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Three-Body Lab is an interactive browser simulator of the classical three-body problem. It uses the same dark sidebar-and-canvas layout as the Physics Playground.

Configurations

• Figure-Eight: a stable periodic orbit where three equal masses share one curve (Moore 1993, Chenciner & Montgomery 2000).
• Lagrange Triangle: an equilateral arrangement that rotates rigidly. It is unstable with equal masses and stable when one mass dominates.
• Butterfly I: an unstable periodic orbit found by Šuvakov & Dmitrašinović in 2013.
• Pythagorean: Burrau’s problem. Masses 3, 4 and 5 start at rest, go through chaotic close encounters, and one body is ejected.
• Hierarchical: a star, planet and moon, showing how a close pair with a distant third stays stable.
• Random Start: random initial conditions that usually turn chaotic.

Features

• Ghost twin: a copy of the system nudged by a tiny amount (1e-6 by default). A log-scale graph tracks how fast the two copies diverge, which shows chaos directly.
• Edit mode: pause the simulation, then drag bodies to move them and drag the arrow tips to set their velocities.
• Controls: speed, trail length, gravitational softening, twin nudge size, and a mass multiplier for each body.
• Live readouts: time, energy and its conservation error, angular momentum, closest separation, steps per frame, and twin separation.
• Navigation: the view auto-fits by default. You can also zoom with the scroll wheel and pan by dragging empty space. Bodies that leave the screen get an edge arrow with their distance.
• Shortcuts: Space to play or pause, R to reset.

Physics: the simulator uses Newtonian gravity with G = 1 and a leapfrog integrator. The time step shrinks automatically when bodies come close. A short note in the sidebar explains the history: Poincaré showed that no general closed-form solution exists, so the problem can only be solved numerically.

Microduck Simulator is an interactive 3D simulator of Pollen Robotics’ Microduck, a bipedal duck robot about 25 cm tall and 800 g. It is built on specifications taken from the official microduck runtime and microduck_rl training repositories.

Real robot details used

• Servos: 15 Dynamixel XL330s. IDs are 20–24 for the left leg, 30–34 for the neck, head and beak, and 10–14 for the right leg. The beak (ID 34) is excluded from every policy’s actions.
• Pose and limits: the actual HOME pose and joint limits. Sliders use the real robot convention (radians, signs as in HOME).
• Policy loop: runs at 50 Hz with a 61-D observation (angular velocity, gravity, joint positions, joint velocities, last action, commands) and a 14-D action, both shown live as a heatmap.
• Training settings: velocity command ranges of vx ±0.4 m/s, vy ±0.3 m/s and ωz ±1.0 rad/s; an air-time reward window of 0.125–0.3 s; 2 cm foot clearance; battery voltage from 6.5 to 8.2 V; ±1° backlash; and a ±0.3 m/s push range.
• Policy names: the top bar shows the real task ID for each mode, such as Mjlab-Velocity-Flat-MicroDuck.

Behaviours

• Walk: drive with the on-screen joystick or W A S D / Q E, and adjust body pose and head commands. Lengthening the gait period pushes air time out of the reward window.
• Roller skates: passive wheels under the feet and a swizzle glide with no foot lift.
• Sit · stand (Y): switched by a posture-flag command.
• Beak pick (G): the phase-encoded command can be seen circling in the observation panel. The duck can pick up the block and carry it while walking.
• Ball kick (K / L): shifts weight onto the support foot, then kicks a 70 mm ball.
• Forward roll (R): rolls over the head and lands back on its feet.
• Falls and stand-up: a push that moves the capture point past 62 mm knocks the robot over, and it gets back up by itself.
• Joint control: sliders for all 15 servos, plus the SIT and FOLD keyframes as presets.
• Quack (Space): opens the beak and plays a sound.

