This provides an attempt at running OCR-D workflows configured and controlled via makefiles using GNU bash, GNU make and GNU parallel.

Makefilization offers the following advantages:

• incremental builds (steps already processed for another configuration or in a failed run need not be repeated) and automatic dependencies (new files will force all their dependents to update)
• persistency of configuration and results
• encapsulation and ease of use
• sharing configurations and repeating experiments
• less writing effort, fast templating
• parallelization across workspaces

Nevertheless, there are also some disadvantages:

• depends on directories (fileGrps) as targets, which is hard to get correct under all circumstances
• must mediate between filesystem perspective (understood by make) and METS perspective
• make cannot handle target names with spaces in them (at all)
  (This means that fileGrp directories must not have spaces. Local file paths may contain spaces though, if the respective processors support that.)

Contents:

• Dependencies
• Installation
• Docker Image
• Usage
  • ocrd-import
  • PAGE transformations and ocrd-page-transform
  • METS transformations
  • ocrd-make
    • LOGLEVEL
    • PAGES
    • TIMEOUT
    • FAILDUMMY
    • METSSERV
    • PAGEWISE
    • Remote distribution
• Customisation
  • Recommendations
  • Example
• Testing
• Results
  • OCR-D ground truth
• Implementation
  • GPU vs CPU parallelism
  • workspace vs page parallelism

To install system dependencies for this package, run...

make deps-ubuntu

...in a privileged context for Ubuntu (like a Docker container).

Or equivalently, install the following packages:

• parallel (GNU parallel)
• make (GNU make)
• xmlstarlet
• bc and sed

Additionally, you must of course install ocrd itself along with its dependencies in the current shell environment. Moreover, depending on the specific configurations you want to use (i.e. the processors it contains), additional modules must be installed. See OCR-D setup guide for instructions.

(Yes, workflow-configuration is already part of ocrd_all, which is also available on Dockerhub.)

Run:

make install

... if you are in a (Python) virtual environment. Otherwise specify the installation prefix directory via environment variable VIRTUAL_ENV.

Assuming $VIRTUAL_ENV/bin is in your PATH, you can then call:

cd WORKSPACE && make [OPTIONS] -f WORKFLOW-CONFIG.mk
make -C WORKSPACE [OPTIONS] -f WORKFLOW-CONFIG.mk

... for processing single workspace directory, or ...

ocrd-make [OPTIONS] -f WORKFLOW-CONFIG.mk WORKSPACE...

... for processing multiple workspaces at once (with the same interface as above).

Where:

• OPTIONS are the usual options controlling GNU make (e.g. -j for parallel processing).
• WORKFLOW_CONFIG.mk is one of the configuration makefiles you find here or created yourself.
• WORKSPACE is a directory with a mets.xml, or all (the default) for all such directories that we can find.

Calling workflows is possible from anywhere in your filesystem, but for the WORKFLOW_CONFIG.mk you may need to:

• either provide the *.mk configurations in the source directory at installation time (to ensure they are installed under the installation prefix and can always be found by file name only)
• or provide full paths at runtime (by absolute path name, or relative to the CWD).

(The previous version of ocrd-make tried to copy or symlink all makefiles to the runtime directory. You can still use those, but should remove the old Makefile.)

Instead of the above native installation steps, you can use the prebuilt image from Docker Hub:

docker pull bertsky/workflow-configuration
docker run -V /path/to/data:/data bertsky/workflow-configuration ocrd-make ...

For general guidance on using Docker with OCR-D, see User Guide.

To create workspaces from directories which contain image files:

ocrd-import DIRECTORY

To get help for the import tool:

ocrd-import --help

Usage: ocrd-import [OPTIONS] [DIRECTORY]

with options:
 -i|--ignore      keep going after unknown file types
 -s|--skip SUFFIX ignore file names ending in given SUFFIX (repeatable)
 -R|--regex EXPR  only include paths matching given EXPR (repeatable)
 -C|--no-convert  do not attempt to convert image file types
 -r|--render DPI  when converting PDFs, render at DPI pixel density
 -P|--nonnum-ids  do not use numeric pageIds but basename patterns
 -B|--basename    only use basename for IDs

Create OCR-D workspace meta-data (mets.xml) in DIRECTORY (or /home/xbert/unsortiert/arbeit/heyer/tools/ocrd_tesserocr), importing...
* all image files (with known file extension or convertible via ImageMagick) under fileGrp OCR-D-IMG
* all .xml files (if they validate as PAGE-XML) under fileGrp OCR-D-SEG-PAGE
...but failing otherwise.

To perform various tasks via XSLT on PAGE-XML files (these all share the same options, including --help):

page-add-nsprefix-pc # adds namespace prefix 'pc:'
page-rm-nsprefix-pc # removes namespace prefix 'pc:'
page-set-nsversion-2019 # update the PAGE namespace schema version to 2019
page-fix-coords # replace negative values in coordinates by zero
page-flatten-tableregions # move recursive TableRegion cells to top level for editing in LAREX
page-unflatten-tableregions # move flattened TableRegion cells back to hierarchy after editing in LAREX
page-move-alternativeimage-below-page # try to push page-level AlternativeImage back to subsegments
page-remove-alternativeimages # remove $which [last] AlternativeImage entries at hierarchy $level [page]
page-remove-metadataitem # remove all MetadataItem entries
page-remove-dead-regionrefs # remove non-existing regionRefs
page-remove-empty-readingorder # remove empty ReadingOrder or groups
page-remove-all-regions # remove all *Region (and TextLine and Word and Glyph) entries
page-remove-text-regions # remove all TextRegion (and TextLine and Word and Glyph) entries
page-remove-regions # remove all *Region (and TextLine and Word and Glyph) entries of $type
page-remove-lines # remove all TextLine (and Word and Glyph) entries
page-remove-words # remove all Word (and Glyph) entries
page-remove-glyphs # remove all Glyph entries
page-remove-textequiv # remove all TextEquiv entries for selected levels and @index
page-rename-id-clashes # reassign new @id of segments that clash with other existing @id
page-ensure-readingorder # generate ReadingOrder hierarchy from recursive document order if empty
page-ensure-textequiv-conf # set TextEquiv/@conf attributes if missing
page-ensure-textequiv-index # set TextEquiv/@index attributes from element order
page-ensure-textequiv-unicode # create empty TextEquiv/Unicode elements if empty
page-sort-textequiv-index # sort TextEquiv by @index
page-textequiv-lines-to-regions # project text from TextLines to TextRegions (concat with LF in between)
page-textequiv-words-to-lines # project text from Words to TextLines (concat with spaces in between)
page-extract-text # extract TextEquiv/Unicode from TextRegion|TextLine|Word|Glyph $level [highest] consecutively, in $order [reading-order], interspersed by $pb and $lb
page-extract-lines # extract TextEquiv/Unicode from TextLine consecutively, in $order [reading-order]
page-extract-words # extract TextEquiv/Unicode from Word consecutively
page-extract-glyphs # extract TextEquiv/Unicode from Glyph consecutively

Usage: NAME [OPTIONS] [FILE]

with options:
 -s name=value    set param NAME to string literal VALUE (repeatable)
 -p name=value    set param NAME to XPath expression VALUE (repeatable)
 -i|--inplace     overwrite input file with result of transformation
 -P|--pretty      pretty-print output (line breaks with indentation)
 -d|--diff        show diff between input and output
 -D|--dump        just print the transformation stylesheet (XSL)
 -h|--help        just show this message

Open PAGE-XML file FILE (or stdin) and apply the XSL transformation "NAME.xsl"
Write the result to stdout, unless...
 -i / --inplace is given - in which case the result is written back to the
                           file silently, or
 -d / --diff is given - in which case the result will be compared to the
                        input and a patch shown on stdout.

