Nidaba is an open source distributed optical character recognition pipeline that makes it easy to preprocess, OCR, and postprocess scans of text documents in a multitude of ways.

Nidaba is a powerful tool allowing you to mix and combine some of the most advanced free image processing and character recognition software and distribute the execution of this software to multiple machines.

Nidaba does a bunch of things for you:


Unless you are installing from a precompiled source you will need the ability to build Python C-based modules from source in order to install NumPy and SciPy. Users on Unix-based platforms such as Ubuntu or Mac OS X will need the traditional C build toolchain installed (e.g. Developer Tools / XCode Tools on the Mac, or the build-essential package on Ubuntu or Debian Linux – basically, anything with gcc, make and so forth) as well as the Python development libraries, often named python-dev or similar:

# apt-get install build-essential python-dev
# apt-get build-dep python-scipy

In addition, some plugins require libraries and executables present on the system to work properly. A good start is install leptonica and tesseract:

# apt-get install libtesseract3 tesseract-ocr-eng libleptonica-dev liblept

Further, a broker is required for celery. For testing purposes we recommend redis:

# apt-get install redis-server

The recommended way to get Nidaba is to install the latest stable release via pip:

$ pip install nidaba

To also install the prerequisites for the nidaba.plugins.kraken plugin it is possible to install the kraken bundle instead:

$ pip install nidaba[kraken]


Deploying python applications can be painful in some circumstances. Unfortunately, nidaba is no exception to this and the build process of several dependencies is currently of a disastrous quality. nidaba should either be installed in disposable virtual machines or if installed on a machine that is intended to run other applications, e.g. a personal laptop or workstation, we strongly urge you to utilize virtualenv.

Alternatively, run pip in the root directory of the git repository:

$ pip install .


Per default no dictionaries and OCR models necessary to runs the tests are installed. To download the necessary files run:

$ python download

Afterwards, the test suite can be run:

$ python nosetests

Tests for plugins calling external programs, at the time only tesseract and ocropus, will be skipped if these aren’t installed.


There are currently two configuration files, one used by the celery framework and one containing the actual nidaba configuration. Both are written in YAML so indentation and whitespace is important. They are installed automatically into sys.prefix/etc/nidaba/{celery,nidaba}.yaml; usually into the root of the virtualenv containing nidaba or /usr/etc/nidaba/.

The former resembles a celery configuration object and may contain all available options. The example file looks like this:

BROKER_URL: 'redis://'

The later contains essential configuration for several subtasks and the overall framework:

storage_path: ~/OCR
redis_url: 'redis://'
  polytonic_greek: {dictionary: [dicts, greek.dic],
                    deletion_dictionary: [dicts, del_greek.dic]}
  latin: {dictionary: [dicts, latin.dic],
                    deletion_dictionary: [dicts, del_latin.dic]}
  greek: [models, greek.pyrnn.gz]
  atlantean: [models, atlantean.pyrnn.gz]
  fraktur: [models, fraktur.pyrnn.gz]
  fancy_ligatures: [models, ligatures.pyrnn.gz]
  default: [models, en-default.pronn]
  fraktur_clstm: [models, fraktur.clstm]
  tesseract: {implementation: capi,
             tessdata: /usr/share/tesseract-ocr}
  ocropus: {}
  kraken: {}
  leptonica: {}


The home directory for nidaba to store files created by OCR jobs, i.e. the location of the shared storage medium. This may differ on different machines in the cluster.


URL to the Redis key-value store which is required in addition to celery’s broker but may be shared.

lang_dicts (optional)

See the spell-checking documentation for further information.

ocropus_models and kraken_models (optional)

See the plugins documentation for further information.

plugins_load (optional)

An associative array of plugins to load with additional configuration data for each plugin. See plugins for more information.


Celery requires a worker retrieving tasks from the message broker to actually execute them. Starting the celery worker server is quite simple and the only requirement is that it is NOT run inside the nidaba directory and the message broker is up and running:

$ nidaba worker

For further worker options have a look at the Celery worker documentation.

