Begin transforming Tesseract into pluggable OCR engine

src/ocrmypdf/_pipeline.py

@@ -379,11 +379,8 @@ def get_orientation_correction(preview, page_context):
 
     """
 
-    orient_conf = tesseract.get_orientation(
-        preview,
-        engine_mode=page_context.options.tesseract_oem,
-        timeout=page_context.options.tesseract_timeout,
-        tesseract_env=page_context.options.tesseract_env,
+    orient_conf = page_context.plugin_manager.hook.get_ocr_engine().get_orientation(
+        preview, page_context.options
     )
 
     correction = orient_conf.angle % 360
@@ -531,22 +528,17 @@ def create_ocr_image(image, page_context):
     return output_file
 
 
-def ocr_tesseract_hocr(input_file, page_context):
+def ocr_engine_hocr(input_file, page_context):
     hocr_out = page_context.get_path('ocr_hocr.hocr')
     hocr_text_out = page_context.get_path('ocr_hocr.txt')
     options = page_context.options
-    tesseract.generate_hocr(
+
+    ocr_engine = page_context.plugin_manager.hook.get_ocr_engine()
+    ocr_engine.generate_hocr(
         input_file=input_file,
         output_hocr=hocr_out,
         output_text=hocr_text_out,
-        languages=options.language,
-        engine_mode=options.tesseract_oem,
-        tessconfig=options.tesseract_config,
-        timeout=options.tesseract_timeout,
-        pagesegmode=options.tesseract_pagesegmode,
-        user_words=options.user_words,
-        user_patterns=options.user_patterns,
-        tesseract_env=options.tesseract_env,
+        options=options,
     )
     return (hocr_out, hocr_text_out)
 
@@ -610,22 +602,17 @@ def render_hocr_page(hocr, page_context):
     return output_file
 
 
-def ocr_tesseract_textonly_pdf(input_image, page_context):
+def ocr_engine_textonly_pdf(input_image, page_context):
     output_pdf = page_context.get_path('ocr_tess.pdf')
     output_text = page_context.get_path('ocr_tess.txt')
     options = page_context.options
-    tesseract.generate_pdf(
+
+    ocr_engine = page_context.plugin_manager.hook.get_ocr_engine()
+    ocr_engine.generate_pdf(
         input_file=input_image,
         output_pdf=output_pdf,
         output_text=output_text,
-        languages=options.language,
-        engine_mode=options.tesseract_oem,
-        tessconfig=options.tesseract_config,
-        timeout=options.tesseract_timeout,
-        pagesegmode=options.tesseract_pagesegmode,
-        user_words=options.user_words,
-        user_patterns=options.user_patterns,
-        tesseract_env=options.tesseract_env,
+        options=options,
     )
     return (output_pdf, output_text)
 

src/ocrmypdf/_plugin_manager.py

@@ -26,9 +26,11 @@ import pluggy
 from ocrmypdf import pluginspec
 
 
-def get_plugin_manager(plugins: List[str]):
+def get_plugin_manager(plugins: List[str], builtins=True):
     pm = pluggy.PluginManager('ocrmypdf')
     pm.add_hookspecs(pluginspec)
+    if builtins:
+        plugins.insert(0, 'ocrmypdf.builtin_plugins')
     for name in plugins:
         if name.endswith('.py'):
             # Import by filename

src/ocrmypdf/_sync.py

@@ -42,8 +42,8 @@ from ocrmypdf._pipeline import (
     is_ocr_required,
     merge_sidecars,
     metadata_fixup,
-    ocr_tesseract_hocr,
-    ocr_tesseract_textonly_pdf,
+    ocr_engine_hocr,
+    ocr_engine_textonly_pdf,
     optimize_pdf,
     preprocess_clean,
     preprocess_deskew,
@@ -176,13 +176,11 @@ def exec_page_sync(page_context):
             )
 
         if options.pdf_renderer == 'hocr':
-            (hocr_out, text_out) = ocr_tesseract_hocr(ocr_image_out, page_context)
+            (hocr_out, text_out) = ocr_engine_hocr(ocr_image_out, page_context)
             ocr_out = render_hocr_page(hocr_out, page_context)
 
         if options.pdf_renderer == 'sandwich':
-            (ocr_out, text_out) = ocr_tesseract_textonly_pdf(
-                ocr_image_out, page_context
-            )
+            (ocr_out, text_out) = ocr_engine_textonly_pdf(ocr_image_out, page_context)
 