To perform the same transformations, but as a workspace processor, use ocrd-page-transform and pass the filename of the transformation as parameter, e.g.:

ocrd-page-transform -P xsl page-extract-lines.xsl -P xslt-params "-s order=reading-order"
ocrd-page-transform -P xsl page-remove-alternativeimages.xsl -P xslt-params "-s level=line -s which=dewarped"
cat <<'EOF' > my-transform.xsl
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:pc="http://schema.primaresearch.org/PAGE/gts/pagecontent/2019-07-15">
  <xsl:output method="xml" standalone="yes" encoding="UTF-8" omit-xml-declaration="no"/>
  <xsl:template match="//pc:Word"/>
  <xsl:template match="node()|text()|@*">
    <xsl:copy>
      <xsl:apply-templates select="node()|text()|@*"/>
    </xsl:copy>
  </xsl:template>
</xsl:stylesheet>
EOF
ocrd-page-transform -P xsl my-transform.xsl

Usage: ocrd-page-transform [OPTIONS]

  apply arbitrary XSL transformation file for PAGE-XML

  > Processor base class and helper functions. A processor is a tool
  > that implements the uniform OCR-D command-line interface for run-
  > time data processing. That is, it executes a single workflow step,
  > or a combination of workflow steps, on the workspace (represented by
  > local METS). It reads input files for all or requested physical
  > pages of the input fileGrp(s), and writes output files for them into
  > the output fileGrp(s). It may take  a number of optional or
  > mandatory parameters. Process the :py:attr:`workspace`  from the
  > given :py:attr:`input_file_grp` to the given
  > :py:attr:`output_file_grp` for the given :py:attr:`page_id` under
  > the given :py:attr:`parameter`.

  > (This contains the main functionality and needs to be overridden by
  > subclasses.)

Options:
  -I, --input-file-grp USE        File group(s) used as input
  -O, --output-file-grp USE       File group(s) used as output
  -g, --page-id ID                Physical page ID(s) to process
  --overwrite                     Remove existing output pages/images
                                  (with --page-id, remove only those)
  --profile                       Enable profiling
  --profile-file                  Write cProfile stats to this file. Implies --profile
  -p, --parameter JSON-PATH       Parameters, either verbatim JSON string
                                  or JSON file path
  -P, --param-override KEY VAL    Override a single JSON object key-value pair,
                                  taking precedence over --parameter
  -m, --mets URL-PATH             URL or file path of METS to process
  -w, --working-dir PATH          Working directory of local workspace
  -l, --log-level [OFF|ERROR|WARN|INFO|DEBUG|TRACE]
                                  Log level
  -C, --show-resource RESNAME     Dump the content of processor resource RESNAME
  -L, --list-resources            List names of processor resources
  -J, --dump-json                 Dump tool description as JSON and exit
  -h, --help                      This help message
  -V, --version                   Show version

Parameters:
   "xsl" [string - REQUIRED]
    File path of the XSL transformation script
   "xslt-params" [string - ""]
    Assignment of XSL transformation parameter values, given as in
    `xmlstarlet` (which differentiates between `-s name=value` for
    literal `value` and `-p name=value` for XPath expression `value`),
    white-space separated.
   "pretty-print" [number - 0]
    Reformat with line breaks and this many spaces of indentation after
    XSL transformation (unless zero).
    "mimetype" [string - "application/vnd.prima.page+xml"]
    MIME type to register the output files under (should correspond to
    `xsl` result)

Besides the transformations for PAGE-XML above, which are wrapped both as OCR-D CLI ocrd-page-transform and standlone CLIs page-..., this module installs some XSL transformations for METS-XML, which are likewise wrapped as standalone CLIs mets-...:

mets-add-nsprefix-mets # add namespace prefix mets:
mets-alias-filegrp # zero-cost copy of fileGrp $input [FULLTEXT] as fileGrp $output [ALTO]
mets-copy-agents # copy all metsHdr/agent from $other-mets [mets.xml]

