[Bezier racing] Two Bezier curves on a path. #manim #animation #bezier #updater

20260810_bezier.py

from manim import *
class Bezier(VGroup):
    def __init__(self, start, handle1, handle2, end,  **kwargs):
        super().__init__(**kwargs)
        self.curve = VMobject(stroke_color=ORANGE,stroke_width=4)
        self.start = Circle(arc_center=start, color=RED, radius=0.08, fill_opacity=0.7)
        self.end   = Circle(arc_center=end, color=BLUE, radius=0.08, fill_opacity=0.7)
        self.handle1 = Circle(arc_center=handle1, color=RED, radius=0.05, fill_opacity=0.0)
        self.handle2 = Circle(arc_center=handle2, color=BLUE, radius=0.05, fill_opacity=0.0)
        self.hline1 = VMobject()
        self.hline2 = VMobject()
        self.add(
            self.curve,
            self.start,
            self.handle1,
            self.handle2,
            self.end,
            self.hline1,
            self.hline2
        )
        self.add_updater(self.updater,call_updater=True)
    def updater(self, grp):
        start = grp.start.get_center()
        handle1 = grp.handle1.get_center()
        handle2 = grp.handle2.get_center()
        end = grp.end.get_center()
        grp.curve.set_anchors_and_handles(anchors1=start, handles1=handle1,handles2=handle2, anchors2=end)
        grp.hline1.become(always_redraw(lambda: Line(self.start.get_center(),self.handle1.get_center(), stroke_color=RED,stroke_width=2)))
        grp.hline2.become(always_redraw(lambda: Line(self.end.get_center(),self.handle2.get_center(), stroke_color=BLUE,stroke_width=2)))

class bezierex2(Scene):
    def construct(self):
        path = VMobject(stroke_width=0.3).set_points_as_corners(
            [
                [-6,3,0],[-2,2,0],[2,-3,0],[6,0,0],[3,2,0],[-3,-3,0],[-6,0,0]
            ]
        ).make_smooth()
        self.add(path)
        bz = Bezier(2*LEFT,LEFT,RIGHT,2*RIGHT)
        bz.a = ValueTracker(0.21)
        def bz_updater(mobj):
            mobj.start.move_to(path.point_from_proportion(mobj.a.get_value()))
            mobj.end.move_to(path.point_from_proportion(mobj.a.get_value()-0.1))
            mobj.handle1.move_to(TangentLine(path, mobj.a.get_value(), length=2, d_alpha=1e-6).get_start())
            mobj.handle2.move_to(TangentLine(path, mobj.a.get_value()-0.1, length=2, d_alpha=1e-6).get_end())
            mobj.updater(mobj)
        bz.add_updater(bz_updater, call_updater=True)

        bz2 = bz.copy()
        bz2.a.set_value(0.11)
        bz2.update()

        self.add(bz,bz2)
        self.wait()
        self.play(
            LaggedStart(
                bz.a.animate(rate_func=rate_functions.linear).set_value(1-0.05),
                bz2.a.animate(rate_func=rate_functions.linear).set_value(0.9-0.05),
                lag_ratio=0.3,
            ),
            run_time=5,
        )
        self.play(
            bz.a.animate.set_value(1),
            bz2.a.animate.set_value(0.9),
            run_time=1,
            rate_func=rate_functions.linear
        )
        self.wait()

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