diff --git a/tests/test_mask.py b/tests/test_mask.py
new file mode 100644
index 0000000..e5ae03e
--- /dev/null
+++ b/tests/test_mask.py
@@ -0,0 +1,154 @@
+import numpy as np
+from PIL import Image, ImageDraw
+
+from adetailer.mask import dilate_erode, has_intersection, is_all_black, offset
+
+
+def test_dilate_positive_value():
+    img = Image.new("L", (10, 10), color="black")
+    draw = ImageDraw.Draw(img)
+    draw.rectangle((3, 3, 5, 5), fill="white")
+    value = 3
+
+    result = dilate_erode(img, value)
+
+    assert isinstance(result, Image.Image)
+    assert result.size == (10, 10)
+
+    expect = np.array(
+        [
+            [0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
+            [0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
+            [0, 0, 255, 255, 255, 255, 255, 0, 0, 0],
+            [0, 0, 255, 255, 255, 255, 255, 0, 0, 0],
+            [0, 0, 255, 255, 255, 255, 255, 0, 0, 0],
+            [0, 0, 255, 255, 255, 255, 255, 0, 0, 0],
+            [0, 0, 255, 255, 255, 255, 255, 0, 0, 0],
+            [0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
+            [0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
+            [0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
+        ],
+        dtype=np.uint8,
+    )
+    assert np.array_equal(np.array(result), expect)
+
+
+def test_offset():
+    img = Image.new("L", (10, 10), color="black")
+    draw = ImageDraw.Draw(img)
+    draw.rectangle((4, 4, 5, 5), fill="white")
+
+    result = offset(img, x=1, y=2)
+
+    assert isinstance(result, Image.Image)
+    assert result.size == (10, 10)
+
+    expect = np.array(
+        [
+            [0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
+            [0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
+            [0, 0, 0, 0, 0, 255, 255, 0, 0, 0],
+            [0, 0, 0, 0, 0, 255, 255, 0, 0, 0],
+            [0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
+            [0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
+            [0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
+            [0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
+            [0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
+            [0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
+        ],
+        dtype=np.uint8,
+    )
+    assert np.array_equal(np.array(result), expect)
+
+
+def test_is_all_black_1():
+    img = Image.new("L", (10, 10), color="black")
+    assert is_all_black(img)
+
+    draw = ImageDraw.Draw(img)
+    draw.rectangle((4, 4, 5, 5), fill="white")
+    assert not is_all_black(img)
+
+
+def test_is_all_black_2():
+    img = np.zeros((10, 10), dtype=np.uint8)
+    assert is_all_black(img)
+
+    img[4:6, 4:6] = 255
+    assert not is_all_black(img)
+
+
+def test_has_intersection_1():
+    arr1 = np.array(
+        [
+            [0, 0, 0, 0],
+            [0, 0, 0, 0],
+            [0, 0, 0, 0],
+            [0, 0, 0, 0],
+        ]
+    )
+    arr2 = arr1.copy()
+    assert not has_intersection(arr1, arr2)
+
+
+def test_has_intersection_2():
+    arr1 = np.array(
+        [
+            [0, 0, 0, 0],
+            [0, 255, 255, 0],
+            [0, 255, 255, 0],
+            [0, 0, 0, 0],
+        ]
+    )
+    arr2 = np.array(
+        [
+            [0, 0, 0, 0],
+            [0, 0, 0, 0],
+            [0, 0, 255, 255],
+            [0, 0, 255, 255],
+        ]
+    )
+    assert has_intersection(arr1, arr2)
+
+    arr3 = np.array(
+        [
+            [255, 0, 0, 0],
+            [0, 0, 0, 0],
+            [0, 0, 0, 255],
+            [0, 0, 255, 255],
+        ]
+    )
+    assert not has_intersection(arr1, arr3)
+
+
+def test_has_intersection_3():
+    img1 = Image.new("L", (10, 10), color="black")
+    draw1 = ImageDraw.Draw(img1)
+    draw1.rectangle((3, 3, 5, 5), fill="white")
+    img2 = Image.new("L", (10, 10), color="black")
+    draw2 = ImageDraw.Draw(img2)
+    draw2.rectangle((6, 6, 8, 8), fill="white")
+    assert not has_intersection(img1, img2)
+
+    img3 = Image.new("L", (10, 10), color="black")
+    draw3 = ImageDraw.Draw(img3)
+    draw3.rectangle((2, 2, 8, 8), fill="white")
+    assert has_intersection(img1, img3)
+
+
+def test_has_intersection_4():
+    img1 = Image.new("RGB", (10, 10), color="black")
+    draw1 = ImageDraw.Draw(img1)
+    draw1.rectangle((3, 3, 5, 5), fill="white")
+    img2 = Image.new("RGBA", (10, 10), color="black")
+    draw2 = ImageDraw.Draw(img2)
+    draw2.rectangle((2, 2, 8, 8), fill="white")
+    assert has_intersection(img1, img2)
+
+
+def test_has_intersection_5():
+    img1 = Image.new("RGB", (10, 10), color="black")
+    draw1 = ImageDraw.Draw(img1)
+    draw1.rectangle((4, 4, 5, 5), fill="white")
+    img2 = np.full((10, 10, 4), 255, dtype=np.uint8)
+    assert has_intersection(img1, img2)