Refactor DPI: fix regressions in test suite

Some called functions are particular about the data format of DPI and
don't like to deal with the Decimal() returned by PyPDF2. Convert to
float and int where needed.

ocrmypdf/main.py

@@ -42,6 +42,8 @@ warnings.simplefilter('ignore', pypdf.utils.PdfReadWarning)
 
 BASEDIR = os.path.dirname(os.path.realpath(__file__))
 
+VECTOR_PAGE_DPI = 400
+
 
 # -------------
 # External dependencies
@@ -409,24 +411,46 @@ def get_pageinfo(input_file, pdfinfo, pdfinfo_lock):
     return pageinfo
 
 
+def get_page_dpi(pageinfo):
+    "Get the DPI when nonsquare DPI is tolerable"
+    xres = max(pageinfo.get('xres', VECTOR_PAGE_DPI), options.oversample or 0)
+    yres = max(pageinfo.get('yres', VECTOR_PAGE_DPI), options.oversample or 0)
+    return (float(xres), float(yres))
+
+
+def get_page_square_dpi(pageinfo):
+    "Get the DPI when we require xres == yres"
+    return float(max(
+        pageinfo.get('xres', VECTOR_PAGE_DPI),
+        pageinfo.get('yres', VECTOR_PAGE_DPI),
+        options.oversample or 0))
+
+
 def is_ocr_required(pageinfo, log):
     page = pageinfo['pageno'] + 1
     ocr_required = True
     if not pageinfo['images']:
-        msg = "{0:4d}: page has no images - {1}"
-
-        if options.force_ocr:
-            # Someone wanted to do this to fix a PDF with text objects but a
-            # broken toUnicode mapping
+        if options.force_ocr and options.oversample:
+            # The user really wants to reprocess this file
+            log.info(
+                "{0:4d}: page has no images - "
+                "rasterizing at {1} DPI because "
+                "--force-ocr --oversample was specified".format(
+                    page, options.oversample))
+        elif options.force_ocr:
+            # Warn the user they might not want to do this
             log.warning(
-                msg.format(
-                    page,
-                    "rasterizing anyway because --force-ocr was specified"))
+                "{0:4d}: page has no images - "
+                "all vector content will be "
+                "rasterized at {1} DPI, losing some resolution and likely "
+                "increasing file size. Use --oversample to adjust the "
+                "DPI.".format(page, VECTOR_PAGE_DPI))
         else:
-            #
-            log.info(msg.format(page,
-                                "skipping all processing on this page"))
+            log.info(
+                "{0:4d}: page has no images - "
+                "skipping all processing on this page".format(page))
             ocr_required = False
+
     elif pageinfo['has_text']:
         msg = "{0:4d}: page already has text! – {1}"
 
@@ -614,9 +638,8 @@ def rasterize_with_ghostscript(
 
     log.debug("Rasterize {0} with {1}".format(
             os.path.basename(input_file), device))
-    xres = max(pageinfo['xres'], options.oversample or 0)
-    yres = max(pageinfo['yres'], options.oversample or 0)
 
+    xres, yres = get_page_dpi(pageinfo)
     ghostscript.rasterize_pdf(input_file, output_file, xres, yres, device, log)
 
 
@@ -637,7 +660,7 @@ def preprocess_deskew(
         return
 
     pageinfo = get_pageinfo(input_file, pdfinfo, pdfinfo_lock)
-    dpi = int(pageinfo['xres'])
+    dpi = get_page_square_dpi(pageinfo)
 
     from . import leptonica
     leptonica.deskew(input_file, output_file, dpi)
@@ -660,7 +683,7 @@ def preprocess_clean(
         return
 
     pageinfo = get_pageinfo(input_file, pdfinfo, pdfinfo_lock)
-    dpi = int(pageinfo['xres'])
+    dpi = get_page_square_dpi(pageinfo)
 
     unpaper.clean(input_file, output_file, dpi, log)
 
@@ -714,10 +737,9 @@ def select_image_for_pdf(
     if all(image['enc'] == 'jpeg' for image in pageinfo['images']):
         # If all images were JPEGs originally, produce a JPEG as output
         im = Image.open(image)
-        dpi = im.info.get(
-            'dpi',
-            (int(pageinfo['xres']), int(pageinfo['yres']))
-        )
+        fallback_dpi = get_page_dpi(pageinfo)
+        dpi = im.info.get('dpi', fallback_dpi)
+        dpi = round(dpi[0]), round(dpi[1])  # Pillow requires integer DPI
         im.save(output_file, format='JPEG', dpi=dpi)
     else:
         re_symlink(image, output_file)
@@ -745,8 +767,8 @@ def select_image_layer(
         re_symlink(page_pdf, output_file)
     else:
         pageinfo = get_pageinfo(image, pdfinfo, pdfinfo_lock)
-        dpi = round(max(pageinfo['xres'], pageinfo['yres'],
-                        options.oversample))
+
+        dpi = round(get_page_square_dpi(pageinfo))
         imgsize = ((img2pdf.ImgSize.dpi, dpi), (img2pdf.ImgSize.dpi, dpi))
 
         layout_fun = img2pdf.get_layout_fun(None, imgsize, None, None, None)
@@ -773,7 +795,7 @@ def render_hocr_page(
         pdfinfo_lock):
     hocr = input_file
     pageinfo = get_pageinfo(hocr, pdfinfo, pdfinfo_lock)
-    dpi = round(max(pageinfo['xres'], pageinfo['yres'], options.oversample))
+    dpi = get_page_square_dpi(pageinfo)
 
     hocrtransform = HocrTransform(hocr, dpi)
     hocrtransform.to_pdf(output_file, imageFileName=None,
@@ -797,7 +819,7 @@ def render_hocr_debug_page(
     image = next(ii for ii in infiles if ii.endswith('.image'))
 
     pageinfo = get_pageinfo(image, pdfinfo, pdfinfo_lock)
-    dpi = round(max(pageinfo['xres'], pageinfo['yres'], options.oversample))
+    dpi = get_page_square_dpi(pageinfo)
 
     hocrtransform = HocrTransform(hocr, dpi)
     hocrtransform.to_pdf(output_file, imageFileName=None,