feat: Introduce automatic language detection in TesseractOcrCliModel (#800)

* feat: Introduce automatic language detection in tesseract_ocr_cli model. Extend unit tests.

Signed-off-by: Nikos Livathinos <nli@zurich.ibm.com>

* docs: Add example how to use "auto" language with tesseract OCR engines

Signed-off-by: Nikos Livathinos <nli@zurich.ibm.com>

* fix: Refactor the TesseractOcrModel and TesseractOcrCliModel to validate if the auto-detected
language is installed in the system and if not fall back to a default option without language.

Signed-off-by: Nikos Livathinos <nli@zurich.ibm.com>

---------

Signed-off-by: Nikos Livathinos <nli@zurich.ibm.com>

docling/models/tesseract_ocr_cli_model.py

@@ -4,7 +4,7 @@ import logging
 import os
 import tempfile
 from subprocess import DEVNULL, PIPE, Popen
-from typing import Iterable, Optional, Tuple
+from typing import Iterable, List, Optional, Tuple
 
 import pandas as pd
 from docling_core.types.doc import BoundingBox, CoordOrigin
@@ -14,6 +14,7 @@ from docling.datamodel.document import ConversionResult
 from docling.datamodel.pipeline_options import TesseractCliOcrOptions
 from docling.datamodel.settings import settings
 from docling.models.base_ocr_model import BaseOcrModel
+from docling.utils.ocr_utils import map_tesseract_script
 from docling.utils.profiling import TimeRecorder
 
 _log = logging.getLogger(__name__)
@@ -28,10 +29,13 @@ class TesseractOcrCliModel(BaseOcrModel):
 
         self._name: Optional[str] = None
         self._version: Optional[str] = None
+        self._tesseract_languages: Optional[List[str]] = None
+        self._script_prefix: Optional[str] = None
 
         if self.enabled:
             try:
                 self._get_name_and_version()
+                self._set_languages_and_prefix()
 
             except Exception as exc:
                 raise RuntimeError(
@@ -73,12 +77,20 @@ class TesseractOcrCliModel(BaseOcrModel):
         return name, version
 
     def _run_tesseract(self, ifilename: str):
-
+        r"""
+        Run tesseract CLI
+        """
         cmd = [self.options.tesseract_cmd]
 
-        if self.options.lang is not None and len(self.options.lang) > 0:
+        if "auto" in self.options.lang:
+            lang = self._detect_language(ifilename)
+            if lang is not None:
+                cmd.append("-l")
+                cmd.append(lang)
+        elif self.options.lang is not None and len(self.options.lang) > 0:
             cmd.append("-l")
             cmd.append("+".join(self.options.lang))
+
         if self.options.path is not None:
             cmd.append("--tessdata-dir")
             cmd.append(self.options.path)
@@ -106,6 +118,63 @@ class TesseractOcrCliModel(BaseOcrModel):
 
         return df_filtered
 
+    def _detect_language(self, ifilename: str):
+        r"""
+        Run tesseract in PSM 0 mode to detect the language
+        """
+        assert self._tesseract_languages is not None
+
+        cmd = [self.options.tesseract_cmd]
+        cmd.extend(["--psm", "0", "-l", "osd", ifilename, "stdout"])
+        _log.info("command: {}".format(" ".join(cmd)))
+        proc = Popen(cmd, stdout=PIPE, stderr=DEVNULL)
+        output, _ = proc.communicate()
+        decoded_data = output.decode("utf-8")
+        df = pd.read_csv(
+            io.StringIO(decoded_data), sep=":", header=None, names=["key", "value"]
+        )
+        scripts = df.loc[df["key"] == "Script"].value.tolist()
+        if len(scripts) == 0:
+            _log.warning("Tesseract cannot detect the script of the page")
+            return None
+
+        script = map_tesseract_script(scripts[0].strip())
+        lang = f"{self._script_prefix}{script}"
+
+        # Check if the detected language has been installed
+        if lang not in self._tesseract_languages:
+            msg = f"Tesseract detected the script '{script}' and language '{lang}'."
+            msg += " However this language is not installed in your system and will be ignored."
+            _log.warning(msg)
+            return None
+
+        _log.debug(
+            f"Using tesseract model for the detected script '{script}' and language '{lang}'"
+        )
+        return lang
+
+    def _set_languages_and_prefix(self):
+        r"""
+        Read and set the languages installed in tesseract and decide the script prefix
+        """
+        # Get all languages
+        cmd = [self.options.tesseract_cmd]
+        cmd.append("--list-langs")
+        _log.info("command: {}".format(" ".join(cmd)))
+        proc = Popen(cmd, stdout=PIPE, stderr=DEVNULL)
+        output, _ = proc.communicate()
+        decoded_data = output.decode("utf-8")
+        df = pd.read_csv(io.StringIO(decoded_data), header=None)
+        self._tesseract_languages = df[0].tolist()[1:]
+
+        # Decide the script prefix
+        if any([l.startswith("script/") for l in self._tesseract_languages]):
+            script_prefix = "script/"
+        else:
+            script_prefix = ""
+
+        self._script_prefix = script_prefix
+
     def __call__(
         self, conv_res: ConversionResult, page_batch: Iterable[Page]
     ) -> Iterable[Page]:
@@ -120,7 +189,6 @@ class TesseractOcrCliModel(BaseOcrModel):
                 yield page
             else:
                 with TimeRecorder(conv_res, "ocr"):
-
                     ocr_rects = self.get_ocr_rects(page)
 
                     all_ocr_cells = []