video/.agents/skills/manimgl-best-practices/references/transit_animation.md

Transit Animations - Reference Guide

Example file: examples/transit_animation.py

User Query Scenarios

This example addresses queries like:

• "Create a planet transit animation"
• "Show loading dots animation"
• "Animate a pendulum swing"
• "Create wave propagation"
• "Show orbital motion"

Scene Thinking Process (3b1b Style)

1. Core Concept

Transit/Periodic Motion: Objects moving along paths, leaving traces, showing periodic behavior. Used for astronomical transits, loading indicators, physics demos.

2. Technical Implementation

Transit with Snapshots

venus.add_updater(lambda m, dt: m.shift(dt * velocity * RIGHT))
copies = VGroup()
for _ in range(n_snapshots):
    self.wait(wait_time)
    copies.add(venus.copy().clear_updaters())
self.play(Transform(copies, VGroup(path)))  # Collapse to line

Orbital Motion with Depth Effect

def update_planet(p):
    a = angle.get_value()
    x = 2.5 * np.cos(a)
    y = 0.5 * np.sin(a)  # Compressed y = tilted orbit
    p.move_to([x, y, 0])
    # Size varies with "depth"
    scale = 0.12 + 0.06 * np.sin(a)
    p.set_width(2 * scale)

Phase-Shifted Oscillation (Loading Dots)

for i, dot in enumerate(dots):
    phase = i * TAU / n_dots
    dot.add_updater(lambda m, p=phase: m.set_y(
        original_y + 0.3 * np.sin(3 * time.get_value() + p)
    ))

Pendulum Physics

omega = np.sqrt(g / length)  # Natural frequency
amplitude = PI / 4
theta.add_updater(lambda m: m.set_value(
    amplitude * np.cos(omega * time.get_value()) * np.exp(-0.05 * time.get_value())
))

3. Scene Variants

Scene	Purpose
TransitOfVenus	Historical astronomical transit
OrbitalTransit	Exoplanet-style orbit with depth
LoadingDots	Classic loading animation
WaveTransit	Wave pulse propagation
PendulumSwing	Damped pendulum with trail

Key Patterns

Pattern: Copy and Freeze

copy = mobject.copy().clear_updaters()  # Snapshot current state

Pattern: Continuous Time Updater

time = ValueTracker(0)
time.add_updater(lambda m, dt: m.increment_value(dt))

Run Commands

manimgl transit_animation.py TransitOfVenus -w
manimgl transit_animation.py LoadingDots -w
manimgl transit_animation.py PendulumSwing -w