Redo OCR: disallow in cases that will damage the output PDF

src/ocrmypdf/_pipeline.py

@@ -251,9 +251,18 @@ def is_ocr_required(pageinfo, log, options):
             log.info(msg.format(page,
                                 "rasterizing text and running OCR anyway"))
             ocr_required = True
-        elif options.redo_ocr and pageinfo.only_ocr_text:
-            log.info(msg.format(page,
-                                "redoing OCR"))
+        elif options.redo_ocr:
+            if pageinfo.only_ocr_text:
+                log.info(msg.format(page,
+                                    "redoing OCR"))
+            else:
+                log.error(
+                    ("%4d: page has both printable and hidden text, so "
+                     "--redo-ocr is not currently possible for this file. "
+                     "Try --force-ocr."),
+                    page
+                )
+                raise PriorOcrFoundError()
             ocr_required = True
         elif options.skip_text:
             log.info(msg.format(page,
@@ -590,7 +599,7 @@ def select_ocr_image(
                          Decimal(bbox[1]) / Decimal(72) * yres,
                          Decimal(bbox[2]) / Decimal(72) * xres,
                          Decimal(bbox[3]) / Decimal(72) * yres]
-            pixcoords = [int(c) for c in pixcoords]
+            pixcoords = [int(round(c)) for c in pixcoords]
             log.debug('blanking %r', pixcoords)
             draw.rectangle(pixcoords, fill=white)
             #draw.rectangle(pixcoords, outline=pink)

src/ocrmypdf/_weave.py

@@ -43,20 +43,29 @@ def _update_page_resources(*, page, font, font_key, procset):
     resources['/ProcSet'] = procset
 
 
-def _strip_old_text(pdf, page):
+def _strip_old_invisible_text(pdf, page, log):
     stream = []
     in_text_obj = False
+    render_mode = 0
+    text_objects = []
 
     page.page_contents_coalesce()
     for operands, operator in pikepdf.parse_content_stream(page, ''):
         if not in_text_obj:
             if operator == pikepdf.Operator('BT'):
                 in_text_obj = True
+                render_mode = 0
             else:
                 stream.append((operands, operator))
         else:
+            if operator == pikepdf.Operator('Tr'):
+                render_mode = operands[0]
+            text_objects.append((operands, operator))
             if operator == pikepdf.Operator('ET'):
                 in_text_obj = False
+                if render_mode != 3:
+                    stream.extend(text_objects)
+                text_objects.clear()
 
     def convert(op):
         try:
@@ -141,7 +150,7 @@ def _weave_layers_graft(
     new_text_layer = pikepdf.Stream(pdf_base, pdf_text_contents)
 
     if strip_old_text:
-        _strip_old_text(pdf_base, base_page)
+        _strip_old_invisible_text(pdf_base, base_page, log)
 
     base_page.page_contents_add(new_text_layer, prepend=True)
 
@@ -370,6 +379,7 @@ def weave_layers(
 
         if text and font:
             # Graft the text layer onto this page, whether new or old
+#            strip_old = context.get_options().redo_ocr
             strip_old = (context.get_options().redo_ocr
                          and pdfinfo[page_num - 1].only_ocr_text)
             _weave_layers_graft(