Usage: NAME [OPTIONS] [FILE]

with options:
 -s name=value    set param NAME to string literal VALUE (repeatable)
 -p name=value    set param NAME to XPath expression VALUE (repeatable)
 -i|--inplace     overwrite input file with result of transformation
 -P|--pretty      pretty-print output (line breaks with indentation)
 -d|--diff        show diff between input and output
 -D|--dump        just print the transformation stylesheet (XSL)
 -h|--help        just show this message

Open METS-XML file FILE (or stdin) and apply the XSL transformation "NAME.xsl"
Write the result to stdout, unless...
 -i / --inplace is given - in which case the result is written back to the
                           file silently, or
 -d / --diff is given - in which case the result will be compared to the
                        input and a patch shown on stdout.

Workflows are processed like software builds: File groups (depending on one another) are the targets to be built in each workspace, and all workspaces are built recursively. A build is finished when all targets exist and none are older than their respective prerequisites (e.g. image files).

To run a configuration...

1. Activate working environment (virtualenv) and change to the target directory.

2. Choose (or create) a workflow configuration makefile.
   (Yes, you can have to look inside and browse its rules!)

3. Execute:

    cd WORKSPACE && make [OPTIONS] -f WORKFLOW-CONFIG.mk # or
    make -C WORKSPACE [OPTIONS] -f WORKFLOW-CONFIG.mk
   

   ... for processing single workspace directory, or ...

    ocrd-make [OPTIONS] -f WORKFLOW-CONFIG.mk all
   

   (The special target all (which is also the default goal) will search for all workspaces in the current directory recursively.) You can also run on a subset of workspaces by passing these as goals on the command line...

    ocrd-make -f WORKFLOW-CONFIG.mk PATH/TO/WORKSPACE1 PATH/TO/WORKSPACE2 ...
   

Running OCR-D workflow configurations on multiple workspaces:

  Usage:
  ocrd-make [OPTIONS] [-f CONFIGURATION] [TARGETS] [VARIABLE-ASSIGNMENTS]

  Options (ocrd-specific):
  -X|--transfer HOST:DIR  run workflow on remote HOST in remote DIR
  --remote-init CMD  run CMD before the workflow on remote host

  Options (make-specific):
  -j|--jobs [N]   number of jobs to run simultaneously
  -l|--load-average|--max-load N  system load limit for -j without N
  -I|--include-dir DIR  extra search directory for included makefiles
  -C|--directory DIR  change to directory before reading makefiles

  Targets (general):
  * help (this message)
  * info (short self-description of the selected configuration)
  * show (print command sequence that would be executed for the selected configuration)
  * server (start workflow server for the selected configuration; control via 'ocrd workflow client')

  Targets (data processing):
  * all (recursively find all directories with a mets.xml, default goal)
  * % (name of the workspace directory, overriding the default goal)

  Variables:
  * LOGLEVEL: override global loglevel for all OCR-D processors
    (if unset, then default/configured logging levels apply)
  * PAGES: override page selection (comma-separated list)
    (if unset, then all pages will be processed)
  * TIMEOUT: per-processor timeout (in seconds or with unit suffix)
    (if unset, then processors may run forever)
  * FAILDUMMY: use ocrd-dummy (just copy -I to -O grp) on processing errors
    (if unset, then failed processors terminate the workflow)
  * METSSERV   start/use/stop METS Servers before/during/after workflows
    (if unset, the METS file will have to be de/serialised between each call)
  * PAGEWISE   call processors separately per page during workflows
    (if unset, processors are called on the whole document)

(This will merely delegate to `make` on the given working directories
from the installation directory "/data/ocr-d/ocrd_all/venv38/share/workflow-configuration".
All options except -C and -I are allowed and passed through.
Options -j and -l are intercepted.)
 

