Weird collision behaviors between hfield and sphere (and maybe other geoms) #2883
Description
Intro
Hi!
I am a PhD student at Zhejiang University. I use MuJoCo for my research on legged locomotion.
I am testing bouncing behaviors with MuJoCo. I found that sometimes the sphere gets sucked into the hfield, especially when the sphere is small (e.g. 0.02), has a large timeconst (e.g. 0.04) or a large velocity (e.g. 3m/s).
For example, this model produces the sucking behavior:
<mujoco model="hfield_sphere_h0.5">
<option timestep=".001"/>
<asset>
<hfield name="hf" nrow="10" ncol="10" size="1 1 0.1 0.1"/>
</asset>
<worldbody>
<geom type="hfield" hfield="hf"/>
<body>
<freejoint/>
<geom pos="0 0 0.5" size="0.02 0.02 0.02" type="sphere"/>
</body>
</worldbody>
</mujoco>
suck-2025-09-29_16.48.55.mp4
This model behaves normally:
<mujoco model="hfield_sphere_h0.3">
<option timestep=".001"/>
<asset>
<hfield name="hf" nrow="10" ncol="10" size="1 1 0.1 0.1"/>
</asset>
<worldbody>
<geom type="hfield" hfield="hf"/>
<body>
<freejoint/>
<geom pos="0 0 0.3" size="0.02 0.02 0.02" type="sphere"/>
</body>
</worldbody>
</mujoco>
no-suck-2025-09-29_16.49.32.mp4
After setting timeconst to 0.04, the model also produces the sucking behavior.
<mujoco model="hfield_sphere_h0.3_timeconst0.04">
<option timestep=".001"/>
<asset>
<hfield name="hf" nrow="10" ncol="10" size="1 1 0.1 0.1"/>
</asset>
<worldbody>
<geom type="hfield" hfield="hf"/>
<body>
<freejoint/>
<geom pos="0 0 0.3" size="0.02 0.02 0.02" type="sphere" solref="0.04 1.0"/>
</body>
</worldbody>
</mujoco>
suck2-2025-09-29_16.50.01.mp4
Decreasing the timestep to 1e-5 does not improve the issue.
Changing the hfield to plane definitely solves the issue.
My setup
MuJoCo C 3.3.6
What's happening? What did you expect?
The bouncing issue. Expected to be the same as that of a plane.
Steps for reproduction
Use simulate to run the above models.
Minimal model for reproduction
No response
Code required for reproduction
No response
Confirmations
- I searched the latest documentation thoroughly before posting.
- I searched previous Issues and Discussions, I am certain this has not been raised before.