[Enhancement]: enable parallel calls to the tesseract CLI

.gitignore

@@ -3,3 +3,5 @@
 build/
 dist/
 pyocr.egg-info/
+venv
+.idea

src/pyocr/tesseract.py

@@ -33,6 +33,9 @@
 
 # CHANGE THIS IF TESSERACT IS NOT IN YOUR PATH, OR IS NAMED DIFFERENTLY
 TESSERACT_CMD = 'tesseract.exe' if os.name == 'nt' else 'tesseract'
+# Specifying the OMP_THREAD_LIMIT=1 parameter is the only way to make the calls to tesseract play nice with multiprocessing
+# For more info see: https://github.com/tesseract-ocr/tesseract/issues/898
+THREAD_LIMIT_CMD = 'OMP_THREAD_LIMIT=1'
 
 TESSDATA_EXTENSION = ".traineddata"
 
@@ -230,7 +233,7 @@ def get_available_builders():
 
 
 def run_tesseract(input_filename, output_filename_base, cwd=None, lang=None,
-                  flags=None, configs=None):
+                  flags=None, configs=None, parallel=True):
     '''
     Runs Tesseract:
         `TESSERACT_CMD` \
@@ -254,7 +257,12 @@ def run_tesseract(input_filename, output_filename_base, cwd=None, lang=None,
     '''
     _set_environment()
 
-    command = [TESSERACT_CMD, input_filename, output_filename_base]
+    tesseract_cmd = TESSERACT_CMD
+
+    if parallel:
+        tesseract_cmd = " ".join([THREAD_LIMIT_CMD, TESSERACT_CMD])
+
+    command = [tesseract_cmd, input_filename, output_filename_base]
 
     if lang is not None:
         command += ['-l', lang]
@@ -325,7 +333,7 @@ def temp_dir():
         shutil.rmtree(path)
 
 
-def image_to_string(image, lang=None, builder=None):
+def image_to_string(image, lang=None, builder=None, parallel=True):
     '''
     Runs tesseract on the specified image. First, the image is written to disk,
     and then the tesseract command is run on the image. Tesseract's result is
@@ -353,7 +361,8 @@ def image_to_string(image, lang=None, builder=None):
         (status, errors) = run_tesseract("input.bmp", "output", cwd=tmpdir,
                                          lang=lang,
                                          flags=builder.tesseract_flags,
-                                         configs=builder.tesseract_configs)
+                                         configs=builder.tesseract_configs,
+                                         parallel=parallel)
         if status:
             raise TesseractError(status, errors)