Models for OCR-D processors
OCR engines rely on pre-trained models for their recognition. Every engine has its own internal format(s) for models. Some support central storage of models at a specific location (tesseract, ocropy, kraken) while others require the full path to a model (calamari).
Since v2.22.0, OCR-D/core comes with a framework for managing processor resources uniformly. This means that processors can delegate to OCR-D/core to resolve specific file resources by name, looking in well-defined places in the filesystem. This also includes downloading and caching file parameters passed as a URL. Furthermore, OCR-D/core comes with a bundled database of known resources, such as models, dictionaries, configurations and other processor-specific data files. This means that OCR-D users should be able to concentrate on fine-tuning their OCR workflows and not bother with implementation details like “where do I get models from and where do I put them”. In particular, users can reference file parameters by name now.
All of the above mentioned functionality can be accessed using the
resmgr command line tool.
What models are available?
To get a list of the resources that the OCR-D/core is aware of:
ocrd resmgr list-available
The output will look similar to this:
ocrd-calamari-recognize - qurator-gt4hist-0.3 (https://qurator-data.de/calamari-models/GT4HistOCR/2019-07-22T15_49+0200/model.tar.xz) Calamari model trained with GT4HistOCR - qurator-gt4hist-1.0 (https://qurator-data.de/calamari-models/GT4HistOCR/2019-12-11T11_10+0100/model.tar.xz) Calamari model trained with GT4HistOCR ocrd-cis-ocropy-recognize - LatinHist.pyrnn.gz (https://github.com/chreul/OCR_Testdata_EarlyPrintedBooks/raw/master/LatinHist-98000.pyrnn.gz) ocropy historical latin model by github.com/chreul
As you can see, resources are grouped by the processors which make use of them.
The word after the list symbol, e.g.
LatinHist.pyrnn.gz, defines the name of the resource, which is a shorthand you can
use in parameters without having to specify the full URL (in brackets after the
The second line of each entry contains a short description of the resource.
Installing known resources
You can install resources with the
ocrd resmgr download command. It expects
the name of the processor as the first argument and either the name or URL of a
resource as a second argument.
Although model distribution is not currently centralised within OCR-D, we are working towards a central model repository.
For example, to install the
LatinHist.pyrnn.gz resource for
ocrd resmgr download ocrd-cis-ocropy-recognize LatinHist.pyrnn.gz # or ocrd resmgr download ocrd-cis-ocropy-recognize https://github.com/chreul/OCR_Testdata_EarlyPrintedBooks/raw/master/LatinHist-98000.pyrnn.gz
NOTE: The special name
* can be used instead of a resource name/url to
download all known resources for this processor. To download all tesseract models:
ocrd resmgr download ocrd-tesserocr-recognize '*'
NOTE: Equally, the special processor
* can be used instead of a processor and a resource
to download all known resources for all installed processors:
ocrd resmgr download '*'
(In either case,
* must be in quotes or escaped to avoid wildcard expansion by the shell.)
Installing unknown resources
If you need to install a resource which OCR-D doesn’t know of, that can be achieved by passings its URL in combination with the
--any-url/-n flag to
ocrd resmgr download:
To install a model for
ocrd-tesserocr-recognize that is located at
ocrd resmgr download -n ocrd-tesserocr-recognize https://my-server/mymodel.traineddata
ocrd-tesserocr-recognize -P model mymodel
List installed resources
ocrd resmgr list-installed command has the same output format as
ocrd resmgr list-available. But instead
of the database, it scans the filesystem locations where data is searched for existing
resources and lists URL and description if a database entry exists.
Whenever the OCR-D/core resource manager encounters an unknown resource in the filesystem or when you install
a resource with
ocrd resmgr download, it will create a new stub entry in the user database, which is found at
$HOME/.config/ocrd/resources.yml and created if it doesn’t exist.
This allows you to use the OCR-D/core resource manager mechanics, including lookup of known resources by name or URL, without relying (only) on the database maintained by the OCR-D/core developers.
NOTE: If you produced or found resources that are interesting for the wider OCR(-D) community, please tell us in the OCR-D gitter chat so we can add it to the database.
Where is the data
The lookup algorithm is defined in our specifications
In order of preference, a resource
<name> for a processor
ocrd-foo is searched at:
XDG_DATA_HOME defaults to
$HOME/.local/share if unset).
We recommend using the
$XDG_DATA_HOME location, which is also the default. But
you can override the location to store data with the
--location option, which can
# will download to $PWD/ocrd-resources/ocrd-anybaseocr-dewarp/latest_net_G.pth ocrd resmgr download --location cwd ocrd-anybaseocr-dewarp latest_net_G.pth # will download to /usr/local/share/ocrd-resources/ocrd-anybaseocr-dewarp/latest_net_G.pth ocrd resmgr download --location system ocrd-anybaseocr-dewarp latest_net_G.pth
Changing the default resource directory
$XDG_DATA_HOME default location is reasonable because
models are usually large files which should persist across different deployments,
both native and containerized, both single-module and ocrd_all.
