added image DPI detection to the tesseract parser.

docs/configuration.rst

@@ -250,6 +250,19 @@ PAPERLESS_OCR_OUTPUT_TYPE=<type>
     If not specified, ``pdfa`` is used. Remember that paperless also keeps
     the original input file as well as the archived version.
 
+PAPERLESS_OCR_IMAGE_DPI=<num>
+    Paperless will OCR any images you put into the system and convert them
+    into PDF documents. This is useful if your scanner produces images.
+    In order to do so, paperless needs to know the DPI of the image.
+    Most images from scanners will have this information embedded and
+    paperless will detect and use that information. In case this fails, it
+    uses this value as a fallback.
+
+    Set this to the DPI your scanner produces images at.
+
+    Default is none, which causes paperless to fail if no DPI information is
+    present in an image.
+
 PAPERLESS_CONSUMER_POLLING=<num>
     If paperless won't find documents added to your consume folder, it might
     not be able to automatically detect filesystem changes. In that case,

paperless.conf.example

@@ -40,6 +40,7 @@
 #PAPERLESS_OCR_LANGUAGE=eng
 #PAPERLESS_OCR_OUTPUT_TYPE=pdfa
 #PAPERLESS_OCR_MODE=skip
+#PAPERLESS_OCR_IMAGE_DPI=300
 #PAPERLESS_CONSUMER_POLLING=10
 #PAPERLESS_CONSUMER_DELETE_DUPLICATES=false
 #PAPERLESS_CONVERT_MEMORY_LIMIT=0

src/paperless/settings.py

@@ -346,6 +346,8 @@ OCR_OUTPUT_TYPE = os.getenv("PAPERLESS_OCR_OUTPUT_TYPE", "pdfa")
 # TODO: validate this.
 OCR_MODE = os.getenv("PAPERLESS_OCR_MODE", "skip")
 
+OCR_IMAGE_DPI = os.getenv("PAPERLESS_OCR_IMAGE_DPI")
+
 # GNUPG needs a home directory for some reason
 GNUPG_HOME = os.getenv("HOME", "/tmp")
 

src/paperless_tesseract/parsers.py

@@ -4,6 +4,7 @@ import subprocess
 
 import ocrmypdf
 import pdftotext
+from PIL import Image
 from django.conf import settings
 from ocrmypdf import InputFileError
 
@@ -60,10 +61,22 @@ class RasterisedDocumentParser(DocumentParser):
 
         return out_path
 
-    def get_text(self):
+    def is_image(self, mime_type):
+        return mime_type in [
+            "image/png",
+            "image/jpeg"
+        ]
 
-        if self._text:
+    def get_dpi(self, image):
-            return self._text
+        try:
+            with Image.open(image) as im:
+                x, y = im.info['dpi']
+                return x
+        except Exception as e:
+            self.log(
+                'warning',
+                f"Error while getting DPI from image {image}: {e}")
+            return None
 
     def parse(self, document_path, mime_type):
         archive_path = os.path.join(self.tempdir, "archive.pdf")
@@ -89,6 +102,22 @@ class RasterisedDocumentParser(DocumentParser):
         elif settings.OCR_MODE == 'force':
             ocr_args['force_ocr'] = True
 
+        if self.is_image(mime_type):
+            dpi = self.get_dpi(document_path)
+            if dpi:
+                self.log(
+                    "debug",
+                    f"Detected DPI for image {document_path}: {dpi}"
+                )
+                ocr_args['image_dpi'] = dpi
+            elif settings.OCR_IMAGE_DPI:
+                ocr_args['image_dpi'] = settings.OCR_IMAGE_DPI
+            else:
+                raise ParseError(
+                    f"Cannot produce archive PDF for image {document_path}, "
+                    f"no DPI information is present in this image and "
+                    f"OCR_IMAGE_DPI is not set.")
+
         try:
             ocrmypdf.ocr(**ocr_args)
             # success! announce results