE=$(timedatectl show --property=Timezone --value)
envsubst '$PAPERLESS_SECRET_KEY:$USERMAP_UID:$USERMAP_GID:$PAPERLESS_TIME_ZONE' < ./docker-compose.env.template > docker-compose.env

You may want to take this opportunity to review paperless-ng's configuration for other environment variables you may want to set.

Push-button paperless integration with scanbd

Scanbd is the scanner button daemon. It polls the scanner for button presses and triggers a user-defined script. We'll use this to integrate our scanner with paperless-ng.

sudo apt install scanbd unpaper python3 python3-venv libtiff-tools

Edit /etc/scanbd/scanbd.conf. Most settings will be left the same, but there are a few that are worth changing:

• Comment out the saned_env { ... } line. It tells saned to use a different folder for its configuration which is unnecessary and confusing.
• The script pointed to under the action scan { section should be changed from test.script to this repo's scanbd_trigger.sh.
• Comment out or remove all include directives that load specific backends. These can be found at the bottom of the file.

Setup the scanbd target script:

python3 -m venv _venv
. _venv/bin/activate
pip install -r requirements.txt
sudo mkdir -p /etc/scanbd/scripts
sudo ln -s $(pwd)/scanbd_trigger.sh /etc/scanbd/scripts/scanbd_trigger.sh
sudo systemctl restart scanbd

TODO

• Better secrets management for PAPERLESS_SECRETS_KEY (as described in jonaswinkler/paperless-ng#1236). For now I'm just saving the key to a git-ignored file.
• Detect and clean up dark edges and corners in scanned document. Ideally this would be done by applying adaptive thresholding to those regions to preserve content.