Scenes

A scene is the static world the vehicle flies in. Pick one by name with nexus run --scene <name>: every run names one, and empty is flat ground. The catalog, nexus_uav/_src/config/catalog.yaml, resolves scenes. A local .usdz path such as --scene /path/to/scene.usdz flies that file directly, which is the scene-development workflow. See Converting scenes.

A scene is data: one self-contained Universal Scene Description (USD) file, ingested the same way as the vehicle USD. The physics adds it to the Newton ModelBuilder. When the vehicle renders, the Kit render peer opens it as its stage, alongside the vehicle. The physics engine takes exactly each prim that authors a UsdPhysics schema, such as the slalom pillars, whose authored physics:collisionEnabled = false makes them cost-only. The renderer shows everything. The USD is the single authority on what it contributes, with no scene kinds, flags, or names in code. A scene type whose world can't live in a stage prim gets live machinery in the Kit peer's program, which claims the scene by its USD content. The cesium globe's token injection and tile streaming live there, in nexus_uav/_src/peers/kit/peer-src/cesium_globe.py.

Any scene flies with any vehicle. Whether it shows depends on the vehicle: a vehicle whose USD authors RTX sensors renders them in the Kit peer, and there the scene is visible. A vehicle with none simulates every physics prim of the same scene without a render.

Scene	Description
empty	Flat ground, no static geometry, the default. A sky-only USD, because every scene carries its own lighting.
slalom	Three obstacle pillars for the sampling Model Predictive Control (MPC) demo, cost-only by authored physics:collisionEnabled = false. The Signed Distance Field (SDF) avoidance cost includes them, and the solver never contacts them. Avoidance is entirely the controller's job.
powerline	An overhead power line on wooden poles beside a road at sunset, 50 m square. The road runs up a smooth 3 m drop and turns 90 degrees. The line follows it past firs, ferns, and boulders. Invisible capsules make the poles, arms and wires collide, while the plants and rocks don't. The benchmark matrix flies an inspection over the line here, see Benchmarking. scripts/assets/author_powerline_scene.py builds it from Poly Haven assets, all CC0, listed with their authors in Scene sources.
cesium	A live-streamed globe of Google Photorealistic 3D Tiles through Cesium for Omniverse. Its origin is near Seattle. An override layer flies it over any location, see Running. Streams only where there is a renderer. Needs CESIUM_ION_TOKEN.

What every scene file carries

The converters and authoring scripts, in scripts/assets/scene_root.py, normalize scene assets onto four conventions:

• One root prim: a plain Xform, the defaultPrim with no transform ops, holding everything. The renderer opens the scene USD as its root stage and places the catalog start with a single translate on this prim, authored on the session layer. That layer is runtime-only, so the asset never changes. The vehicle composes in at the origin.
• Its own sky: a dome plus sun under the root, the dome plain or, as in powerline, carrying a sky-only image. There is no in-code sky: a scene USD is the single authority on its own lighting.
• Its render recipe, when it needs one: stage render settings in the root layer's customLayerData.renderSettings, which Kit auto-applies when the stage opens. The cesium scene uses one for its fog, tone-map, and bloom grade. No applier code.
• Where it sits on Earth: a PhysicsScene prim at /World/PhysicsScene that applies NexusSceneAPI and authors nexus:geodeticOrigin, the latitude, longitude, and altitude of the world origin, which is the start point. It anchors the sensors, the Global Positioning System (GPS) among them, and a Cesium globe. A run fails on a scene whose PhysicsScene authors no origin, and an override layer moves it. The prim sits under /World, never at /PhysicsScene, where a vehicle keeps its own, so a layer that moves the origin composes over the scene alone. For the cesium globe the altitude is the WGS84 ellipsoidal height of the street. For a flat scene it's the height over mean sea level.

The cesium scene is exempt from the root-prim rule: its geographic reference anchors it, with no start, and each /Cesium* prim lives at an authored absolute path. Its /World holds only its PhysicsScene.

Placement: start

The converters normalize scene assets to Z-up meters with the XY origin at the center of the scan's bounding box. The asset doesn't bake in where the drone starts. The catalog field start places a scene:

• start: [x, y, z]: the scene-frame point the runtime moves to the world origin, so the drone, placed at the origin on the z=0 physics ground, starts on that surface. scripts/assets/spawn_site.py suggests one. Tweaking it takes a catalog edit, no reconversion. The physics ground is an endless plane at the start's height, so no ground a vehicle touches can lie below the start: a scene with a slope starts at its foot.

Scene sources

The powerline scene's Poly Haven assets and their authors:

• modular_electricity_poles, by James Ray Cock.
• Rob Tuytel and Rico Cilliers made fir_tree_01 and fern_02.
• pine_roots, tree_stump_01, dead_tree_trunk and aerial_grass_rock, by Rob Tuytel.
• boulder_01 and dry_branches_medium_01, by Rico Cilliers.
• rock_moss_set_02, by Kless Gyzen.
• asphalt_track, by Dimitrios Savva.
• belfast_sunset_puresky, the sky, by Greg Zaal, Dimitrios Savva, and Jarod Guest.