Add a Camera.forward() getter (world-space viewing direction)

src/multicam_sim/cameras.py

@@ -170,6 +170,15 @@ def centre(self) -> FloatArray:
         """Camera centre ``C = -R^T @ t`` in world coordinates."""
         return camera_centre(self.rotation(), self.translation())
 
+    def forward(self) -> FloatArray:
+        """World-space unit viewing direction.
+
+        ``R``'s rows are the camera axes in world coordinates in the order
+        ``[right, down, forward]`` (OpenCV, +z forward), so the forward axis is
+        the third row — already unit length for an orthonormal ``R``.
+        """
+        return np.asarray(self.rotation()[2], dtype=np.float64)
+
     def projection_matrix(self) -> FloatArray:
         """The ``3x4`` matrix ``P = K [R | t]``."""
         return projection_matrix(self.intrinsics.matrix(), self.rotation(), self.translation())

tests/test_cameras.py

@@ -123,3 +123,20 @@ def test_intrinsics_rejects_non_positive_size(width: int, height: int) -> None:
 def test_intrinsics_direct_construction_still_works() -> None:
     intr = Intrinsics.from_focal(100.0, 640, 480)
     assert (intr.width, intr.height) == (640, 480)
+
+
+def test_camera_forward_is_unit_length() -> None:
+    intr = Intrinsics.from_focal(100.0, 640, 480)
+    eye = np.array([1.0, 2.0, 3.0])
+    target = np.array([4.0, 0.0, 1.0])
+    cam = Camera.look_at(0, intr, eye, target)
+    assert np.linalg.norm(cam.forward()) == pytest.approx(1.0)
+
+
+def test_camera_forward_points_from_eye_to_target() -> None:
+    intr = Intrinsics.from_focal(100.0, 640, 480)
+    eye = np.array([1.0, 2.0, 3.0])
+    target = np.array([4.0, 0.0, 1.0])
+    cam = Camera.look_at(0, intr, eye, target)
+    expected = (target - eye) / np.linalg.norm(target - eye)
+    np.testing.assert_allclose(cam.forward(), expected, atol=1e-9)