def __init__(self, groups=None):
        super(GroupedRequirements, self).__init__(NamedRequirements)
        self.groups = groups or self.default_groups
        self.filename = None
        self.create_default_groups()
def _read_flow(self, file_name):
        return _read_16bits_png_with_flow_and_valid_mask(file_name)