Fix of ImageMobject opacity bug (#4089)

* Fix issue 4072.

* Added a unittest for the interpolate function in utils/bezier.py

Author: henrikmidtiby

manim/utils/bezier.py

@@ -1059,7 +1059,7 @@ def interpolate(
             * ``alpha`` is a :class:`float`, the return is another :class:`~.Point3D`.
             * ``alpha`` is a :class:`~.ColVector`, the return is a :class:`~.Point3D_Array`.
     """
-    return start + alpha * (end - start)
+    return (1 - alpha) * start + alpha * end
 
 
 def integer_interpolate(

tests/module/utils/test_bezier.py

@@ -10,6 +10,7 @@
     _get_subdivision_matrix,
     get_quadratic_approximation_of_cubic,
     get_smooth_cubic_bezier_handle_points,
+    interpolate,
     partial_bezier_points,
     split_bezier,
     subdivide_bezier,
@@ -215,3 +216,18 @@ def test_get_quadratic_approximation_of_cubic() -> None:
             ]
         ),
     )
+
+
+def test_interpolate() -> None:
+    """Test that :func:`interpolate` handles interpolation of both float and uint8 values."""
+    start = 127.0
+    end = 25.0
+    alpha = 0.2
+    val = interpolate(start, end, alpha)
+    assert np.allclose(val, 106.6000000)
+
+    start = np.array(127, dtype=np.uint8)
+    end = np.array(25, dtype=np.uint8)
+    alpha = 0.09739
+    val = interpolate(start, end, alpha)
+    assert np.allclose(val, np.array([117.06622]))