Panels

• Live readout: state, commanded and actual speed, trunk tilt, support, capture point and battery.
• Policy I/O: the 61-D observation and 14-D action.
• Foot contact: a contact timeline and the last air time for each foot.
• Servo table: target, actual and error for all 15 servos.
• Controls and overlays: camera presets, a support-polygon and centre-of-mass overlay, backlash and battery settings, and time scale.

Limitations: the motion comes from a kinematic generator (gait phases plus inverse kinematics), not from the trained ONNX policy or MuJoCo physics. Leg geometry is approximate, so poses can differ somewhat from the real robot, especially when the real SIT and FOLD keyframe values are applied directly.

Verified: headless Chromium ran every mode without script errors. Walking air time was 180 ms, inside the reward window. Push → fall → stand-up, block pickup and ball kick all worked, and there is no horizontal scroll at phone width.

Sources:

• pollen-robotics/microduck
• pollen-robotics/microduck_rl

This is an interactive laboratory designed to help users explore and understand charge-carrier flow inside semiconductors from multiple perspectives.

• Electron Flow — Visualizes the movement of electrons and holes, including thermal motion, drift, and scattering, in real time using Canvas.
• 4 Visualization Modes — Explore semiconductor behavior through Carrier Drift, Crystal Lattice, Thermal Motion, and Electric Field views.
• Material Models — Compare Intrinsic, N-type, and P-type semiconductors.
• Energy Bands — Visualize the concepts of the Valence Band, Band Gap, and Conduction Band.
• P–N Junction — Explore P-type and N-type regions, the depletion region, and the basic concept of electrical bias.
• MOSFET Channel — Follow the process Gate Voltage → Electric Field → Channel Formation → Carrier Drift → Drain Current.
• Concept Map — Understand the relationship Crystal Lattice → Energy Bands → Charge Carriers → Electric Field → Electrical Current.
• Interactive Parameters — Directly adjust Electric Field, Carrier Density, Temperature, Scattering, and Hole Fraction.
• Simulation Controls — Experiment with Inject Carriers, Pause, Reverse Field, and Reset.
• Real-Time Telemetry — Monitor Electrons, Holes, Drift Index, Scatter Events, and Current Trend as the simulation runs.
• Electron vs. Conventional Current — Visually observe the difference between the electron drift direction and the conventional current direction.
• Educational Conceptual Simulator — This is not a TCAD or SPICE tool for real semiconductor process or device design. It is designed as an educational conceptual simulator for understanding the fundamental principles of semiconductor charge transport.

Reed–Solomon Lab is an interactive browser simulator of Reed–Solomon error correction. It runs a real encoder and decoder over GF(2⁸), and every step of the repair updates live as you damage bytes.

Pipeline (top to bottom)

• Status and budget bar: shows how much of the correction budget 2e + s ≤ n − k is used. Errors cost 2 slots and erasures cost 1. The result is labeled Clean, Corrected, Decoder failure or Miscorrected.
• Step 1 – Encode: message bytes plus parity bytes form the codeword. The generator polynomial g(x) is shown.
• Step 2 – Channel: click received bytes to add errors (random values) or erasures (flagged positions). You can also add random hits or bursts.
• Step 3 – Syndromes: non-zero syndromes show that damage was detected.
• Step 4 – Berlekamp–Massey: computes the error locator Λ(x), plus the erasure locator Γ(x) when erasures are present.
• Step 5 – Chien search: a bar chart of |Λ| per byte position. Roots, which mark the error locations, are highlighted green.
• Step 6 – Forney: a table of error values and the XOR fix applied at each position.
• Step 7 – Decoded: repaired bytes are highlighted, and the sent, received and decoded text are compared side by side.

Scenarios: Sandbox, QR Code 1-M RS(26,16), Audio CD C2 RS(28,24), DVB-T RS(204,188) and Deep Space RS(255,223). Each comes with notes on how that code is used in practice. You can also edit the message and the number of parity symbols.

Verified: 4,000 random fuzz tests within the correction limit all decoded exactly. Damage beyond the limit never crashes the decoder; it is reported as a failure or a miscorrection.