[OVPHYSX] Improve USD integration to avoid disk dumps

[AntoineRichard]

Summary

Replace the disk-based USD export in `OvPhysxManager` with an in-memory transfer. Currently the USD stage is exported to a temp file and reloaded — this disk round-trip is unnecessary overhead.

Current Flow

1. `stage.Export(tmp_path)` — write USD to disk
2. `physx.add_usd(tmp_path)` — read USD from disk

Desired Flow

1. `stage.ExportToString()` — serialize to string in memory
2. `physx.add_usd_from_string(string)` — load directly from memory

OVPhysX Blocker for @marcodiiga

Add `physx.add_usd_from_string()` (or `add_usd_from_bytes()`) to the ovphysx Python API. The C++ side likely already parses USD from a buffer — this just needs a Python binding.

IsaacLab-Side Changes

~10 lines in `ovphysx_manager.py` — replace file export/load with string export/load, remove temp file cleanup.

Spec

Full design spec: `docs/superpowers/specs/2026-04-20-ovphysx-usd-integration-design.md`

Parent issue: #5315

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📋 Full Design Spec (click to expand)

OVPhysX USD Integration Improvement — Design Spec

Issue: #5324 — [OVPHYSX] Improve USD integration to avoid disk dumps
Date: 2026-04-20
Status: Draft

Summary

Currently, OvPhysxManager exports the USD stage to a temporary file on disk and then loads it back into the ovphysx PhysX instance. This disk round-trip is unnecessary overhead and introduces temp file management complexity. The goal is to pass the USD stage directly to ovphysx without writing to disk.

Current Problem

From OvPhysxManager._warmup_and_load():

1. The in-memory USD stage is exported to a temp .usda file via stage.Export()
2. ovphysx loads the file via physx.add_usd(temp_path)
3. The temp file persists until process exit

Issues:

• Performance: Disk I/O is slow for large scenes (many environments, complex assets)
• Reliability: Temp file may not be cleaned up on crash
• Containerization: Some container environments have limited temp storage
• Security: USD file on disk may contain sensitive scene data

Desired State

Pass the USD stage directly to ovphysx via an in-memory API, eliminating the disk round-trip entirely.

Approach Options

Option A: In-memory USD string (ovphysx API addition)

Add an ovphysx API that accepts a USD string/bytes instead of a file path:

# Current
physx.add_usd("/tmp/exported_scene.usda")

# Proposed
usd_string = stage.ExportToString()
physx.add_usd_from_string(usd_string)

Or using flatbuffers/binary format:

usd_bytes = stage.Flatten().ExportToString()  # or binary format
physx.add_usd_from_bytes(usd_bytes)

Option B: Shared memory / file descriptor

Use a file descriptor or shared memory segment instead of a named file:

import io
buffer = io.BytesIO()
stage.Export(buffer)  # if pxr supports stream export
physx.add_usd_from_fd(buffer.fileno())

Risk: USD's Export() may only support file paths, not streams.

Option C: Direct scene graph transfer

Instead of going through USD serialization at all, transfer the scene graph directly:

• Iterate prims in the USD stage
• Call ovphysx APIs to create physics objects directly
• Skip serialization entirely

Risk: High complexity, essentially reimplementing USD loading in Python.

Recommendation

Option A (in-memory USD string) is the cleanest. It requires @marcodiiga to add add_usd_from_string() or similar to the ovphysx Python API. The IsaacLab side change is minimal (replace stage.Export(path) + physx.add_usd(path) with stage.ExportToString() + physx.add_usd_from_string(string)).

Blocker for @marcodiiga: Add an in-memory USD loading API to ovphysx. The C++ side likely already parses USD from a buffer — this just needs a Python binding that passes the buffer directly instead of reading from a file path.

Implementation Steps

1. @marcodiiga adds physx.add_usd_from_string() to ovphysx wheel
2. Update OvPhysxManager._warmup_and_load():
   • Replace stage.Export(tmp_path) with stage.ExportToString()
   • Replace physx.add_usd(tmp_path) with physx.add_usd_from_string(usd_string)
   • Remove temp file creation and cleanup
3. Remove the atexit temp file cleanup handler

Dependencies

• Requires ovphysx API addition (add_usd_from_string or equivalent)
• Requires PR ovphysx backend integration #4852 merged
• Independent of all other specs

Estimated Scope

• IsaacLab-side changes: ~10 lines in ovphysx_manager.py
• ovphysx-side: new Python binding for in-memory USD loading