Gravitas

Deterministic fixed-point physics for lockstep games and simulations.

Gravitas gives .NET simulations a physics runtime that can replay the same inputs into the same results. It combines fixed-point 2D, 3D, and explicit mixed-dimension physics with context-owned worlds, stable collision ordering, continuous collision detection, queries, constraints, replay hashes, and renderer-neutral diagnostics.

Why Gravitas?

• Lockstep by design. Authoritative physics uses deterministic fixed-point math and explicit fixed-frame phases.
• One runtime, three paths. Run pure 2D, pure 3D, both side by side, or deliberate 2D/3D contacts through mixed mode.
• A complete collision stack. Build with primitives, convex and concave meshes, compounds, materials, manifolds, response islands, CCD, joints, and ragdolls.
• No engine lock-in. Hosts provide transforms and lifecycle calls through a small adapter boundary; Gravitas owns no renderer, scene graph, or ECS.
• Tools without hidden state. Context-local queries, replay hashes, diagnostic events, and debug-draw commands make simulation behavior visible without changing it.

Install

dotnet add package Gravitas

Prefer the dependency-light serialization profile?

dotnet add package Gravitas.Lean

Package	Profile
Gravitas	Standard package with built-in MemoryPack support
Gravitas.Lean	Same core physics API without the direct MemoryPack runtime dependency

Both packages target netstandard2.1 and net8.0.

The Core Loop

Gravitas makes simulation ownership visible. A host owns commands and timing; one GravitasWorldContext owns the physics world and all of its runtime services.

using Gravitas;

using GravitasWorldContext context = GravitasWorldContext.CreateOwned();
context.SetFrameRate(60);

// Configure GridForge coverage, then bind host agents, bodies, and colliders.
// Apply the frame's deterministic commands before advancing physics.
context.Simulate();
context.LateSimulate();

var replayHash = context.ComputeReplayHash();

// Presentation is separate from authoritative simulation state.
context.Visualize();
context.LateVisualize();

The full Getting Started guide creates the grid, host adapter, collider, and dynamic body behind that loop.

What You Can Build

Area	Highlights
3D physics	Dynamic, kinematic, and static bodies; primitive, mesh, and compound colliders; grounding; CCD; constraints; ragdolls
2D physics	First-class X/Z planar bodies, angular response, compounds, support detection, CCD, constraints, and ragdolls
Mixed physics	Finite 2D slabs embedded in 3D, cross-dimension contacts, constrained response, queries, CCD, and diagnostics
Queries	Closest-hit, all-hit, overlap, raycast, swept-shape, cone-volume, mixed, and batch APIs
Determinism	Stable runtime ownership, replay hashes, explicit serialization boundaries, and caller-owned hot-path buffers

Documentation

• Getting Started — build and step your first world.
• GitHub Wiki — understand host integration, dimensions, collisions, queries, replay, and diagnostics.
• API Reference — browse public namespaces, types, and members.
• Coverage Report — inspect the current test-suite coverage.
• Contributing — build, test, and prepare a focused change.

Built On The LSF Stack

Gravitas is the physics layer in a modular deterministic simulation stack:

Library	Responsibility
FixedMathSharp	Fixed-point scalars, transforms, and full-domain geometry
SwiftCollections	Low-allocation collections, pools, and spatial structures
GridForge	Explicit voxel worlds, traversal, and partition backing
Chronicler	Deterministic state transfer and replay infrastructure

Contributing

Contributions are welcome. Start with CONTRIBUTING.md; the deeper deterministic, physics, testing, and performance rules live in AGENTS.md.

For questions and discussion, join the community on Discord.

License

Gravitas is available under the MIT License. See NOTICE and COPYRIGHT for attribution details.