omsi-launcher

Archived: This project is not maintained anymore, feel free to fork and maintain further while respecting the license

WIP OMSI 2 game launcher & manager built using Kotlin and Compose.

WARNING: This is unstable software. Create Backups of your game files before you use it.

Why use this?

Omsi is optimized extremely poorly for usage with a lot of AddOns, as a single AddOn doesn't exist in a single location, it exists in a lot of locations (maps, Vehicles, Sceneryobjects, Splines, Scripts), this combined with OMSI seemingly checking your entire OMSI folder each time some UI refreshes the game gets increasingly more laggy the more addons/mods you add.

A simple solution is to not have as many addons installed, or having multiple installations of the game itself for different groups of addons.

This tool helps you to create multiple installation, without taking up more storage by symlinking common resources to a base installation of the game, therefore each individual instance just takes the space of the individual addons it has installed

How to launch an instance

Whilst it is theoretically possible to launch Omsi through Steam, it is recommended to let the launcher call Steam instead.

What is "omsi-launcher elevation helper" and why does it request admin permissions?

omsi-launcher elevation helper is a tool we use to temporarily launch a elevated process for some file system operations, this is required as Windows decided that it is necessary to be an administrator in order to create symlinks. People interested in source code can click here

How does this work?

When starting Omsi through Steam, Steam just calls <SteamLibrary>\OMSI 2\Launcher.exe it doesn't care what that file is. So when starting an instance through the launcher it will create a symlink at <SteamLibrary>\OMSI 2 pointing to the instance you want to launch, this way Steam will actually run <selected instance>\Omsi.exe

Nerdy explanation of how we store which AddOns are enabled for each instance

Steam keeps track of which AddOns you have installed at <library>\steamapps\appmanifest_252530.acf (252530 being Omsi's Steam id)

Since Steam overwrites this file if it is a symlink, we needed a different approach.

Instead, when creating the base game installation we know copy the base manifest into <launcher>\game\manifest.acf, each time we create a new instance, we copy that file into <instance>\manifest.acf

Then if you launch an instance we check:

• Does <Steam library>\OMSI 2 already symlink to that instance
  • If yes:
    • Copy appmanifest_252530.acf into <instance>\manifest.acf
  • If not (but symlink):
    • Resolve that symlink to the previous instance
    • Copy the current appmanifest_252530.acf to the resolved instance
    • Copy the manifest of the instance we want to start to appmanifest_252530.acf
  • If not (but directory)
    • Copy the manifest of the instance we want to start to appmanifest_252530.acf

That way no only OMSI will only see the selected AddOns (as they're the only addons physically present in the instance folder) also Steam will show them and not download any additional ones

Folder structure

Base game: When starting the launcher for the first time, it asks you to set up your "base game folder", this will contain the actual base game and addons you want to share between all other addons (OmniNavigation, bus DLCs)

Individual Instance: Each instance of Omsi has its own installation folder, during the creation of the instance, the launcher will go through all contents of the base game folder and create a symlink pointing to the base game. The launcher will only symlink files and not directories, those will instead be created and their contents will be symlinked. Using this method addon installers can write to base game locations, without writing their contents to the base game folders

Developer folder structure

This documents internal configuration files of the launcher, if you are not a dev working on the project, just stop reading

%APPDATA%\omsi-launcher\config.json: This currently only stores the actual installation location of the launcher, set during setup <installation>\instances.json: This stores all set instances with their launch flags and patch version

Trivia we found whilst making this

• You can symlink <steam library>\OMSI 2 to wherever you want and Steam will launch it.
• You can symlink anything in your OMSI 2 directory and Omsi will load everything
• Steam stores it's install location at: HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Valve\Steam Value: InstallPath
• Omsi stores it's install location at: HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\aerosoft\OMSI 2 Value: Product_Path