[Question] Terrain curriculum issue: Terrain level does not exceed 6

[H-Hisamichi]

Question

Hello everyone,

I have been trying to resolve an ongoing issue where the terrain level does not go above 6 when using the terrain curriculum. This problem occurs not only in my custom environment but also in the default RL demo.

In Fig. 1, the black line represents the terrain level progression for "Isaac-Velocity-Rough-Anymal-D-v0" using the default parameters. The red line shows the progression when the terrain sampling range is halved. As you can see, the level converges to 6 regardless of the terrain's difficulty. Furthermore, the graph does not look like simple hard clipping; rather, it exhibits a curve similar to a PD controller's response.

Fig. 1:

I have reviewed the terrain_levels_vel implementation but could not find any incorrect processing. Considering the possibility of the RL agent falling into a local optimum, I also tried adjusting PPO parameters such as entropy_coef and learning_rate, but observed no improvement.

Is it the expected behavior for the terrain level to be restricted to a maximum of 6 in the terrain curriculum?

Thank you in advance for your insights.

Build Info

Describe the versions that you are currently using:

• Isaac Lab Version: [2.3.1]
• Isaac Sim Version: [5.1.0]

[RandomOakForest]

Thank you for posting this. I moved your post to our Discussions for follow-up. You are not misusing the curriculum, but its current design makes mid‑level saturation a very plausible outcome—so your observation is consistent with known behaviour, even though it is not the ideal target behaviour.

Practical workarounds you can try now

If you want terrain difficulty to keep increasing beyond level 6 in your experiments, consider:

• Increasing commanded speeds and biasing command sampling toward forward heading velocities so the robot reliably exits its current grid cell during episodes.
• Reducing episode length or reset conditions that bring the robot back near the origin before it has traversed enough distance to trigger a level increase.
• Implementing or swapping in a custom curriculum function that:
  • Uses velocity tracking error or success metrics instead of L2 distance, or
  • Uses cumulative signed distance along commanded heading instead of absolute position, so oscillating commands don’t cancel out progress.

These do require editing or replacing terrain_levels_vel in your task config but do not require changes to PPO itself.