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Methods for modifying model parameters at runtime. Useful for domain randomization in RL training.

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setGravity(g)

Change the gravity vector. 
api.setGravity([0, 0, -9.81]);    // Earth gravity (Z-up)
api.setGravity([0, 0, -1.62]);    // Moon gravity
api.setGravity([0, 0, 0]);        // Zero gravity
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g
[number, number, number]
required
Gravity vector in m/s^2.

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setTimestep(dt)

Change the simulation timestep. 
api.setTimestep(0.001);  // 1ms (more stable, slower)
api.setTimestep(0.005);  // 5ms (faster, less stable)
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dt
number
required
Timestep in seconds.
Changing the timestep affects simulation stability. Smaller values are more stable but slower. Large values may cause the simulation to explode.

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setBodyMass(name, mass)

Change a body’s mass. 
api.setBodyMass("block", 0.5);   // 500g
api.setBodyMass("block", 2.0);   // 2kg
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name
string
required
Body name.
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mass
number
required
New mass in kg.

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setGeomFriction(name, friction)

Change a geom’s friction coefficients. 
api.setGeomFriction("floor", [1.0, 0.005, 0.0001]);  // High friction
api.setGeomFriction("ice", [0.1, 0.001, 0.00001]);    // Low friction
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name
string
required
Geom name.
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friction
[number, number, number]
required
MuJoCo friction coefficients: [slide, torsional, rolling].

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setGeomSize(name, size)

Change a geom’s size. 
api.setGeomSize("box", [0.05, 0.05, 0.05]);  // 10cm cube
api.setGeomSize("ball", [0.03, 0, 0]);        // 3cm radius sphere
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name
string
required
Geom name.
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size
[number, number, number]
required
New size (semantics depend on geom type — see MuJoCo docs).

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Example: Domain Randomization

function DomainRandomizer() {
  const { api } = useMujoco();

  function randomize() {
    // Randomize gravity
    const gz = -9.81 + (Math.random() - 0.5) * 2;
    api.setGravity([0, 0, gz]);

    // Randomize object mass
    api.setBodyMass("cube", 0.05 + Math.random() * 0.2);

    // Randomize floor friction
    const friction = 0.5 + Math.random() * 1.0;
    api.setGeomFriction("floor", [friction, 0.005, 0.0001]);

    // Reset to apply changes
    api.reset();
  }

  return <button onClick={randomize}>Randomize</button>;
}