code: def _validate_pixel_density(self):
          
          for f in [f for f in self.mets.find_files() if f.mimetype.startswith('image/')]:
              if not f.local_filename and not self.download:
                  self.report.add_notice("Won't download remote image <%s>" % f.url)
                  continue
              exif = self.workspace.resolve_image_exif(f.url)
              for k in ['xResolution', 'yResolution']:
                  v = exif.__dict__.get(k)
                  if v is None or v <= 72:
                      self.report.add_error("Image %s: %s (%s pixels per %s) is too low" % (f.ID, k, v, exif.resolutionUnit))
docs: Validate image pixel density
  
          See `spec <https://ocr-d.github.io/mets#pixel-density-of-images-must-be-explicit-and-high-enough>`_.