def test_otsu_negative_int(self):
     image = self.image - 2
     assert threshold_otsu(image) == 0

 def test_otsu_float_image(self):
     image = np.float64(self.image)
     assert 2 <= threshold_otsu(image) < 3

def test_otsu_astro_image():
    img = skimage.img_as_ubyte(data.astronaut())
    with expected_warnings(['grayscale']):
        assert 109 < threshold_otsu(img) < 111

 def test_otsu(self):
     assert threshold_otsu(self.image) == 2

def test_otsu_coins_image():
    coins = skimage.img_as_ubyte(data.coins())
    assert 106 < threshold_otsu(coins) < 108

def test_otsu_coins_image_as_float():
    coins = skimage.img_as_float(data.coins())
    assert 0.41 < threshold_otsu(coins) < 0.42

def test_otsu_camera_image():
    camera = skimage.img_as_ubyte(data.camera())
    assert 86 < threshold_otsu(camera) < 88

def test_otsu_one_color_image():
    img = np.ones((10, 10), dtype=np.uint8)
    with pytest.raises(ValueError):
        threshold_otsu(img)

def test_otsu_astro_image():
    img = skimage.img_as_ubyte(data.astronaut())
    assert 109 < threshold_otsu(img) < 111

def test_otsu_lena_image():
    img = skimage.img_as_ubyte(data.lena())
    assert 140 < threshold_otsu(img) < 142

def test_otsu_lena_image():
    img = skimage.img_as_ubyte(data.lena())
    with expected_warnings(['grayscale']):
        assert 140 < threshold_otsu(img) < 142