OpenSE4

An open-source engine reimplementation for Space Empires IV Deluxe.

OpenSE4 is a new engine, written from scratch in C++23, with Vulkan rendering and an OpenGL fallback. It is being built to play Space Empires IV Deluxe faithfully, the way OpenXcom plays UFO: Enemy Unknown or OpenTTD plays Transport Tycoon Deluxe.

• You need your own copy of Space Empires IV Deluxe. It is available on Steam. OpenSE4 reads the game's data files, art and sound from your installation at runtime. None of the original game ships with this project.

• Written from specs, nothing copied. The rules are reimplemented from the manual, the game's documented data formats, observation of the running game and, since 2026-09-29, analysis of the original executable. Findings are written up as plain-language specs and the code is written from those; no code, data, art or text from the original is copied or shipped. See docs/CLEANROOM.md.

• Playable. Every part of the classic rules is implemented:

  • economy and population;
  • ships and movement;
  • combat: strategic and tactical, with replays and the combat simulator;
  • research, intelligence, diplomacy and events;
  • computer players.

  The game has the classic windows, sound and music, runs hotseat, network (with UPnP) and PBEM multiplayer, and includes a dedicated server. Every rule has been checked against the original. docs/PARITY_PLAN.md tracks what remains:

  • small details the original leaves open, where the engine makes its own choice;
  • a few differences in how the computer players expand, still being measured.

opense4 finds your install on its own, or takes its location with --classic-dir. Without an install it explains where it looked and exits: there is no game to play without your copy. docs/ENGINE.md is the map of the engine's code.

Screenshots

A star system, a hundred turns in	Designing a warship	A guided tutorial

The screenshots show OpenSE4 running on an installed copy of Space Empires IV Deluxe: the art is the original game's, read from that install.

What OpenSE4 adds

The rules and the classic windows stay as they were. Around them, OpenSE4 brings the game to today's machines and adds what a newcomer or a multiplayer group needs.

Modern screens and systems

• Widescreen: the 1024x768 layout stretches to use the whole width, up to 21:9, with a larger system view and galaxy map. Classic 4:3 with bars is a setting away.
• Any resolution: the classic layouts are scaled to any window or screen, windowed, borderless or fullscreen. There are options for sharp pixels, whole-number scaling and a larger text size.
• Native on Linux and Windows, built for macOS too, with Vulkan rendering and an OpenGL fallback. A Windows installer and a Linux desktop entry come with the release.

Learning the game

• Seven guided tutorials:
  • every step points at what to use and waits until you have done it;
  • close a window the lesson still needs and it shows you how to reopen it;
  • steps that take several turns count your progress;
  • you can leave a lesson and resume it later.
• Five training games with goals to meet: a land rush, a research sprint, holding a fortress, a first treaty and a conquest.
• A built-in manual of 24 chapters, linked from the lessons. Shift+F1 opens the page for the window in front.

Playing together

• Network play over TCP:
  • simultaneous turns, or one player after another (an OpenSE4 extension);
  • the router is set up automatically over UPnP, and games on the local network are found on their own;
  • connections are encrypted, and the game remembers each host's key, so an impostor can't pose as your host;
  • passwords never travel.
• Dropped connections and desyncs: a dropped player reconnects without losing the turn. A copy of the game that drifts from the host's is detected, reported and repaired.
• Play by e-mail, through mail or a shared folder: each player's turn file holds only that player's view, encrypted for them and signed by the host.
• A dedicated server (opense4-server) hosts network games and processes e-mail turns without a player at the keyboard.
• The same game on every machine: the engine is deterministic, so a Linux host and a Windows player see exactly the same results. Hotseat play on one machine works too.

Comfort

• Hotkeys you can change: the classic keys work, and every one can be rebound (Settings → Controls).
• Small additions:
  • the Galaxy Map can show jump distances;
  • Help can search;
  • combat replays can list each turn's events.
• Mods: opense4-datacheck checks a modded data set and names the file, line and record of anything it doesn't understand.

Building

Full instructions for Linux, Windows and macOS are in docs/BUILDING.md. In short, you need:

• a C++23 compiler (GCC 14+, Clang 18+, MSVC 17.10+ or Apple Clang 21), CMake 3.25+ and Ninja;
• SDL3, which CMake fetches if the system has none;
• glslc, from the Vulkan SDK or shaderc.

cmake --preset debug              # or: release, asan
cmake --build --preset debug
ctest --preset debug
./build/debug/opense4

On Arch Linux, install them with pacman -S cmake ninja sdl3 shaderc vulkan-headers.

Setting up the game data

OpenSE4 reads your installed copy of Space Empires IV Deluxe in place. docs/SETUP.md explains:

• how to get the files, including on Linux and macOS;
• how OpenSE4 finds them;
• how to check a data set or a mod with opense4-datacheck.

Multiplayer

Games are hosted from the client or with the dedicated opense4-server.

• TCP: direct play over the network, with simultaneous turns or one player after another. Connections are encrypted, the game remembers each host's key, and a dropped player reconnects without losing the turn. The default port is 6720, and the router is forwarded automatically over UPnP when the router allows it.
• Hotseat: several players on one machine.
• PBEM: turn files passed by mail or a shared folder. Each player's turn file holds only their own view, encrypted for them and signed by the host.

See docs/MULTIPLAYER.md.

