'Scenarios are: 1) no pHYs chunk in PNG stream, 2) units specifier
other than 1; 3) px_per_unit is 0; 4) px_per_unit is None'

@pytest.fixture(params=[((-1), (-1)), (0, 1000), (None, 1000), (1, 0), (1, None)])
def no_dpi_fixture(self, request, chunks_):

    (units_specifier, px_per_unit) = request.param
    if (units_specifier == (-1)):
        chunks_.pHYs = None
    else:
        chunks_.pHYs.horz_px_per_unit = px_per_unit
        chunks_.pHYs.vert_px_per_unit = px_per_unit
        chunks_.pHYs.units_specifier = units_specifier
    png_parser = _PngParser(chunks_)
    return png_parser
