Object Placement Instability in SimplerEnv Multi-Object Scenes

[MstarLioning]

Dear SimplerEnv Authors,

We are encountering object placement instability when creating custom multi-object scenes using sapien.Pose. Objects placed at identical coordinates show inconsistent stability across episodes. We would appreciate guidance on proper multi-object placement design and improvement suggestions.

Problem Summary

Issue: Identical objects at same positions sometimes remain stable, sometimes fall through table

Reproduction Steps

def _initialize_actors(self):
    """Improved stable object placement algorithm - resolving object dropping issues"""
    
    # Predefined safe positions - manually specified known safe positions  
    safe_positions = [
        (-0.30, 0.10), (-0.25, 0.10), (-0.20, 0.10),
        (-0.30, 0.20), (-0.25, 0.20), (-0.20, 0.20),
        # ... additional positions
    ]
    
    # Stage 1: Locked placement with motion constraints
    for i, obj in enumerate(all_objects):
        x, y = safe_positions[i]
        z = self.scene_table_height + object_specific_height
        
        obj.set_pose(sapien.Pose([x, y, z], euler2quat(0, 0, 0)))
        obj.lock_motion(0, 0, 0, 1, 1, 0)  # Lock x,y rotation only
        obj.set_damping(object_specific_damping, object_specific_damping)

Questions

1. Are there recommended physics parameters per object type for stable placement?
2. What is the proper settling time after object placement?
3. Should we use different placement strategies for different object shapes?
4. Is there a deterministic way to ensure consistent placement across episodes?

Thank you for your excellent work and any guidance you can provide.

Best regards

[xuanlinli17]

All "strange" object movements are definitely caused by penetration due to initialization mistakes, especially if something falls through the table. Remove visual matching (i.e., render raw sim backgrounds) by passing in rgb_overlay_path=None in env creation to see what happened.