Assemble or edit LunCoSim's 3D world: load scenes, spawn objects, place, move, rotate, scale, tune, or clear them. USE THIS SKILL for requests such as "put a lander near that crater", "spawn rovers", "load the Moon scene", "add rocks", "move this", "set its mass or material", or "build a scene with X and Y". For the agent mid-code: `LoadScene`, `SpawnEntity`, `MoveEntity`, `TransformEntity`, `SetObjectProperty`, a catalog `entry_id`, a `.usda` file, or a placement/ lighting issue. Project-specific: USD is the source of truth projected to ECS; use the fixed Y-up, right-handed, -Z-forward metre frame, root-qualified `lunco://` or `twin://` paths, and catalogued spawnables. Do not use `SetDocumentSource` for live edits. Use author-scenario for behavior and authoring-vessel-controllers for GNC. Use edit-usd-assembly for editing the reusable rover, lander, payload, or sensor assembly itself in a visible Editor session.
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name
build-usd-scene
description
Assemble or edit LunCoSim's 3D world: load scenes, spawn objects, place, move, rotate, scale, tune, or clear them. USE THIS SKILL for requests such as "put a lander near that crater", "spawn rovers", "load the Moon scene", "add rocks", "move this", "set its mass or material", or "build a scene with X and Y". For the agent mid-code: `LoadScene`, `SpawnEntity`, `MoveEntity`, `TransformEntity`, `SetObjectProperty`, a catalog `entry_id`, a `.usda` file, or a placement/ lighting issue. Project-specific: USD is the source of truth projected to ECS; use the fixed Y-up, right-handed, -Z-forward metre frame, root-qualified `lunco://` or `twin://` paths, and catalogued spawnables. Do not use `SetDocumentSource` for live edits. Use author-scenario for behavior and authoring-vessel-controllers for GNC. Use edit-usd-assembly for editing the reusable rover, lander, payload, or sensor assembly itself in a visible Editor session.
Build & edit USD scenes
The 3D world is OpenUSD, projected to Bevy ECS — USD is the source of truth,
the ECS scene is its projection. You build the world by authoring USD (via
commands that apply reversible ops), not by mutating ECS directly. Drive it over
the API (--api, port 4101; launch per test-via-api).
This skill owns mounting and scene placement. For authoring the reusable
assembly itself, switch to the interactive Assembly Editor runbook,
which keeps a headful preview visible and routes every edit through the
document/journal command boundary.
Before assembling a scene, choose its world/time contract. The complete option
matrix is in assets/tutorials/README.md;
the short version is below.
Choose the scene's lighting and time model
Scene contract
Author
Use it when
Fixed instructional world
A real DistantLight reference such as lunco://lighting/sun.usda, with an authored rotation; omit LunCoEpochAPI and SolarSystem.
Teaching UI, spawning, or basic controls where changing sunlight is not the subject.
Ephemeris world
Apply LunCoEpochAPI, author double lunco:time:epochJd = …, and reference lunco://celestial/solar_system.usda under SolarSystem; author the site anchor on the scene root when needed.
Teaching a real lunar day, Earth tracking, orbital motion, or any feature whose result depends on celestial time.
Existing world
Reference or payload the authoritative scene that already owns gravity, lighting, time, and celestial content.
Adding a lesson or assembly whose subject is behaviour, not scenery.
UI-only lesson
Omit the payload; the tutorial launcher clears an outgoing lesson scene before showing the UI-only lesson.
Teaching menus, commands, or workbench concepts.
Do not combine a fixed light with an implicit orbital provider. LunCoEpochAPI
without an authored lunco:time:epochJd is a lint error (epoch-api-missing-time):
set the epoch on the same scene root, or remove the celestial opt-in. A fixed
light is a complete scene contract, not a temporary fallback.
For tutorial payloads, use the fixed contract for onboarding scenes such as
first_drive.usda, and the ephemeris contract for driving_basics.usda and
slope_test.usda. rover_variants.usda reuses driving_basics.usda, while
the lander mission reuses scenes/luncosim/lander_ops.usda; do not copy their
environment or silently choose a second clock.
The one coordinate frame (spec 009)
The engine runs in one fixed canonical frame: Y-up, right-handed, −Z-forward,
SI metres, f64. Any external asset (USD upAxis/metersPerUnit, glTF, Blender)
is converted once, at the importer — never branch on convention in your own
placement math. A position you pass to SpawnEntity is Y-up metres.
The command surface
Command
Params
Does
LoadScene
{path, root_prim}
Load a USD scene. path is a root-qualified lunco://… or twin://… address. root_prim empty = the stage's defaultPrim.
ClearScene
{}
Tear down the current scene.
RestartScene
{}
Reload/reset the current scene.
SpawnEntity
{target, entry_id, position:[x,y,z], rotation?}
Instance a catalogued prefab. entry_id comes from the spawn catalog (list_bundled / ListBundled).
