Optimize VMobject.pointwise_become_partial() (#3760)

* Optimize VMobject.pointwise_become_partial()

* selftransformation -> self

* Small factorization of nppc

manim/mobject/types/vectorized_mobject.py

@@ -1830,60 +1830,91 @@ def pointwise_become_partial(
         a: float,
         b: float,
     ) -> Self:
-        """Given two bounds a and b, transforms the points of the self vmobject into the points of the vmobject
-        passed as parameter with respect to the bounds. Points here stand for control points of the bezier curves (anchors and handles)
+        """Given a 2nd :class:`.VMobject` ``vmobject``, a lower bound ``a`` and
+        an upper bound ``b``, modify this :class:`.VMobject`'s points to
+        match the portion of the Bézier spline described by ``vmobject.points``
+        with the parameter ``t`` between ``a`` and ``b``.
 
         Parameters
         ----------
         vmobject
-            The vmobject that will serve as a model.
+            The :class:`.VMobject` that will serve as a model.
         a
-            upper-bound.
+            The lower bound for ``t``.
         b
-            lower-bound
+            The upper bound for ``t``
 
         Returns
         -------
-        :class:`VMobject`
-            ``self``
+        :class:`.VMobject`
+            The :class:`.VMobject` itself, after the transformation.
+
+        Raises
+        ------
+        TypeError
+            If ``vmobject`` is not an instance of :class:`VMobject`.
         """
-        assert isinstance(vmobject, VMobject)
+        if not isinstance(vmobject, VMobject):
+            raise TypeError(
+                f"Expected a VMobject, got value {vmobject} of type "
+                f"{type(vmobject).__name__}."
+            )
         # Partial curve includes three portions:
-        # - A middle section, which matches the curve exactly
-        # - A start, which is some ending portion of an inner cubic
-        # - An end, which is the starting portion of a later inner cubic
+        # - A middle section, which matches the curve exactly.
+        # - A start, which is some ending portion of an inner cubic.
+        # - An end, which is the starting portion of a later inner cubic.
         if a <= 0 and b >= 1:
             self.set_points(vmobject.points)
             return self
-        bezier_quads = vmobject.get_cubic_bezier_tuples()
-        num_cubics = len(bezier_quads)
+        num_curves = vmobject.get_num_curves()
+        if num_curves == 0:
+            self.clear_points()
+            return self
 
-        # The following two lines will compute which bezier curves of the given mobject need to be processed.
-        # The residue basically indicates de proportion of the selected bezier curve that have to be selected.
-        # Ex : if lower_index is 3, and lower_residue is 0.4, then the algorithm will append to the points 0.4 of the third bezier curve
-        lower_index, lower_residue = integer_interpolate(0, num_cubics, a)
-        upper_index, upper_residue = integer_interpolate(0, num_cubics, b)
+        # The following two lines will compute which Bézier curves of the given Mobject must be processed.
+        # The residue indicates the proportion of the selected Bézier curve which must be selected.
+        #
+        # Example: if num_curves is 10, a is 0.34 and b is 0.78, then:
+        # - lower_index is 3 and lower_residue is 0.4, which means the algorithm will look at the 3rd Bézier
+        #   and select its part which ranges from t=0.4 to t=1.
+        # - upper_index is 7 and upper_residue is 0.8, which means the algorithm will look at the 7th Bézier
+        #   and select its part which ranges from t=0 to t=0.8.
+        lower_index, lower_residue = integer_interpolate(0, num_curves, a)
+        upper_index, upper_residue = integer_interpolate(0, num_curves, b)
 
-        self.clear_points()
-        if num_cubics == 0:
-            return self
+        nppc = self.n_points_per_curve
+        # If both indices coincide, get a part of a single Bézier curve.
         if lower_index == upper_index:
-            self.append_points(
-                partial_bezier_points(
-                    bezier_quads[lower_index],
-                    lower_residue,
-                    upper_residue,
-                ),
+            # 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)],
+                lower_residue,
+                upper_residue,
             )
         else:
-            self.append_points(
-                partial_bezier_points(bezier_quads[lower_index], lower_residue, 1),
+            # Allocate space for (upper_index-lower_index+1) Bézier curves.
+            self.points = np.empty((nppc * (upper_index - lower_index + 1), self.dim))
+            # 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)],
+                lower_residue,
+                1,
             )
-            for quad in bezier_quads[lower_index + 1 : upper_index]:
-                self.append_points(quad)
-            self.append_points(
-                partial_bezier_points(bezier_quads[upper_index], 0, upper_residue),
+            # 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[
+                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)],
+                0,
+                upper_residue,
             )
+
         return self
 
     def get_subcurve(self, a: float, b: float) -> Self: