def METHOD_NAME(self):
    # Returns the survey ID or "None" if it cannot be retrieved
    # In this function assume that the rotation is correct already.
    paper_width = self.obj.sheet.survey.defs.paper_width
    paper_height = self.obj.sheet.survey.defs.paper_height
    # Search for the barcode in the lower left corner.
    code = \
        read_barcode(self.obj.surface.surface, self.obj.matrix.mm_to_px(),
                     paper_width / 2,
                     paper_height - self.obj.sheet.survey.defs.corner_mark_bottom - defs.code128_vpad - defs.code128_height - 5,
                     paper_width / 2,
                     self.obj.sheet.survey.defs.corner_mark_bottom + defs.code128_vpad + defs.code128_height + 5)
    if code is None or not code.isdigit() or len(code) <= 4:
        return None
    return int(code[:-4])