IW4L

IW4L is a Call of Duty runtime written from scratch in Rust, on bevy and wgpu. Point it at a copy of MW2 you already own and it loads that install's data into its own engine.

The on-disk layouts came out of reverse engineering the original binaries and reading public technical references.

This whole project is written by an LLM.

Status

No releases yet: you build it from this tree. Nothing here is stable either — the API, the config format, caches and the wire protocol all change between commits, so everyone in a session needs to be on the same one. Expect bugs and desyncs.

The network side is for arranged playtests among people who already agreed to play; it has never been vetted for lobbies full of strangers. IW4L sends nothing home, and diagnostic files sit on your disk until you attach them to a report.

Architecture

assets	MW2 zones read natively; MW3 and Black Ops land in the same asset_iw4 IR.
shaders	Retail D3D9 SM3 tokens translated to WGSL, so no DirectX at runtime.
rendering	One sorted drawsurf list; only the tess emitters fork per surface type.
physics	Fixed 17 ms step on its own accumulator. Framerate changes nothing about how a body falls.
simulation	One TickInput → sim::step → Snapshot funnel for server, prediction and replay.
network	Custom p2p wire over UDP: deltas, reliability, reconciliation. A QUIC master only introduces peers.
platforms	Linux, macOS (Metal) and a portable Windows build.

Retail protocols, the original ABI and patched executables are out of scope. IW4L clients talk to IW4L clients.

Game data

IW4L_GAMES points at the folder holding your game trees. No assets ship in this repository or in any release, and IW4L is unaffiliated with the rights holders of the original games.

Those trees are read only. IW4L never patches them, swaps files in them or writes anything back; caches, settings, demos and logs land in iw4l-artifacts/ next to the IW4L binary.

Build and run

System packages first: docs/BUILD.md (Fedora / Debian / Arch — compiler, ALSA, udev, X11/Wayland headers; macOS — Xcode command line tools).

cp .env.example .env          # IW4L_GAMES — folder containing the game trees
make map mp_boneyard          # run a map
make map mp_boneyard CMDS='spawn assault; wait 2s; quit'
make help                     # every recipe

Live runs use [profile.play], a development build with optimizations turned on. PROFILE=release builds the real release binary. docs/WINDOWS.md covers Windows.

Documentation

Implementation notes live under docs/, one short file per area. Start at docs/INDEX.md.

file	about
docs/BUILD.md	system packages per distro, macOS, Windows cross prerequisites
docs/RUN.md	running the game, console scripts, commands and traps
docs/PERF.md	Perfetto tracing and performance analysis
docs/RENDER.md	rendering pipeline
docs/MAP-LOAD.md	map loading and asset installation
docs/ENTITIES.md	simulation data flow and entity taxonomy
docs/SIM-STEP.md	simulation step architecture
docs/ANIM.md	animation system
docs/WINDOWS.md	portable Windows build
docs/DEPLOY.md	release, publishing and deployment
docs/MASTER.md	running your own master server

Contributing and support

A personal, experimental project. Bug reports are welcome and get no promised fix date. CONTRIBUTING.md says what a useful report contains and how changes get reviewed; security reports go to SECURITY.md.

Acknowledgements

IW4L ships none of the code below. It was read against all of it.

• OpenAssetTools and its iw4x-x64 fork — modding tools whose asset-structure headers document the on-disk layouts IW4L reads.
• IW4x — a custom client for MW2 (2009), a cross-reference for asset and protocol behaviour.
• KisakCOD — an open-source CoD4 reimplementation, a cross-reference for engine structure one generation over in the same family.
• Ghidra — the framework the original binaries were read with.

License

IW4L is licensed under the Apache License 2.0, and NOTICE holds the copyright notices, the licences of the projects above and the bundled fonts. That licence covers IW4L's own source code. Call of Duty, Modern Warfare, Black Ops and the related assets, trademarks and intellectual property belong to their respective owners.