Add rudimentary support for combining OCR layer with existing content

It appears to be very fragile due to weaknesses in PyPDF. Better
option is probably to use pdftk's watermark feature.

src/ocrpage.py

@@ -6,6 +6,8 @@ import os.path
 import fileinput
 import re
 from parse import parse
+import PyPDF2 as pypdf
+import shutil
 
 from subprocess import Popen, check_call, PIPE, CalledProcessError, \
     TimeoutExpired
@@ -73,6 +75,9 @@ parser.add_argument(
 parser.add_argument(
     'skip_big', type=int,
     help="Skip OCR for pages that are very large")
+parser.add_argument(
+    'exact_image', type=int,
+    help="Use original page from PDF without re-rendering")
 parser.add_argument(
     'tess_cfg_files', default='', nargs='*',    # Implemented
     help="Tesseract configuration")
@@ -225,11 +230,11 @@ def setup_working_directory(input_file, soft_link_name):
         pass
 
 
-@active_if(not ocr_required)
+@active_if(not ocr_required or (ocr_required and options.exact_image))
 @transform(setup_working_directory,
            formatter(),
-           os.path.join(options.tmp_fld, '%04i.skip.pdf' % pageno))
-def skip_ocr(
+           os.path.join(options.tmp_fld, '%04i.page.pdf' % pageno))
+def extract_single_page(
         input_file,
         output_file):
     args_pdfseparate = [
@@ -526,27 +531,25 @@ def ocr_tesseract(
             raise CalledProcessError(p.returncode, args_tesseract)
 
         if os.path.exists(output_file + '.html'):
-            # Tesseract 3.02 appends suffix ".html" on its own
-            re_symlink(output_file + ".html", output_file,
-                       logger, logger_mutex)
+            # Tesseract 3.02 appends suffix ".html" on its own (.hocr.html)
+            shutil.move(output_file + '.html', output_file)
         elif os.path.exists(output_file + '.hocr'):
-            # Tesseract 3.03 appends suffix ".hocr" on its own
-            re_symlink(output_file + ".hocr", output_file,
-                       logger, logger_mutex)
+            # Tesseract 3.03 appends suffix ".hocr" on its own (.hocr.hocr)
+            shutil.move(output_file + '.hocr', output_file)
 
-            # The filename gets inserted to hocr
-            # but Tesseract does not verify that it is escaped XML
-            # it's not necessary so strip it out
-            regex_nested_single_quotes = re.compile(
-                r"""title='image "([^"]*)";""")
-            with fileinput.input(files=(output_file,), inplace=True) as f:
-                for line in f:
-                    line = regex_nested_single_quotes.sub(
-                        r"""title='image " ";""", line)
-                    print(line, end='')  # stdout is redirected here
+        # Tesseract inserts source filename into hocr file without escaping
+        # it. This could break the XML parser. Rewrite the hocr file,
+        # replacing the filename with a space.
+        regex_nested_single_quotes = re.compile(
+            r"""title='image "([^"]*)";""")
+        with fileinput.input(files=(output_file,), inplace=True) as f:
+            for line in f:
+                line = regex_nested_single_quotes.sub(
+                    r"""title='image " ";""", line)
+                print(line, end='')  # fileinput.input redirects stdout
 
 
-@active_if(ocr_required)
+@active_if(ocr_required and not options.exact_image)
 @merge([unpack_with_ghostscript, convert_to_png,
         deskew_imagemagick, deskew_leptonica, cleaned_to_png],
        os.path.join(options.tmp_fld, "%04i.image_for_pdf" % pageno))
@@ -567,7 +570,7 @@ def select_image_for_pdf(infiles, output_file):
         re_symlink(input_file, output_file, logger, logger_mutex)
 
 
-@active_if(ocr_required)
+@active_if(ocr_required and not options.exact_image)
 @merge([ocr_tesseract, select_image_for_pdf],
        os.path.join(options.tmp_fld, '%04i.rendered.pdf' % pageno))
 def render_page(infiles, output_file):
@@ -590,7 +593,34 @@ def render_text_output_page(input_file, output_file):
                          showBoundingboxes=True, invisibleText=False)
 
 
-@merge([render_page, skip_ocr],
+@active_if(ocr_required and options.exact_image)
+@transform(ocr_tesseract, suffix(".hocr"), ".hocr.pdf")
+def render_hocr_blank_page(input_file, output_file):
+    dpi = round(max(pageinfo['xres'], pageinfo['yres']))
+
+    hocrtransform = HocrTransform(input_file, dpi)
+    hocrtransform.to_pdf(output_file, imageFileName=None,
+                         showBoundingboxes=False, invisibleText=True)
+
+
+@active_if(ocr_required and options.exact_image)
+@merge([render_hocr_blank_page, extract_single_page],
+       os.path.join(options.tmp_fld, "%04i.merged.pdf") % pageno)
+def merge_hocr_with_original_page(infiles, output_file):
+    with open(infiles[0], 'rb') as hocr_input, \
+            open(infiles[1], 'rb') as page_input, \
+            open(output_file, 'wb') as output:
+        hocr_reader = pypdf.PdfFileReader(hocr_input)
+        page_reader = pypdf.PdfFileReader(page_input)
+        writer = pypdf.PdfFileWriter()
+
+        the_page = hocr_reader.getPage(0)
+        the_page.mergePage(page_reader.getPage(0))
+        writer.addPage(the_page)
+        writer.write(output)
+
+
+@merge([render_page, merge_hocr_with_original_page, extract_single_page],
        os.path.join(options.tmp_fld, '%04i.ocred.pdf' % pageno))
 def select_final_page(infiles, output_file):
     re_symlink(infiles[-1], output_file, logger, logger_mutex)
@@ -600,7 +630,7 @@ cmdline.run(options)
 
 # parser.add_argument(
 #     'tess_cfg_files',
-#     help="Specific configuration files to be used by Tesseract during OCRing")
+#   help="Specific configuration files to be used by Tesseract during OCRing")
 
 
 def main():