To get help:

[ocrd-]make help

To get a short description of the chosen configuration:

[ocrd-]make -f CONFIGURATION.mk info

To see the command sequence that would be executed for the chosen configuration (in the format of ocrd process):

[ocrd-]make -f CONFIGURATION.mk show

To run a workflow server for the command sequence that would be executed for the chosen configuration (to be controlled via ocrd workflow client or HTTP):

[ocrd-]make -f CONFIGURATION.mk server

To spawn a new configuration file, in the directory of the source repository, do:

ocrd-make NEW-CONFIGURATION.mk

Furthermore, you can add any options that make understands (see make --help or info make 'Options Summary'). For example,

• -n or --dry-run to just simulate the run
• -q or --question to just check whether anything needs to be built at all
• -s or --silent to suppress echoing recipes
• -j or --jobs to run on workspaces in parallel
• -l or --max-load to set the maximum load level in parallel mode
• -B or --always-make to consider all targets out-of-date (i.e. unconditionally rebuild)
• -o or --old-file to consider some target up-to-date w.r.t. its prerequisites (i.e. unconditionally keep) but older than its dependents (i.e. unconditionally ignore)
• -W or --new-file to consider some target newer than its dependents (i.e. unconditionally update them)

For example, to rebuild anything after the fileGrp OCR-D-BIN, do:

ocrd-make -f CONFIGURATION.mk -W OCR-D-BIN all

You can also use that pattern to specify any fileGrp other than the .DEFAULT_GOAL of your configuration as the overall target. For example, to build anything up to the fileGrp OCR-D-SEG-LINE, do:

ocrd-make -f CONFIGURATION.mk .DEFAULT_GOAL=OCR-D-SEG-LINE all

If you run make in the workspace directly instead of having ocrd-make do it recursively, then no all target exists and you can directly set the target fileGrp to replace .DEFAULT_GOAL:

make -C WORKSPACE -f CONFIGURATION.mk -W OCR-D-BIN
make -C WORKSPACE -f CONFIGURATION.mk OCR-D-SEG-LINE

There are 6 special variables and 1 additional option:

To override the default (or configured) log levels for all processors and libraries, use LOGLEVEL. For example, to get debugging everywhere, do:

ocrd-make -f CONFIGURATION.mk all LOGLEVEL=DEBUG
make -C WORKSPACE -f CONFIGURATION.mk LOGLEVEL=DEBUG

To process only a subset of pages in all fileGrps, set PAGES. For example, to only consider pages PHYS_0005 through PHYS_0007, do:

ocrd-make -f CONFIGURATION.mk all PAGES=PHYS_0005..PHYS_0007
make -C WORKSPACE -f CONFIGURATION.mk PAGES=PHYS_0005..PHYS_0007

The variable gets interpreted as the usual --page-id parameter by processors, so it supports range expressions, comma-separated lists and regular expressions.

If the METS provides physical page labels (@ORDER or @ORDERLABEL), then these work as well:

ocrd-make -f CONFIGURATION.mk all PAGES=5..7
make -C WORKSPACE -f CONFIGURATION.mk PAGES=5..7

To set an upper limit on the time each processor may take to run, use TIMEOUT. Set a numeric value in seconds, or post-fix with a temporal unit, as in timeout(1).

Beware that useful values may vary widely, depending on the processor and parameters (esp. whether GPUs are used), the input image size, and any PAGES setting.

In the case of PAGEWISE=1, this applies to single-page calls.

If FAILDUMMY=1, then timed out calls (as with any other cause of failure) will be caught be ocrd-dummy, which may take up additional time.

To handle errors gracefully, set FAILDUMMY=1. This will run a ocrd-dummy on the respective file groups and pages, which effectively copies the input to the output annotation (so subsequent steps can continue on these pages).

Without this, a failed step causes the workflow to stop for that target workspace, removing output pages already processed successfully (unless PAGEWISE=1).