     return PageResult(
         pageno=page_context.pageno,

src/ocrmypdf/builtin_plugins/__init__.py

@@ -0,0 +1,18 @@
+# © 2020 James R. Barlow: github.com/jbarlow83
+#
+# This file is part of OCRmyPDF.
+#
+# OCRmyPDF is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# OCRmyPDF is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with OCRmyPDF.  If not, see <http://www.gnu.org/licenses/>.
+
+from ocrmypdf.builtin_plugins.tesseract_ocr import *

src/ocrmypdf/builtin_plugins/tesseract_ocr.py

@@ -0,0 +1,68 @@
+# © 2020 James R. Barlow: github.com/jbarlow83
+#
+# This file is part of OCRmyPDF.
+#
+# OCRmyPDF is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# OCRmyPDF is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with OCRmyPDF.  If not, see <http://www.gnu.org/licenses/>.
+
+from ocrmypdf import hookimpl
+from ocrmypdf.exec import tesseract
+from ocrmypdf.pluginspec import OcrEngine
+
+
+class TesseractOcrEngine(OcrEngine):
+    def languages(self):
+        return tesseract.get_languages()
+
+    def get_orientation(self, input_file, options):
+        return tesseract.get_orientation(
+            input_file,
+            engine_mode=options.tesseract_oem,
+            timeout=options.tesseract_timeout,
+            tesseract_env=options.tesseract_env,
+        )
+
+    def generate_hocr(self, input_file, output_hocr, output_text, options):
+        tesseract.generate_hocr(
+            input_file=input_file,
+            output_hocr=output_hocr,
+            output_text=output_text,
+            languages=options.language,
+            engine_mode=options.tesseract_oem,
+            tessconfig=options.tesseract_config,
+            timeout=options.tesseract_timeout,
+            pagesegmode=options.tesseract_pagesegmode,
+            user_words=options.user_words,
+            user_patterns=options.user_patterns,
+            tesseract_env=options.tesseract_env,
+        )
+
+    def generate_pdf(self, input_file, output_pdf, output_text, options):
+        tesseract.generate_pdf(
+            input_file=input_file,
+            output_pdf=output_pdf,
+            output_text=output_text,
+            languages=options.language,
+            engine_mode=options.tesseract_oem,
+            tessconfig=options.tesseract_config,
+            timeout=options.tesseract_timeout,
+            pagesegmode=options.tesseract_pagesegmode,
+            user_words=options.user_words,
+            user_patterns=options.user_patterns,
+            tesseract_env=options.tesseract_env,
+        )
+
+
+@hookimpl
+def get_ocr_engine():
+    return TesseractOcrEngine()

src/ocrmypdf/pluginspec.py

@@ -15,9 +15,11 @@
 # You should have received a copy of the GNU General Public License
 # along with OCRmyPDF.  If not, see <http://www.gnu.org/licenses/>.
 
+from abc import ABC, abstractmethod
 from argparse import ArgumentParser, Namespace
+from collections import namedtuple
 from pathlib import Path
-from typing import Optional
+from typing import AbstractSet, Optional
 
 import pluggy
 from PIL import Image
@@ -87,3 +89,35 @@ def filter_page_image(page: 'PageContext', image_filename: Path) -> Path:
     Note that the ocrmypdf image optimization stage may ultimately chose a
     different format.
     """
+
+
+OrientationConfidence = namedtuple('OrientationConfidence', ('angle', 'confidence'))
+
+
+class OcrEngine(ABC):
+    @abstractmethod
+    def languages(self) -> AbstractSet[str]:
+        """Returns set of languages that are supported."""
+
+    @abstractmethod
+    def get_orientation(
+        self, input_file: Path, options: Namespace
+    ) -> OrientationConfidence:
+        """Returns the orientation of the image."""
+
+    @abstractmethod
+    def generate_hocr(
+        self, input_file: Path, output_hocr: Path, output_text: Path, options: Namespace
+    ) -> None:
+        pass
+
+    @abstractmethod
+    def generate_pdf(
+        self, input_file: Path, output_pdf: Path, output_text: Path, options: Namespace
+    ) -> None:
+        pass
+
+
+@hookspec(firstresult=True)
+def get_ocr_engine() -> OcrEngine:
+    pass