Allow accessing ghost vectors in :class:.LinearTransformationScene (#3435)

* Fix CSV reader adding empty files

Fixes issue #3311

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Added LinearTransformationScene.ghost_vectors

* Added test and prevented empty VGroups as ghost vectors

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Fixed typo in example

* Added ability to join together multiple renders

* Revert "Added ability to join together multiple renders" (wrong branch)

This reverts commit dee29c3.

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: JasonGrace2282

manim/scene/vector_space_scene.py

@@ -572,7 +572,7 @@ def __init__(self, **kwargs):
                     self,
                     show_coordinates=True,
                     leave_ghost_vectors=True,
-                    *kwargs
+                    **kwargs
                 )
 
             def construct(self):
@@ -616,6 +616,8 @@ def __init__(
             },
         }
 
+        self.ghost_vectors = VGroup()
+
         self.foreground_plane_kwargs = {
             "x_range": np.array([-config["frame_width"], config["frame_width"], 1.0]),
             "y_range": np.array([-config["frame_width"], config["frame_width"], 1.0]),
@@ -741,6 +743,13 @@ def add_moving_mobject(
         mobject.target = target_mobject
         self.add_special_mobjects(self.moving_mobjects, mobject)
 
+    def get_ghost_vectors(self) -> VGroup:
+        """
+        Returns all ghost vectors ever added to ``self``. Each element is a ``VGroup`` of
+        two ghost vectors.
+        """
+        return self.ghost_vectors
+
     def get_unit_square(
         self, color: str = YELLOW, opacity: float = 0.3, stroke_width: float = 3
     ):
@@ -996,8 +1005,11 @@ def get_piece_movement(self, pieces: list | tuple | np.ndarray):
         """
         start = VGroup(*pieces)
         target = VGroup(*(mob.target for mob in pieces))
-        if self.leave_ghost_vectors:
-            self.add(start.copy().fade(0.7))
+        # don't add empty VGroups
+        if self.leave_ghost_vectors and start.submobjects:
+            # start.copy() gives a VGroup of Vectors
+            self.ghost_vectors.add(start.copy().fade(0.7))
+            self.add(self.ghost_vectors[-1])
         return Transform(start, target, lag_ratio=0)
 
     def get_moving_mobject_movement(self, func: Callable[[np.ndarray], np.ndarray]):

tests/test_linear_transformation_scene.py

@@ -0,0 +1,24 @@
+from manim import RIGHT, UP, LinearTransformationScene, Vector, VGroup
+
+__module_test__ = "vector_space_scene"
+
+
+def test_ghost_vectors_len_and_types():
+    scene = LinearTransformationScene()
+    scene.leave_ghost_vectors = True
+
+    # prepare vectors (they require a vmobject as their target)
+    v1, v2 = Vector(RIGHT), Vector(RIGHT)
+    v1.target, v2.target = Vector(UP), Vector(UP)
+
+    # ghost_vector addition is in this method
+    scene.get_piece_movement((v1, v2))
+
+    ghosts = scene.get_ghost_vectors()
+    assert len(ghosts) == 1
+    # check if there are two vectors in the ghost vector VGroup
+    assert len(ghosts[0]) == 2
+
+    # check types of ghost vectors
+    assert isinstance(ghosts, VGroup) and isinstance(ghosts[0], VGroup)
+    assert all(isinstance(x, Vector) for x in ghosts[0])