Command Line Interface

The simplest way to add jobs to the pipeline is using the nidaba command line utility. It is automatically installed during the installation procedure.

Configuration and Plugins

The config subcommand is used to inspect the current

$ nidaba config
{'kraken_models': {'goodell': ['models', 'goodell.pyrnn.pronn'],
                   'loeb_2013': ['models', 'loeb.pronn'],
                   'migne': ['models', 'migne.pronn'],
                   'migne_2013': ['models', 'migne_2013.pronn'],
                   'non-teubner': ['models', 'non-teubner-german-serif.pronn'],
                   'omnibus': ['models', 'omnibus.pyrnn.pronn'],
                   'porson': ['models', 'porson.pyrnn.pronn'],
                   'rahlfs': ['models', 'rahlfs.pyrnn.pronn'],
                   'teubner': ['models', 'teubner-serif.pyrnn.pronn']},
 'lang_dicts': {'latin': {'deletion_dictionary': ['dicts', 'del_latin.dic'],
                          'dictionary': ['dicts', 'latin.dic']},
                'polytonic_greek': {'deletion_dictionary': ['dicts',
                                    'dictionary': ['dicts', 'greek.dic']}},
 'ocropus_models': {'atlantean': ['models', 'atlantean.pyrnn.gz'],
                    'fancy_ligatures': ['models', 'ligatures.pyrnn.gz'],
                    'fraktur': ['models', 'fraktur.pyrnn.gz'],
                    'greek': ['models', 'greek.pyrnn.gz']},
 'plugins_load': {'kraken': {},
                  'tesseract': {'implementation': 'capi',
                                'tessdata': '/home/mittagessen/git'}},
 'redis_url': 'redis://',
 'storage_path': '~/OCR'}

To see which plugins are available and enabled the plugins subcommand may be used:

$ nidaba plugins
ocropus (disabled)
tesseract (enabled)
kraken (disabled)
leptonica (disabled)


The worker subcommand is a shorthand to start up a celery worker. In fact

$ nidaba worker

is equivalent to:

$ celery -A nidaba worker

except that the latter will accept additional command line parameters.


The batch subcommand is used to add a job to the pipeline. A rather minimal invocation looks like this:

$ nidaba batch -b sauvola:whsize=10,factor=0.35 -l tesseract -o tesseract:languages=\[grc,eng\],extended=True -- 0007.tif

It converts the input file input.png to grayscale, binarizes it using the Sauvola algorithm, creates a page segmentation of the image using tesseract’s algorithm, and finally runs everything through tesseract with the English and Greek language models, creating a TEI XML file.

There are several groups of options, each associated with a particular set of functions of the pipeline. An option may appear multiple times to define additional execution paths similar to adding new branches to the leaves of a tree where each leaf is one result of the, logically, preceding group of options. All paths through this tree will then be executed by the workers in the cluster.

Each option must follow the same format algorithm:param1=val1,param2=\[val2,val3\],...,paramN=N where parameters are divided by , and lists of parameter values are contained in \[val1,val2\] statements. Additionally, there is a helper preamble file: which must be followed by a valid path to a file. This file will be copied to the common storage medium and its new location will be used instead of the path when executing the function.

Available option groups in order of processing are:

–binarize / -b
Defines binarization parameters. See the binarization documentation for more details.
–segmentation / -l
Defines page segmentation parameters. See the segmentation documentation for more details.
–ocr / -o
Defines OCR parameters. See the OCR documentation for more details.
–postprocessing / -p
Defines parameters of various postprocessing methods. See the spell-checking documentation for more details.
–stats / -s
Defines parameters of metrics. See the Metrics documentation for more details.

There is one additional option which is technically part of another option group but can’t be included in other groups for several reasons:

A switch to indicate that input files are already 8bpp grayscale and conversion to grayscale is unnecessary.

