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Glossary

This glossary defines the terms as they are used in this document. Where a term has a broader meaning in another standard, the definition here is the LunCoSim-specific one.

A–C

Asset address. A logical URI that names bytes managed by the asset system. lunco:// addresses the installed engine library; twin:// addresses a file inside the open Twin. Authored files should not use physical cache paths.

Authority. The right to write a control input or structural fact. Selection is not authority. A session must claim an endpoint before its input reaches it.

Avian. The rigid-body physics engine used by the LunCoSim runtime. Avian integrates bodies and joints; it is not where a particular rover or mission is authored.

Build. The perspective used to compose the live scene from assemblies and entities. It edits the running world, not an isolated document.

Communication point. A simulation time at which Modelica participants exchange inputs and outputs. Values are held between communication points.

Composition arc. A USD relationship that combines layers or prims: subLayers, references, payloads, inherits, specializes, and variants.

Connection. An authored USD relationship that carries a value from an outputs: property to an inputs: property. The runtime derives a SimConnection from it.

Cosimulation. Running two or more participants together on a shared clock, exchanging values at communication points. LunCoSim uses the term for the Modelica-to-physics loop and for the broader participant fabric.

D–H

Default prim. The root prim a USD stage declares as its reference target. LunCoSim uses it as a stable root for composition and projection.

Dock panel. A workbench panel placed in a dock rectangle. Runtime UI surfaces can align to a dock panel but do not own the dock tree.

Editor. The perspective that opens one explicit USD document in an isolated preview. Saving and undo apply to that document, not to the live scene.

Entity. A runtime object projected from the composed scene. An entity has a stable identity, a name, components, and possibly ports. The entity tree is a runtime view, not the authored prim hierarchy.

Exposure. A typed, named snapshot published by an engine producer for presentation consumers. Runtime UI templates and API readers consume exposures; they do not query ECS state directly.

Fixed step. A simulation update at a deterministic interval. The task kernel, physics, and realtime-safe Modelica programs advance on fixed steps.

FMU. A Functional Mock-up Unit packaged for FMI. LunCoSim can import FMI 3.0 Co-Simulation FMUs with scalar Float64 ports on desktop builds.

glTF. An external mesh and scene format. A .gltf or .glb file is referenced from a USD payload or reference and loaded by the glTF importer; it is not a USD layer.

HUD. A heads-up display surface over the viewport. LunCoSim has both workbench HUDs (graphs, performance, input overlay) and authored runtime surfaces (rover, camera, terrain progress).

I–M

Input hold. A live value written to a port by a session. It overrides the authored source until released. SetPorts creates a hold; ReleasePort and ReleaseControl remove it.

Lunica. The Modelica perspective and workbench. It provides the model browser, diagram, text editor, compile and run controls, plots, and experiments.

Manifest. twin.toml, the file that makes a folder a Twin. It declares the project identity, entry scene, source roots, and settings.

Modelica participant. A compiled Modelica class admitted into the running simulation. It has typed inputs and outputs and advances at communication points.

Modelica source root. A directory declared in [modelica].paths where the compiler looks for .mo sources.

N–R

OpenUSD. The scene description and composition standard used for the world. LunCoSim uses standard USD physics schemas and a small set of LunCoSim schema extensions.

Perspective. A task layout over the same Twin. The five main layouts are View, Build, Analyze, Editor, and Lunica.

Port. A typed scalar input or output at a component boundary. A port has a name, direction, kind, and owner. Ports are the vocabulary of the connection fabric.

Prim. A node in a USD stage. A prim can carry schemas, attributes, relationships, and children.

Program. A Modelica model or Rhai scenario attached to a prim through LunCoProgramAPI. The program is a child prim, so deleting the prim removes the behavior.

Projection. The runtime process that turns a composed USD stage into live entities, components, physics bodies, and renderable objects.

Realtime-safe. A declaration (lunco:program:realtimeSafe = true) that a program may drive force or torque ports on a client-predicted body. It is a promise that the program is fixed-step deterministic, not a quality rating.

Rhai. The embedded scripting language used for scenario policy, tutorial flow, and authored tests.

Rumoca. The Modelica compiler and runtime used by LunCoSim. It parses, type-checks, compiles, and integrates the authored models.

S–Z

Scene test. A headless, deterministic run of an authored Rhai observer against a scene. The exit code is the verdict: 0 pass, 1 fail, 2 no verdict.

Shader look. A material intent that names a WESL source, parameter values, and texture layers. It is projected from an authored USD material or created by a tool.

SimConnection. The runtime representation of an authored USD connection. It carries a typed value from a producer to a consumer at a communication point.

Stage. A composed USD scene. The stage is inert until the runtime projects it.

SysML v2. The textual systems-modeling language used for requirements, logical structure, typed values, and verification links. LunCoSim supports a bounded profile; see SysML v2.

Texture layer. A named raster role a shader can sample: Height, Albedo, Mineral, Surface, Normal, ShadowCache, and related roles. Texture slots are the scarce resource; scalar layers should usually share a packed channel.

Twin. A project folder containing twin.toml. It is the unit of saving, versioning, sharing, and opening.

Typed command. A write operation with a reflected name and validated parameters. The UI, Rhai, and HTTP API use the same command surface.

Typed query. A read operation with a reflected name and typed result. A missing value is not silently replaced with zero.

USD. See OpenUSD.

Verdict. The typed result a test scenario publishes. A test that ends without a verdict is not a pass.

WESL. The shader source language used for LunCoSim materials. A WESL material declares its parameters in a struct Material block.

World hold. A runtime state in which simulation advancement is paused because required preparation is incomplete. A world hold is not a slow world.