code: def resolution(file_, resolution_string):
      
      if sorl_settings.THUMBNAIL_DUMMY:
          dummy_source = sorl_settings.THUMBNAIL_DUMMY_SOURCE
          source = dummy_source.replace('%(width)s', '(?P<width>[0-9]+)')
          source = source.replace('%(height)s', '(?P<height>[0-9]+)')
          source = re.compile(source)
          try:
              resolution = decimal.Decimal(resolution_string.strip('x'))
              info = source.match(file_).groupdict()
              info = {dimension: int(int(size) * resolution) for (dimension, size) in info.items()}
              return dummy_source % info
          except (AttributeError, TypeError, KeyError):
              # If we can't manipulate the dummy we shouldn't change it at all
              return file_
  
      filename, extension = os.path.splitext(file_)
      return '%s@%s%s' % (filename, resolution_string, extension)
docs: A filter to return the URL for the provided resolution of the thumbnail.