User sensor's dim does not change model's signature

[davidhozic]

Intro

Hi!

I am a researcher at University of Ljubljana, I use MuJoCo for my research.

My setup

MuJoCo 3.9.0 (C API), x86, Fedora 44 KDE

What's happening? What did you expect?

Changing the dimension of the mjSENS_USER sensor type does not result in the compiled-model's signature being different. As a result, models with different nsensordata end up having the same signature, despite having a different memory layout.

Steps for reproduction

1. Use different sensor's dim and type of sensor mjSENS_USER.

Minimal model for reproduction

No model needed (actually two different ones are needed for comparisson), specification creation is fine.

Code required for reproduction

Python:

import mujoco as mj


spec = mj.MjSpec()

world = spec.worldbody

sensor = spec.add_sensor()
sensor.type = mj.mjtSensor.mjSENS_USER
sensor.dim = 100
sensor.name = "user_sensor"

m = spec.compile()
print(f"M1 nsensordata: {m.nsensordata}")
model_1_signature = m.signature

spec.delete(spec.sensor("user_sensor"))

sensor = spec.add_sensor()
sensor.type = mj.mjtSensor.mjSENS_USER
sensor.dim = 200
sensor.name = "user_sensor"

m = spec.compile()
print(f"M2 nsensordata: {m.nsensordata}")
model_2_signature = m.signature

assert model_1_signature != model_2_signature

Confirmations

• I searched the latest documentation thoroughly before posting.
• I searched previous Issues and Discussions, I am certain this has not been raised before.

[rajatmohan22]

Ah, I can fix this.
The signature currently hashes only the sensor type, which explains the bug. For native sensors the dimension is implied by type, but user sensors own their dim, so I’m adding dim into the sensor signature entry and a C++ regression test that compiles dim=100 vs dim=200.