To use METS Servers for each workspace, set METSSERV=1. On each workspace, this will

• start a METS Server in the background, using local Unix Domain Socket mets.sock, then
• run the workflow, having processors communicate via the socket, and finally
• stop the METS Server.

The METS Server avoids the cost of de/serialisation of the METS between processor calls, and thus increases efficiency. It also allows calling processors for pages independently (because the server synchronises METS updates, which the filesystem mets.xml cannot).

So a very useful combination is METSSERV=1 PAGEWISE=1. (In that combination, the top-level number of jobs, -j, and load-level, -l, will also be distributed to the page-wise calls; see below).

To run processors on each page individually, set PAGEWISE=1. For each workflow step that needs an update, this will call make recursively with PAGES set to each single page ID. (The top-level PAGES setting is still respected, i.e. it only splits up the requested pages.)

This is most useful in combination with FAILDUMMY=1 (for per-page error handling) and METSSERV=1 (for parallel distribution).

To run jobs on another machine (which has ocrd-make and the respecive OCR-D processors installed), transferring the workflow configuration file and workspace directories prior to execution, and the results afterwards, use -X or --transfer.

It takes as argument the remote host name and remote working directory, separated by a colon. In case the installation on the remote side needs initialization after login, use --remote-init followed by the respective command.

Example:

ocrd-make -j 4 --remote-init ". ~/.bash_profile" -X user@host.domain:/local -f CONFIGURATION.mk all

To write new configurations, first choose a (sufficiently descriptive) makefile name, and spawn a new file for that: make -C workflow-configuration NEW-CONFIGURATION.mk (or copy from an existing configuration).

Next, edit the file to your needs: Write rules using file groups as prerequisites/targets in the normal GNU make syntax. The first target defined must be the default goal that builds the very last file group for that configuration, or else a variable .DEFAULT_GOAL pointing to that target must be set anywhere in the makefile.

• Keep the comments and the include Makefile directive in the file.

• Change/customize at least the info target, and the INPUT and OUTPUT name/rule.

• Copy/paste rules from the existing configurations.

• Define variables with the names of all target/prerequisite file groups, so rules and dependent targets can re-use them (and the names can be easily changed later).

• Try to utilise the provided static pattern rule (which takes the target as output file group and the prerequisite as input file group) for all processing steps. The rule covers any OCR-D compliant processor with no more than 1 output file group. Use it by simply defining the target-specific variable TOOL (and optionally PARAMS or OPTIONS) and giving no recipe whatsoever.

• When any of your processors use GPU resources, you must prevent races for GPU memory during parallel execution.

  You can achieve this by simply setting GPU = 1 for that target when using the static pattern rule, or by using sem --id OCR-D-GPUSEM when writing your own recipes.

  Alternatively, you can either prevent using GPUs globally by (un)setting CUDA_VISIBLE_DEVICES=, or prevent running parallel jobs (on multiple CPUs) by passing -j.

INPUT = OCR-D-GT-SEG-LINE

$(INPUT):
	ocrd workspace find -G $@ --download
	ocrd workspace find -G OCR-D-IMG --download # just in case

# You can use variables for file group names to keep the rules brief:
BIN = $(INPUT)-BINPAGE

# This is how you use the pattern rule from Makefile (included below):
# The prerequisite will become the input file group,
# the target will become the output file group,
# the recipe will call the executable given by TOOL,
# also generating a JSON parameter file from PARAMS:
$(BIN): $(INPUT)
$(BIN): TOOL = ocrd-olena-binarize
$(BIN): PARAMS = "impl": "sauvola-ms-split"
# or equivalently:
$(BIN): OPTIONS = -P impl sauvola-ms-split

# You can also use the file group names directly:
OCR-D-OCR-TESS: $(BIN)
OCR-D-OCR-TESS: TOOL = ocrd-tesserocr-recognize
OCR-D-OCR-TESS: PARAMS = "textequiv_level": "glyph", "model": "frk+deu"
# or equivalently:
OCR-D-OCR-TESS: OPTIONS = -P textequiv_level glyph -P model frk+deu

