子时间轴机制

JAnim 引入了子时间轴机制，使得在主时间轴中可以嵌套子时间轴，从而实现复杂逻辑的分离，以及更好的可复用性。

from janim.imports import *

class DrawingStarInNumberPlane(Timeline):
    def construct(self):
        plane = NumberPlane(faded_line_ratio=1)
        star = Star(outer_radius=2)
        dot = Dot(star.points.get_start(), color=YELLOW)

        state = self.camera.copy()

        self.play(Create(plane, lag_ratio=0.05))
        self.play(
            self.camera.anim.points.move_to(dot).scale(0.3),
            FadeIn(dot, scale=0.8),
            lag_ratio=0.3
        )
        self.play(
            MoveAlongPath(dot, star),
            Create(star, auto_close_path=False),
            Follow(self.camera, dot, ORIGIN),
            duration=3
        )
        self.play(
            self.camera.anim.become(state),
            star.anim.set(color=YELLOW, fill_alpha=0.5),
            duration=2
        )


class Texts(Timeline):
    def construct(self):
        txt = Text(
            'I just wrote some text for demonstration.\n'
            'It has no real meaning—it\'s only meant to show that \n'
            'this text is completely decoupled from another Timeline.',
            stroke_color=BLACK,
            stroke_alpha=1,
            stroke_background=True
        )
        txt.points.to_border(UL)

        self.forward()
        self.play(Write(txt, duration=3))


class MainTimeline(Timeline):
    def construct(self):
        tl1 = DrawingStarInNumberPlane().build().to_item().show()
        tl2 = Texts().build().to_item(keep_last_frame=True).show()

        self.forward(tl1.duration)

参考：

to_item()

在上面这个例子中，我们向主时间轴 MainTimeline 中插入了两个子时间轴 DrawingStarInNumberPlane 和 Texts 。

子时间轴的引入使得每个子时间轴都可以独立地控制自己的动画，我们可以在主时间轴中更灵活地组织和管理动画逻辑。

提示

上面这个例子中，文字不随另一个时间轴的动画而移动，这个效果也可以使用 fix_in_frame() 方法来实现，你可以另行参阅。

并且子时间轴在主时间轴中作为一个整体物件，可以更加方便地传入 FrameEffect 及其派生类应用复杂的视觉效果：

class MainTimeline(Timeline):
    def construct(self):
        tl1 = DrawingStarInNumberPlane().build().to_item(keep_last_frame=True).show()

        effect = SimpleFrameEffect(
            tl1,
            shader='''
            f_color = texture(fbo, v_texcoord);

            vec3 col = 0.5 + 0.5 * cos(time * 1.5 + v_texcoord.xyx + vec3(0,2,4));
            f_color.rgb *= col;
            ''',
            uniforms=['float time']
        )
        frameclip = TransformableFrameClip(effect)

        tl2 = Texts().build().to_item(keep_last_frame=True).show()

        self.play(
            DataUpdater(
                effect,
                lambda data, p: data.apply_uniforms(time=p.elapsed)
            ),
            duration=tl1.duration
        )
        self.forward()
        self.play(
            frameclip.anim.clip.set(scale=0.5, x_offset=-0.2, y_offset=-0.1)
        )
        self.play(
            Write(frameclip.create_border_rect())
        )

参考：

SimpleFrameEffect TransformableFrameClip

并且，子 Timeline 机制让 Timeline 有了极高的可复用性：

from janim.imports import *

class GraphDemonstration(Timeline):
    def __init__(self, f, x_range, typ_code):
        super().__init__()
        self.f = f
        self.x_range = x_range
        self.typ_code = typ_code

    def construct(self):
        axes = Axes(axis_config=dict(include_numbers=True))
        graph = axes.get_graph(self.f, self.x_range, color=RED, stroke_radius=0.05)

        typ = TypstMath(
            self.typ_code,
            stroke_color=BLACK,
            stroke_alpha=1,
            stroke_background=True
        ).show()
        typ.points.scale(1.6).to_border(UP)

        def dots_updater(p):
            points = graph.current().points
            return Group(
                Dot(points.get_start()),
                Dot(points.get_end()),
                fill_color=BLACK,
                stroke_alpha=1,
            )

        self.forward()
        self.play(
            Create(axes, lag_ratio=0.05)
        )
        self.play(
            Create(graph),
            ItemUpdater(None, dots_updater)
        )


class MainTimeline(Timeline):
    def construct(self):
        params_list = [
            (lambda x: x**2, (-1, 1.5), 'f(x) = x^2'),
            (lambda x: x**3, (-1.5, 1.5), 'f(x) = x^3'),
            (lambda x: math.atan(x), (-2, 2), 'f(x) = tan^(-1) x')
        ]

        width = 1 / len(params_list)
        clip = 0.5 - width / 2

        for i, params in enumerate(params_list):
            offset = (-clip + i * width, 0)
            tl = GraphDemonstration(*params).build().to_item(keep_last_frame=True).show()
            frameclip = TransformableFrameClip(tl, clip=(clip, 0, clip, 0), offset=offset).show()

        self.forward(4)

备注

有待完善关于 to_playback_control_item() 的说明