actuator and sensor delays

[thowell]

actuator and sensor delays

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MuJoCo vs MuJoCo Warp: Delay Implementation Differences

API Signature Differences

All shape changes are driven by GPU batching over nworld parallel worlds.

Parameter	MuJoCo C	MuJoCo Warp	Reason
values	1D (nsample,)	2D (nworld, nsample)	Per-world initial values
phase	scalar float	1D (nworld,)	Per-world phase
time	scalar float	1D (nworld,)	Per-world simulation time
result	scalar return / output param	(nworld,) or (nworld, dim)	One result per world
interp	default -1	no default	Explicit specification required

Sensor Computation: Conditional vs Always-Compute

MuJoCo C: Conditional Computation

Each sensor goes through compute_or_read_sensor(), which skips computation for delayed sensors:

• delay > 0: reads directly from history buffer, never calls mj_computeSensor
• interval > 0, condition not met: reads stale value from buffer
• interval > 0, condition met: computes fresh value

MuJoCo Warp: Always Compute, Then Overwrite

All sensors are computed unconditionally via a single kernel launch, then apply_sensor_delay() runs as a second pass:

1. Compute all sensor values (including delayed ones)
2. Insert fresh values into history buffers
3. Overwrite sensordata with delayed values from history

Justification

GPU kernels execute uniformly across all threads — per-sensor branching (compute vs skip) creates warp divergence and is inefficient. Computing everything then overwriting is the standard GPU tradeoff: redundant computation is cheaper than divergent control flow. The final result is identical.

Public API Functions

MuJoCo C	MuJoCo Warp	Description
mj_readCtrl(m, d, id, time, interp)	read_ctrl(m, d, ctrlid, time, interp, result)	Read delayed ctrl for 1 actuator
mj_readSensor(m, d, id, time, buf, interp)	read_sensor(m, d, sensorid, time, interp, result)	Read delayed sensor for 1 sensor
mj_initCtrlHistory(m, d, id, times, values)	init_ctrl_history(m, d, ctrlid, times, values)	Initialize actuator history buffer
mj_initSensorHistory(m, d, id, times, values, phase)	init_sensor_history(m, d, sensorid, times, values, phase)	Initialize sensor history buffer