pybullet: two (potentially related) issues with restoring sleep state

[mjlbach]

Reproduction

The reproduction scripts, including assets, for the bug are located here:
https://drive.google.com/file/d/1-ym77X1yYny8EzQ-QRBPmW--U6NY4X2U/view?usp=sharing

The reproduction scripts are titled bug_1.py and bug_2.py.

Issue 1 (bug_1.py)

p.saveState on a state containing awake objects does not result in the awake state being restored on p.restoreState. The state of an object on which you apply force must be restored manually by calling p.changeDynamics with the activation state and stepping physics with p.step()

Issue 2 (bug_2.py)

In the same problem setup as 1:

• apply a force on object 3
• it hits object 2
• both objects go to sleep on the floor
• Teleport object 2 far away
• Let both objects go to sleep
• restoreState

After 4 restoreState calls, object 2 will no longer wake up as object 3 approaches, so object 3 passes through object 2 without triggering a collision. The workaround is not to use restoreState, but call resetBasePositionAndOrientation to manually reset the state.

cc @fxia22 @ChengshuLi

[erwincoumans]

Indeed. Save/restore state is not compatible with sleeping/deactivation at the moment. For now, it is best to disable sleeping if you rely on determinism and save/restore state.

[mjlbach]

I see! Thank you so much for clarifying Erwin! Feel free to close the issue, I just wanted to documented/provide a test-case if this was an untracked bug.

[erwincoumans]

You are welcome, and thanks a lot for clear description of reproduction of both bugs! I've been busy with Brax and tiny-differentiable-simulator projects, hope to sometimes come back to PyBullet to improve things like this.