[Question] Randomize number of spawned objects on reset

[hritik-roy-chowdhury]

Is there a way to randomize the number of, e.g, spawned cubes on a table on event reset? For instance, using the manager-based workflow, typically the first step is to initialize the configurations and spawn methods of each entity in the scene in a separate scene configuration class. However, only these entities are ever interacted with and from what I can see, it is not possible to replicate them.

[RandomOakForest]

Thank you for posting this. It is a great question for our Discussions section. I'll move the post there for follow up. Here is a summary to consider.

To randomize the number of spawned objects, pre-spawn a fixed maximum number of cubes and dynamically activate/deactivate subsets via events. This avoids adding/removing objects during simulation (see #1783) but achieves variability through state toggling.

Implementation Example

Scene Configuration (pre-spawn maximum cubes):

from omni.isaac.lab.assets import RigidObjectCfg
from omni.isaac.lab.managers import SceneEntityCfg

MAX_CUBES = 10  # Maximum cubes per env

cube_cfg = RigidObjectCfg(
    prim_path="/World/envs/env_.*/Cube_.*",
    spawn=usd_utils.FileUsdCfg(usd_path="/path/to/cube.usd"),
    init_state=RigidObjectCfg.InitialStateCfg(pos=(0.0, 0.0, 0.1)),
)
scene_cfg = {"cubes": SceneEntityCfg(cube_cfg, count=MAX_CUBES)}

Event Configuration (randomize active cubes per reset):

from omni.isaac.lab.managers import EventTermCfg
import torch

def randomize_cube_count(env):
    # Randomly select active cubes (0 to MAX_CUBES)
    num_active = torch.randint(0, MAX_CUBES+1, (env.num_envs,))
    
    for env_idx in range(env.num_envs):
        for cube_idx in range(MAX_CUBES):
            # Toggle visibility/collision based on active count
            active = cube_idx < num_active[env_idx]
            env.scene["cubes"].set_visibility(env_idx, cube_idx, active)
            env.scene["cubes"].set_collision_enabled(env_idx, cube_idx, active)

env_cfg.events.randomize_count = EventTermCfg(
    func=randomize_cube_count,
    mode="reset",
    params={}
)

Key Details

1. Fixed Pre-Spawning: All cubes are spawned once during initialization, avoiding runtime object addition/removal.
2. Dynamic Activation: At each reset, randomly enable visibility and collision for a subset of cubes, creating the illusion of variable counts.
3. Physics Consistency: Disabled cubes ignore collisions and rendering, minimizing performance impact.

[kaikwan]

Hi @RandomOakForest ,

Thanks for the answer!

How can I disable collisions dynamically during resets? People in this thread also couldn't figure it out #1736 (comment)

I've tried

prim_path = asset.cfg.prim_path.replace("env_.*", f"env_{cur_env}")

# Modify physics properties and apply pose and velocity

schemas.modify_collision_properties(
    prim_path,
    schemas_cfg.CollisionPropertiesCfg(
        collision_enabled=False,
    ),
)

But I got a bunch of errors

2025-07-08 04:32:24 [9,300ms] [Error] [omni.physx.plugin] PhysX error: PxRigidDynamic::setGlobalPose(): it is illegal to call this method if PxSceneFlag::eENABLE_DIRECT_GPU_API is enabled!, FILE /builds/omniverse/physics/physx/source/physx/src/NpRigidDynamic.cpp, LINE 102
2025-07-08 04:32:24 [9,300ms] [Error] [omni.physx.plugin] PhysX error: PxRigidDynamic::setLinearVelocity(): it is illegal to call this method if PxSceneFlag::eENABLE_DIRECT_GPU_API is enabled!, FILE /builds/omniverse/physics/physx/source/physx/src/NpRigidDynamic.cpp, LINE 239
2025-07-08 04:32:24 [9,300ms] [Error] [omni.physx.plugin] PhysX error: PxRigidDynamic::setAngularVelocity(): it is illegal to call this method if PxSceneFlag::eENABLE_DIRECT_GPU_API is enabled!, FILE /builds/omniverse/physics/physx/source/physx/src/NpRigidDynamic.cpp, LINE 287