MoveEntity
{…}
Reposition an existing entity.
TransformEntity
{entity_id, translation, rotation}
Set an existing entity's complete active-frame pose as one undoable USD edit.
SetObjectProperty
{entity_id:u64, property, value}
Set a named property (both strings; value is coerced by property type).
SelectEntity
{…}
Select (drives the gizmo/inspector).
SetPorts
{target, writes:[[name,val]]}
Poke an input port (e.g. drive a spawned rover) — see author-scenario for behaviour.
Discover the live set with DiscoverSchema; discover spawnables with list_bundled.
Recipe
Base:LoadScene {path:"lunco://scenes/…/foo.usda", root_prim:""} for an existing
scene, or start from the loaded default and add to it. ClearScene first if
replacing.
What can I spawn?list_bundled → pick an entry_id.
Place it:SpawnEntity {entry_id, position:[x,y,z], rotation?} (Y-up metres).
The response data carries the new entity id.
Adjust:MoveEntity or TransformEntity / SetObjectProperty (colour, mass, material, scale) /
SelectEntity to inspect.
Confirm:CaptureScreenshot → target/x.png → Read it (see
inspect-simulation for reading state back).
Persist: to make it permanent, author it into the .usda scene file under
assets/scenes/ (the runtime edits are USD ops; save them into the layer).
Verify the contract: run target/debug/luncosim --validate <scene.usda>;
for a composed world, run the authored Rhai scene gate and inspect its
verdict. --validate catches parse/composition/lint failures but does not
prove that the light remains stable during runtime.
Gotchas
Before typing lunco:, name the standard field this would be. If USD owns the concept, use its schema. A vendor namespace is correct only for semantics USD does not define, and it must cover only that new concept. For a parametric surface of revolution, author only the LunCo-specific profile/shape fields; patch sampling and degree belong to the standard patch attributes.
A Modelica program under a child Scope has no script-readable ports through the owner hierarchy. Apply LunCoProgramAPI to the entity whose script reads the outputs, or connect the child program's outputs explicitly by USD prim path. The restriction is on script traversal, not on USD port connections.
A moving part is a JOINT, not a script that rewrites a transform. Author a UsdPhysics joint between two bodies and let the solver provide contact, limits, reaction forces, and joint ports. Do not duplicate dimensions or motion state in a script.
A commandable sprung mechanism is a JOINT DRIVE — author it, then read it back. Apply UsdPhysicsDriveAPI:linear and author physics:type = "force" with drive:linear:physics:stiffness (N/m) and :damping (N·s/m). The canonical USD-to-Avian reader preserves that SI force law and uses Avian's implicit SpringDamper realization when the driven body has a positive authored mass; it derives equivalent frequency and damping ratio for stable integration, without creating a second spring or changing the authored coefficients. Its stroke and its load then come off the joint's own displacement and force ports. No Modelica model and no rhai script restates the spring — a second spelling of one spring puts two writers on one fact, and which wins becomes a function of load order. Author physics:type explicitly: the coefficients mean newtons under "force", while an "acceleration" drive is mass-normalized and has no honest newton readback at all, so its force port reads nothing. Missing mass, angular coefficient drives, and negative coefficients are not repaired by a fallback; the USD linter reports the invalid or conditionally unstable authoring.
A physical landing member uses the standard joint drive. Apply PhysicsDriveAPI:linear to a PhysicsPrismaticJoint; author physics:type = "force" with stiffness, damping, and . The native Avian prismatic joint is the sole axial mechanism, and its measured and output-only are the public state. Do not duplicate standard fields under ; missing or invalid fields fail projection and are never replaced by a target, force cap, or solver-resolution workaround.
Anti-patterns
❌ Passing a bare or absolute filesystem path to LoadScene.
❌ Guessing an entry_id instead of list_bundled.
❌ SetDocumentSource to build a scene incrementally — use the typed scene commands and their USD operation path.
❌ Branching placement math on up-axis/units — the frame is fixed; convert only at the importer.
❌ Mutating ECS Transform directly and expecting USD to remember it — author the USD.
maxForce
displacement
force
lunco:*
The joint's axis carries the SIGN, and it is the only place that does. For a drive, force = stiffness * (targetPosition - displacement) under the authored force convention. A landing leg's compression must read NEGATIVE displacement, which fixes the axis to point the way the mechanism EXTENDS. physics:axis can only name "X"|"Y"|"Z", so a raked or reversed axis is carried by physics:localRot0 (quaternions are (w, x, y, z)), and the limits follow it: a landing leg is lowerLimit = -stroke, upperLimit = 0, rest at 0. Get this right in the joint and nothing downstream needs a sign fixup; get it wrong and every consumer grows one.