Running

opense4 [--renderer=auto|vulkan|opengl] [--classic-dir=DIR] [--quick-start[=RACE]] [--seed=N]

--help lists every option. With --renderer=auto (the default) the game tries Vulkan 1.3 and falls back to OpenGL 3.3 if Vulkan is missing or unsuitable.

The game has the original's two screen layouts, 800x600 and 1024x768, drawn with your install's art, fonts and mouse pointers and scaled to the window. Like the original, OpenSE4 picks 800x600 when the desktop is at most 800 pixels wide and 1024x768 otherwise. --layout=800x600 or --layout=1024x768 forces one for a run, and Settings → Graphics → Screen layout keeps the choice (forcing a layout is OpenSE4's addition).

./build/debug/opense4                                # auto-detects a Steam install
./build/debug/opense4 --classic-dir=/path/to/se4     # or point at it
./build/debug/opense4 --quick-start=Terran           # skip the intro
./build/debug/opense4-datacheck                      # validate an installed or modded data set
./build/debug/opense4-server --players=2 --ai=3      # host a network game without playing

On Windows, the release's -setup.exe installs OpenSE4 for all users. It goes into Program Files with a Start menu entry and, if you choose, a desktop shortcut, and you remove it from Apps & features. The zip holds the same programs to run from any folder. Saved games and settings live in %APPDATA%\OpenSE4 either way.

On Linux, the game can appear in the desktop's application list (GNOME, KDE Plasma and other freedesktop.org desktops):

• Release package: run ./install-desktop-entry.sh in the unpacked folder. It adds the entry and icon for your user and starts the game from that folder. Run it again after moving the folder; --uninstall removes the entry.
• Source build: cmake --install build/release --prefix ~/.local (or /usr/local, with sudo) installs the programs, the desktop entry, the icons and the AppStream metadata.

The intro has the original's buttons: Quick Start, New Game (the full game and empire setup), Resume Game (the last game you saved), Load Game, Tutorial, Scenario, Credits and Quit Game. Tutorial and Scenario open OpenSE4's own guided lessons and training games (docs/LEARNING.md); Multiplayer, Settings and the Manual sit at the top right. During a game the Game Menu's Learn button opens them and Shift+F1 shows the manual page for the window in front. In the game, the classic hotkeys work: F1–F12 open the windows and End Turn, and letter and Ctrl keys give orders while their button is lit. Pointing at a button names it and its key at the top of the system view; Help → Hotkeys lists every key as bound, and Settings → Controls changes them. Autosaves are named AutoSav0 to AutoSav9, and each player's statistics, history and log files go to History/ in the user data folder and travel with saved games. See docs/SETUP.md and docs/MULTIPLAYER.md.

Headless screenshots, used for testing, work with SDL's offscreen driver:

SDL_VIDEO_DRIVER=offscreen ./build/debug/opense4 --quick-start=Terran --turns=30 --open=colonies --screenshot=shot.png
SDL_VIDEO_DRIVER=offscreen ./build/debug/opense4 --quick-start=Terran --turn-style=simultaneous --turns=20 --screenshot=sim.png
SDL_VIDEO_DRIVER=offscreen ./build/debug/opense4 --quick-start=Terran --turns=10 --open=none --screenshot=main.png  # no Log on top
SDL_VIDEO_DRIVER=offscreen ./build/debug/opense4 --quick-start=Terran --open=help:hotkeys --screenshot=keys.png
SDL_VIDEO_DRIVER=offscreen ./build/debug/opense4 --quick-start=Terran --layout=800x600 --size=800x600 --screenshot=small.png

Modding

Mods for the original game are replacement data files, and sometimes replacement art. OpenSE4 plays whatever data set the game directory holds: apply the mod to a copy of the game directory and pass that copy with --classic-dir. Run opense4-datacheck on it first. It loads every data file and reports anything it does not understand with the file, line and record. See docs/SETUP.md.

Layout

src/core      math, deterministic RNG, typed ids, logging
src/datafile  reader for the classic "Key := Value" data format
src/ruleset   typed model of a complete classic data set
src/game      classic-rules engine (implemented from docs/spec/)
src/net       multiplayer: sessions, protocol, UPnP port mapping
src/server    opense4-server: dedicated host and PBEM turn processor
src/assets    runtime access to the installed classic art
src/gfx       RHI with Vulkan and OpenGL backends, 2D batch renderer, ImGui bridge
src/client    the app shell and the classic client (windows, front end, multiplayer)
tools/        opense4-datacheck, opense4-observe (drive the original), cleanroom_check.py
docs/spec/    rules specs, written in our own words
shaders/      GLSL shared by both backends
assets/       fonts (Noto Sans, SIL OFL)
packaging/    Linux desktop entry, application icon and AppStream metadata
tests/        doctest unit tests (our own fixtures only)

License

OpenSE4 is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. See LICENSE.

The Noto Sans fonts are under the SIL Open Font License (see assets/fonts/OFL.txt). Bundled third-party libraries keep their own licences, which the release packages list in THIRD_PARTY_NOTICES.txt. The GPL covers only OpenSE4's own code and content; the original game's files stay the property of their owners and are never part of OpenSE4.

Trademarks

Space Empires is a trademark of its respective owner. OpenSE4 is an independent project. It is not affiliated with, endorsed by or sponsored by Strategy First or Malfador Machinations. The name is used only to say which game this engine is for.