# This uses more than 1 input file group and no output file group,
# which works with the standard recipe as well (but mind the ordering):
EVAL: $(INPUT) OCR-D-OCR-TESS
EVAL: TOOL = ocrd-cor-asv-ann-evaluate

# Because the first target in this file was $(BIN),
# we must override the default goal to be our desired overall target:
.DEFAULT_GOAL = EVAL

# ALWAYS necessary:
include Makefile

To run ocrd-import and ocrd-make (in various modes) on sample data, in the installation directory do:

make test

This is also used by the CI.

🚧 these results are no longer meaningful and should be updated!

For the data_structure_text/dta repository, which includes both layout and text annotation down to the textline level, but very coarse segmentation, the following character error rate (CER) was measured:

Hence, it appears that consistently (across different OCRs) ...

• denoising with Ocropy (with noise_maxsize=3.0) does not help
• deskewing with Ocropy on the page level usually helps
• additional deskewing and flipping with Tesseract on the region level usually deteriorates
• binarization with sauvola-ms-split is better than wolf

However, this result is still preliminary. Both the processor implementations evolve and the GT annotations get fixed over time.

To make writing (and reading) configurations as simple as possible, they are expressed as rules operating on METS file groups (i.e. workspace-local). For convenience, the most common recipe pattern involving only 1 input and 1 output file group via some OCR-D CLI is available via static pattern rule, which merely takes the target-specific variables TOOL (the CLI executable) and optionally PARAMS (a JSON-formatted list of parameter assignments) or OPTIONS (a white-space separated list of parameter assignments). Custom rules are possible as well. If the makefile does not start with the overall target, it must specify its .DEFAULT_GOAL, so callers can run without knowledge of the target names.

Rules that are not configuration-specific (like the static pattern rule) are all shared by including a common Makefile at the end of configuration makefiles (which gets copied from workflow.mk at install time).

make always operates on the level of the workspace directory (i.e. only one at a time), where targets are fileGrps and the default goal is the maximum fileGrp.

For running entire collections of workspaces (possibly in parallel), recursive make has been abandoned in favour of the parallel-based bash script ocrd-make. Its command-line interface looks like make, but the targets are workspaces and the default goal is all (which recursively finds all workspaces).

🚧 we should explain the use of GNU parallel here.

When executing workflows in parallel across workspaces (with --jobs) on multiple CPUs, it must be ensured that not too many OCR-D processors which use GPU resources are running concurrently (to prevent over-allocation of GPU memory). Thus, make needs to know:

1. which processors (have/want to) use GPU resources, and
2. how many such processors can run in parallel.

It can then synchronize these processors with a semaphore. This is achieved by expanding the static pattern rule with a synchronisation mechanism (based on GNU parallel). Workflow configurations can use that by setting the target-specific variable GPU to a non-empty value for the respective rules. (Custom recipes will have to use sem --id OCR-D-GPUSEM.)

That way, races are prevented, but also GPUs cannot become the bottleneck: When all GPUs are busy, processors will fall back to CPU.

When executing workflows in parallel across workspaces (with --jobs) on multiple CPUs, it must be ensured that OCR-D processors do not use local multiprocessing facilities themselves (to prevent over-allocation of CPUs).

In the current state of affairs, OCR-D processors cannot be run in parallel across pages via multiprocessing. (At least, they are never implemented that way.) That may change in the future with a new OCR-D API. But still, many processors do already use libraries like OpenMP or OpenBLAS which use multiprocessing locally within pages. This can be controlled via environment variables like OMP_THREAD_LIMIT.

This is achieved by exporting these variables to all recipes with a value of 1 when -j is used, or half the number of physical CPUs (unless NTHREADS is explicitly given) otherwise.