Batch status

The status subcommand is used to retrieve the status of a job. It takes the return value of a previously executed batch command. A currently running command will return pending and some additional information about the batch:

$ nidaba status 90ae699a-7172-44ce-a8bf-5464bccd34d0
Status: pending

4/4 tasks completed. 0 running.

Output files:

When the job has been processed the status command will return a list of paths containing the final output:

$ nidaba status 90ae699a-7172-44ce-a8bf-5464bccd34d0
Status: success (final)

4/4 tasks completed. 0 running.

Output files:

0007.tif → 0007_ocr.tesseract_eng_grc.tif.xml
0009.tif → 0009_ocr.tesseract_eng_grc.tif.xml

On failure the subtasks that failed and their error message will be printed:

$ nidaba status 90ae699a-7172-44ce-a8bf-5464bccd34d0
Status: failed (final)

1/2 tasks completed. 0 running.

Output files:


nidaba.ocr.kraken (0007.tif): IOError: [Errno 2] No such file or directory: '/home/mittagessen/OCR/t/e/u/b/n/e/r/-/s/e/r/i/f/-/2/0/1/3/-/1/2/-/1/6/-/1/1/-/2/6/-/0/0/0/6/7/0/0/0/p/y/r/n/n/p/r/o/n/n'

A batch may contain failed and running tasks at the same time. To be able to distinguish failed batches which are still running and finalized failed batches the status command will add the (final) indicator on the latter.

It is also possible to increase the verbosity of the error output:

$ nidaba status -vv 90ae699a-7172-44ce-a8bf-5464bccd34d0
Status: failed (final)

288/384 tasks completed. 0 running.

Output files:


nidaba.ocr.kraken (0046.tif, {u'model': u'teubner'}):   File "/home/mittagessen/git/nidaba/nidaba/tasks/", line 91, in __call__
    ret = super(NidabaTask, self).__call__(*args, **nkwargs)
  File "/home/mittagessen/envs/nidaba/local/lib/python2.7/site-packages/celery/app/", line 438, in __protected_call__
    return*args, **kwargs)
  File "/home/mittagessen/git/nidaba/nidaba/plugins/", line 152, in ocr_kraken
    rnn = models.load_any(model)
  File "/home/mittagessen/git/kraken/kraken/lib/", line 110, in load_any
    seq = load_pyrnn(fname)
  File "/home/mittagessen/git/kraken/kraken/lib/", line 211, in load_pyrnn
    with io.BufferedReader(of(fname, 'rb')) as fp:
IOError: [Errno 2] No such file or directory: '/home/mittagessen/OCR/t/e/u/b/n/e/r/-/s/e/r/i/f/-/2/0/1/3/-/1/2/-/1/6/-/1/1/-/2/6/-/0/0/0/6/7/0/0/0/p/y/r/n/n/p/r/o/n/n'


Every open source project lives from the generous help by contributors that sacrifice their time and nibada is no different.

Here are a few hints and rules to get you started:

  • No contribution is too small; please submit as many fixes for typos and grammar bloopers as you can!
  • Don’t ever break backward compatibility.
  • Always add tests and docs for your code.
  • This is a hard rule; patches with missing documentation won’t be merged. If a feature is not documented, it doesn’t exist.
  • Obey PEP8 and PEP257.
  • Write good commit messages.
  • Ideally, squash your commits, i.e. make your pull requests just one commit.
  • If you’re not comfortable with using git, please use git format-patch and send me the resulting diff.

If you have something great but aren’t sure whether it adheres – or even can adhere – to the rules above: please submit a pull request anyway!

In the best case, we can mold it into something, in the worst case the pull request gets politely closed. There’s absolutely nothing to fear.

Thank you for considering to contribute to nibada! If you have any question or concerns, feel free to reach out to us.

Licensing and Authorship

This project is licensed under GPL2.0+, so feel free to use and adapt as long as you respect the license. See LICENSE for details.

nidaba is written as part of the Open Greek and Latin Project at the Humboldt Chair of Digital Humanities at the University of Leipzig.