Fix :meth:VMobject.pointwise_become_partial failing when vmobject is self (#4193)

* Fix Line failing with buff and path_arc
- make copy of vmobject.points if necessary

* change self to self.copy() in Line, and remove the previous change in pointwise_become_partial()

* Revert to first approach: copy points if self is vmobject

* Add unit test for pointwise_become_partial and Line

---------

Co-authored-by: Francisco Manríquez Novoa <[email protected]>

Author: irvanalhaq9

manim/mobject/types/vectorized_mobject.py

@@ -1932,12 +1932,18 @@ def pointwise_become_partial(
         upper_index, upper_residue = integer_interpolate(0, num_curves, b)
 
         nppc = self.n_points_per_curve
+
+        # Copy vmobject.points if vmobject is self to prevent unintended in-place modification
+        vmobject_points = (
+            vmobject.points.copy() if self is vmobject else vmobject.points
+        )
+
         # If both indices coincide, get a part of a single Bézier curve.
         if lower_index == upper_index:
             # Look at the "lower_index"-th Bézier curve and select its part from
             # t=lower_residue to t=upper_residue.
             self.points = partial_bezier_points(
-                vmobject.points[nppc * lower_index : nppc * (lower_index + 1)],
+                vmobject_points[nppc * lower_index : nppc * (lower_index + 1)],
                 lower_residue,
                 upper_residue,
             )
@@ -1947,19 +1953,19 @@ def pointwise_become_partial(
             # Look at the "lower_index"-th Bezier curve and select its part from
             # t=lower_residue to t=1. This is the first curve in self.points.
             self.points[:nppc] = partial_bezier_points(
-                vmobject.points[nppc * lower_index : nppc * (lower_index + 1)],
+                vmobject_points[nppc * lower_index : nppc * (lower_index + 1)],
                 lower_residue,
                 1,
             )
             # If there are more curves between the "lower_index"-th and the
             # "upper_index"-th Béziers, add them all to self.points.
-            self.points[nppc:-nppc] = vmobject.points[
+            self.points[nppc:-nppc] = vmobject_points[
                 nppc * (lower_index + 1) : nppc * upper_index
             ]
             # Look at the "upper_index"-th Bézier curve and select its part from
             # t=0 to t=upper_residue. This is the last curve in self.points.
             self.points[-nppc:] = partial_bezier_points(
-                vmobject.points[nppc * upper_index : nppc * (upper_index + 1)],
+                vmobject_points[nppc * upper_index : nppc * (upper_index + 1)],
                 0,
                 upper_residue,
             )

tests/module/mobject/geometry/test_unit_geometry.py

@@ -5,8 +5,12 @@
 import numpy as np
 
 from manim import (
+    DEGREES,
+    LEFT,
+    RIGHT,
     BackgroundRectangle,
     Circle,
+    Line,
     Polygram,
     Sector,
     Square,
@@ -142,3 +146,36 @@ def test_changing_Square_side_length_updates_the_square_appropriately():
 def test_Square_side_length_consistent_after_scale_and_rotation():
     sq = Square(side_length=1).scale(3).rotate(np.pi / 4)
     assert np.isclose(sq.side_length, 3)
+
+
+def test_line_with_buff_and_path_arc():
+    line = Line(LEFT, RIGHT, path_arc=60 * DEGREES, buff=0.3)
+    expected_points = np.array(
+        [
+            [-0.7299265, -0.12999304, 0.0],
+            [-0.6605293, -0.15719695, 0.0],
+            [-0.58965623, -0.18050364, 0.0],
+            [-0.51763809, -0.19980085, 0.0],
+            [-0.51763809, -0.19980085, 0.0],
+            [-0.43331506, -0.22239513, 0.0],
+            [-0.34760317, -0.23944429, 0.0],
+            [-0.26105238, -0.25083892, 0.0],
+            [-0.26105238, -0.25083892, 0.0],
+            [-0.1745016, -0.26223354, 0.0],
+            [-0.08729763, -0.26794919, 0.0],
+            [0.0, -0.26794919, 0.0],
+            [0.0, -0.26794919, 0.0],
+            [0.08729763, -0.26794919, 0.0],
+            [0.1745016, -0.26223354, 0.0],
+            [0.26105238, -0.25083892, 0.0],
+            [0.26105238, -0.25083892, 0.0],
+            [0.34760317, -0.23944429, 0.0],
+            [0.43331506, -0.22239513, 0.0],
+            [0.51763809, -0.19980085, 0.0],
+            [0.51763809, -0.19980085, 0.0],
+            [0.58965623, -0.18050364, 0.0],
+            [0.6605293, -0.15719695, 0.0],
+            [0.7299265, -0.12999304, 0.0],
+        ]
+    )
+    np.testing.assert_allclose(line.points, expected_points)

tests/module/mobject/types/vectorized_mobject/test_vectorized_mobject.py

@@ -526,3 +526,25 @@ def test_proportion_from_point():
     abc.scale(0.8)
     props = [abc.proportion_from_point(p) for p in abc.get_vertices()]
     np.testing.assert_allclose(props, [0, 1 / 3, 2 / 3])
+
+
+def test_pointwise_become_partial_where_vmobject_is_self():
+    sq = Square()
+    sq.pointwise_become_partial(vmobject=sq, a=0.2, b=0.7)
+    expected_points = np.array(
+        [
+            [-0.6, 1.0, 0.0],
+            [-0.73333333, 1.0, 0.0],
+            [-0.86666667, 1.0, 0.0],
+            [-1.0, 1.0, 0.0],
+            [-1.0, 1.0, 0.0],
+            [-1.0, 0.33333333, 0.0],
+            [-1.0, -0.33333333, 0.0],
+            [-1.0, -1.0, 0.0],
+            [-1.0, -1.0, 0.0],
+            [-0.46666667, -1.0, 0.0],
+            [0.06666667, -1.0, 0.0],
+            [0.6, -1.0, 0.0],
+        ]
+    )
+    np.testing.assert_allclose(sq.points, expected_points)