Wire a physical part to PHYSICS, and flight software to SENSORS. Contact, contact force, position and velocity are collider/body ports — they exist because the thing exists, with nothing to author. Sensors (lunco:sensor:range / :imu / :contact) are authored INSTRUMENTS that read those physics and add mount offset, range limits and out-of-range behaviour; they are what a GNC model should see, because a computer knows only what its instruments report. Getting it backwards is not a style question: gate a landing leg's behaviour on the ALTIMETER — whose datum sits above the pads — and a hand-copied constant has to restate that offset, lighting the legs before touchdown. A constant in a .mo that exists only to translate between two prims' positions means the wire is wrong. (USD has no standard sensor schema at all — core UsdPhysics stops at bodies/colliders/joints, and Omniverse invents its own too: PhysxContactReportAPI, IsaacContactSensor. lunco:sensor:* is the legitimate vendor-namespace case.)
Publish the physical quantity, not the driving term. A strut that reports the force pressed onto it reads fully loaded while it is still in the air; the honest number is the spring's own reaction, which is exactly zero until compression starts. Take it from the joint that integrates the spring — PrismaticJoint's force port — rather than re-deriving it. When a visualization "happens too early", suspect something is publishing an input rather than a result.
Bevy renders an axis-Y Cone with its APEX UP. Verify the authored axis and orientation in a render before adding a corrective rotation.
rhai has no float pow. Exponentiation is registered under the OPERATOR name ** only (packages/arithmetic.rs), so pow(x, 0.7) throws Function not found: pow (f64, f64) every tick — and because a scenario's error is per-tick and non-fatal, the rest of that function silently never runs. Use x ** 0.7.
DUPLICATE NAMES ARE SILENT — check them before debugging unrelated rendering. Two prims with the same name in one parent, or the same property authored twice on one prim, can be accepted without a diagnostic and change the composed result. Search the parent scope and property name before investigating shaders.
A procedural camera background is not a sphere. Author the existing
environment/starfield_sky.usda pattern: an Xform with
MaterialBindingAPI and bool lunco:surface:skybox = true. USD projection
stamps ProceduralSkybox and the renderer uses its fullscreen background
pass, so there is no radius, culling volume, collision surface, or mesh
vertex shader to maintain. Use UsdLuxDomeLight for a textured environment
light; it is a separate lighting contract.
Custom-shader inputs are snake_case — the ShaderMaterial reflection binds the WGSL struct's field names (star_density, point_size, brightness). A camelCase inputs:starDensity is a dead wire: no error, no effect, and hours of "why does tuning the sky do nothing".
Exposure and illuminance only mean something together. The frame's brightness is illuminance / 2^EV100, so a scene that copies a DistantLight intensity from one file and an exposureEv100 from another lands stops away from either. Author both on purpose: the sun prim's inputs:intensity and the LunCoEnvironment prim's lunco:env:exposureEv100.
Celestial time is an explicit scene choice. A SolarSystem reference makes body poses ephemeris-driven; LunCoEpochAPI makes the scene responsible for choosing the mission epoch. Author both the API and lunco:time:epochJd together. If the lesson needs repeatable light but not astronomy, use the fixed DistantLight contract instead.
LoadScene path must be root-qualified — use lunco://scenes/luncosim/lander_ops.usda for a shipped asset or twin://<name>/… for an opened Twin. Use OpenFile for a filesystem path.
Spawn entry_id must be in the catalog — an unknown id logs unknown entry '…' and no-ops. List first with list_bundled.
Empty spawn path / root_prim → the defaultPrim sentinel: an empty path means "the stage's default prim". A stage without defaultPrim is an invalid scene mount and fails visibly.
Spawns land ON the terrain surface. Placement samples the terrain height oracle (analytic, so it works even before a streamed/CDLOD collider tile bakes) — a spawn over un-baked terrain rests on the ground instead of free-falling. The GUI click path terrain-fits the asset's composed UsdPhysics collision footprint (slope-aligned, and it considers a physics obstacle under the chassis). An asset without a defaultPrim or collision footprint is rejected visibly; placement never invents dimensions or a lift. The API SpawnEntity path uses the explicit position supplied by the caller, so pass a real Y.
One spawn = one entity. In a single-player (Standalone) session a SpawnEntity instantiates exactly one rover; it is not also re-projected from the document (that path is suppressed to avoid a double-instantiation / vanish-on-reload).
Gizmo / selection frame: on a static-USD select, the selectable root is tagged SelectableRoot in the world frame — not GridAnchor. If the gizmo grabs the wrong thing or the wrong frame, that tag is why.
Never SetDocumentSource for live scene building — it replaces the whole source and cancels in-flight work. Submit typed scene commands (SpawnEntity/MoveEntity/TransformEntity/SetObjectProperty), and let their handlers lower the intent through the existing USD operation path.
USD → ECS is a projection, so authored changes flow one way — edit the USD (via ops), and the ECS scene reconciles. Don't hand-mutate ECS transforms expecting them to persist.