Moreover, that variable can easily be overridden during installation.
However, there are use cases where
system or even
cwd should be
used as location to store resources, hence the
Notes on specific processors
Ocropy / ocrd_cis
An Ocropy model is simply the neural network serialized with Python’s pickle
mechanism and is generally distributed in a gzipped form, with a
extension and can be used as such, no need to unarchive.
To use a specific model with OCR-D’s ocropus wrapper in
ocrd_cis and more specifically, the
ocrd-cis-ocropy-recognize processor, use the
ocrd-cis-ocropy-recognize -I OCR-D-SEG-LINE -O OCR-D-OCR-OCRO -P model fraktur-jze.pyrnn.gz
NOTE: Model must be downloade before with
ocrd resmgr download ocrd-cis-ocropy-recognize fraktur-jze.pyrnn.gz
Calamari / ocrd_calamari
Calamari models are Tensorflow model directories. For distribution, this
directory is usually packed to a tarball or ZIP file. Once downloaded, these
containers must be unpacked to a directory again.
ocrd resmgr handles this
for you, so you just need the name of the resource in the database.
The Calamari-OCR project also maintains a repository of models.
To use a specific model with OCR-D’s calamari wrapper
ocrd_calamari and more specifically,
ocrd-calamari-recognize processor, use the
# To use the "default" model, i.e. the one trained on GT4HistOCR by QURATOR ocrd-calamari-recognize -I OCR-D-SEG-LINE -O OCR-D-OCR-CALA # To use your own trained model ocrd-calamari-recognize -I OCR-D-SEG-LINE -O OCR-D-OCR-CALA -P checkpoint_dir /path/to/modeldir # or, to be able to control which checkpoints to use: ocrd-calamari-recognize -I OCR-D-SEG-LINE -O OCR-D-OCR-CALA -P checkpoint '/path/to/modeldir/*.ckpt.json'
Tesseract / ocrd_tesserocr
Tesseract models are single files with a
Since tesseract only supports model lookup in a single directory, models should
only be stored in a single location. If the default location (
not the place you want to use for tesseract models, consider changing the default location
in the OCR-D config file.
NOTE: For reasons of efficiency and to avoid duplicate models, all
reuse the resource directory for
TESSDATA_PREFIX environment variable is set when any of the tesseract processors
are called, it will be the location to look for resources instead of the default.
OCR-D’s Tesseract wrapper,
ocrd_tesserocr and more
ocrd-tesserocr-recognize processor, expects the name of the
model(s) to be provided as the
model parameter. Multiple models can be
combined by concatenating with
+ (which generally improves accuracy but always slows processing):
# Use the deu and frk models ocrd-tesserocr-recognize -I OCR-D-SEG-LINE -O OCR-D-OCR-TESS -P model 'deut+frk' # Use the Fraktur model ocrd-tesserocr-recognize -I OCR-D-SEG-LINE -O OCR-D-OCR-TESS -P Fraktur
Models and Docker
We recommend keeping all downloaded resources in a persistent host directory,
separate of the
ocrd/* Docker container and data directory, and mounting that
resource directory into a specific path in the container alongside the data directory.
The host resource directory can be empty initially. Each time you run the Docker container,
your processors will access the host directory to resolve resources, and you can download
additional models into that location using
The following will assume (without loss of generality) that your host-side data
path is under
./data, and the host-side resource path is under
To download models to
./models in the host FS and
/usr/local/share/ocrd-resources in Docker:
docker run --user $(id -u) \ --volume $PWD/models:/usr/local/share/ocrd-resources \ ocrd/all \ ocrd resmgr download ocrd-tesserocr-recognize eng.traineddata\; \ ocrd resmgr download ocrd-calamari-recognize default\; \ ...
To run processors, as usual do:
docker run --user $(id -u) --workdir /data \ --volume $PWD/data:/data \ --volume $PWD/models:/usr/local/share/ocrd-resources \ ocrd/all ocrd-tesserocr-recognize -I IN -O OUT -P model eng
This principle applies to all
ocrd/* Docker images, e.g. you can replace
ocrd/all above with
ocrd/tesserocr as well.
With the pretrained models mentioned above, good results can be obtained for many originals. Nevertheless, the recognition rate can usually be improved significantly by fine-tuning an existing model or even training a new model on your own particular originals.
For training Tesseract models,
tesstrain can be used. As it is
not included in
ocrd_all, you will still have to install it, first. For information on the setup and the
training process itself see the Readme in the GithHub Repository.
tesstrain only allows you to train models for Tesseract, with
you can train models for four engines compatible with OCR-D - namely Tesseract, Ocropus, Kraken and Calamari - at once.
Especially if you want to use several OCR engines for your workflows or are not sure which OCR engine will give you the best
results, this might be particularly effective for you. Just like
tesstrain it is not included in
you will still have to install it, first. For information on the setup and the training process itself see the
Readme in the GithHub Repository.