diff --git a/bookmark/image_processing.py b/bookmark/image_processing.py
new file mode 100644
index 0000000..87f4f71
--- /dev/null
+++ b/bookmark/image_processing.py
@@ -0,0 +1,28 @@
+import cv2
+import numpy as np
+
+
+def crop_to_book(frame: np.ndarray, reference_frame: np.ndarray, margin: int = 20) -> np.ndarray:
+    """
+    Crops `frame` down to the bounding box of the largest region that
+    differs from `reference_frame` (the book), padded by `margin` pixels
+    and clamped to the frame bounds. Raises ValueError if no difference
+    is found.
+    """
+    diff = cv2.absdiff(frame, reference_frame)
+    gray = cv2.cvtColor(diff, cv2.COLOR_BGR2GRAY)
+    _, mask = cv2.threshold(gray, 30, 255, cv2.THRESH_BINARY)
+    contours, _ = cv2.findContours(mask, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
+    if not contours:
+        raise ValueError("No book detected in frame (no difference from reference)")
+
+    largest = max(contours, key=cv2.contourArea)
+    x, y, w, h = cv2.boundingRect(largest)
+    frame_h, frame_w = frame.shape[:2]
+
+    x0 = max(0, x - margin)
+    y0 = max(0, y - margin)
+    x1 = min(frame_w, x + w + margin)
+    y1 = min(frame_h, y + h + margin)
+
+    return frame[y0:y1, x0:x1]
diff --git a/tests/test_image_processing.py b/tests/test_image_processing.py
new file mode 100644
index 0000000..27afef1
--- /dev/null
+++ b/tests/test_image_processing.py
@@ -0,0 +1,44 @@
+import numpy as np
+import pytest
+
+from bookmark.image_processing import crop_to_book
+
+
+def make_reference(size=100, color=200):
+    return np.full((size, size, 3), color, dtype=np.uint8)
+
+
+def make_book_frame(size=100, ref_color=200, book_color=50, box=(30, 30, 20, 20)):
+    frame = np.full((size, size, 3), ref_color, dtype=np.uint8)
+    x, y, w, h = box
+    frame[y:y + h, x:x + w] = book_color
+    return frame
+
+
+def test_crop_raises_when_no_difference_from_reference():
+    reference = make_reference()
+    with pytest.raises(ValueError):
+        crop_to_book(reference.copy(), reference)
+
+
+def test_crop_returns_region_around_book_with_margin():
+    reference = make_reference()
+    frame = make_book_frame(box=(30, 30, 20, 20))
+
+    cropped = crop_to_book(frame, reference, margin=10)
+
+    # book occupies x:[30,50) y:[30,50), with margin=10 expected crop is
+    # roughly x:[20,60) y:[20,60) -> 40x40
+    assert cropped.shape[0] == pytest.approx(40, abs=2)
+    assert cropped.shape[1] == pytest.approx(40, abs=2)
+
+
+def test_crop_clamps_margin_to_frame_bounds():
+    reference = make_reference()
+    frame = make_book_frame(box=(0, 0, 20, 20))
+
+    cropped = crop_to_book(frame, reference, margin=10)
+
+    # book touches top-left corner, margin should clamp instead of going negative
+    assert cropped.shape[0] <= 30
+    assert cropped.shape[1] <= 30