MeshReplacer

A BepInEx plugin for FUMES that replaces vehicle meshes at runtime using Unity AssetBundles. Lets you drive a completely different body shape — the Caro pickup conversion is the reference implementation.

Game: FUMES (Unity 2021.1.29f1, IL2CPP)
BepInEx: 6.0.0-be.755 (IL2CPP build)

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Features

• Replace any mesh on a vehicle hierarchy by name
• Per-vehicle JSON config — drop multiple mods side-by-side
• Supports both the Cars list (ready-to-drive) and the Body list (garage assembly)
• Patches VehicleBody fields: lamp positions, grill/interior/engine pivots, cargo hooks, light shaft transforms
• Patches hardpoint positions (weapon/cargo slots) before the game places weapons in Vehicle.Awake
• Patches antenna position
• Material-slot fixer: trims excess material slots every frame so the skin system stays stable
• F3 key cycles submesh→material mapping (diagnostic)

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Installation

1. Install BepInEx IL2CPP bleeding edge build (build 755 or compatible) into the FUMES directory.
2. Copy MeshReplacer.dll to BepInEx/plugins/.
3. Create the mod data directory: BepInEx/plugins/meshreplacer/.
4. For each custom vehicle, create a subdirectory and add a vehicle.json plus AssetBundle files:

BepInEx/plugins/
  MeshReplacer.dll
  meshreplacer/
    caro-pickup/
      vehicle.json
      carobody          ← AssetBundle (no extension)
      caroreardoor      ← AssetBundle
      ...

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vehicle.json format

{
  "id": "body-caro-pickup",         // unique ID, used as license plate marker
  "baseBodyId": "body-caro",        // existing BodyType to clone from
  "displayName": "Caro Pickup",
  "vehicleMarker": "CaroModel",     // root GameObject name on the vehicle

  "meshReplacements": [
    {
      "bundle": "carobody",                      // AssetBundle filename (relative to this folder)
      "candidates": ["Caro_Pickup(6)"],          // asset names to try in order
      "target": "CaroModel",                     // GameObject to replace mesh on
      "isBody": true,                            // mark as the primary body mesh
      "fixMaterialSlots": false
    },
    {
      "bundle": "caroreardoor",
      "candidates": ["Caro_Pickup-reardoor"],
      "target": "CaroDoorRear",
      "isBody": false,
      "fixMaterialSlots": true                   // trim excess material slots each frame
    }
  ],

  "vehicleBody": {
    "grillPosition":          [0, 0.72, 1.72],
    "interiorCameraPosition": [0, 1.3,  0.2],
    "enginePosition":         [0, 0.65, 0.9],
    "frontLampsColor":        [1, 0.95, 0.8, 1],
    "frontLampsLightPosition":[0, 0.726, 1.388],
    "frontLampsShafts": [
      { "position": [ 0.6, 0.245, 2.618], "scale": [2, 1.304, 1.402] },
      { "position": [-0.6, 0.245, 2.618], "scale": [2, 1.304, 1.402] }
    ],
    "cargoStrapsHooks": [[0.6, 0.3, 0], [-0.6, 0.3, 0]],
    "lamps": [
      { "front": true,  "bulb": [ 0.59, 0.36,  1.74] },
      { "front": true,  "bulb": [-0.59, 0.36,  1.74] },
      { "front": false, "bulb": [-0.66, 0.39, -2.10] },
      { "front": false, "bulb": [ 0.66, 0.39, -2.10] }
    ]
  },

  "hardpoints": [
    { "index": 0, "position": [0,  1.52, -0.1]  },
    { "index": 1, "position": [0,  0.3,  -1.4]  },
    { "index": 3, "position": [0, 10.225, 1.63] }
  ],

  "antennaPosition": [0.44, 0.515, -1.8]
}

All vehicle.json fields are optional except id and baseBodyId. Fields set to null or omitted are left unchanged.

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Building from source

Prerequisites

• .NET 6 SDK
• FUMES installed via Steam (for the interop DLLs)
• BepInEx 6.0.0-be.755 unpacked in the game directory

Steps

1. Clone the repository.
2. Copy the required DLLs into the libs/ directory at the repo root (see Required DLLs).
3. Build:

dotnet build MeshReplacer/MeshReplacer.csproj --configuration Release

The output is MeshReplacer/bin/Release/net6.0/MeshReplacer.dll.

Required DLLs

Copy these files into the libs/ folder before building. They are generated by BepInEx from the game's IL2CPP binary and are not included in the repository.

File	Source
BepInEx.Core.dll	FUMES/BepInEx/core/
BepInEx.Unity.IL2CPP.dll	FUMES/BepInEx/core/
Il2CppInterop.Runtime.dll	FUMES/BepInEx/core/
0Harmony.dll	FUMES/BepInEx/core/
Assembly-CSharp.dll	FUMES/BepInEx/interop/
Il2Cppmscorlib.dll	FUMES/BepInEx/interop/
UnityEngine.dll	FUMES/BepInEx/interop/
UnityEngine.AssetBundleModule.dll	FUMES/BepInEx/interop/
UnityEngine.CoreModule.dll	FUMES/BepInEx/interop/
UnityEngine.InputLegacyModule.dll	FUMES/BepInEx/interop/
UnityEngine.PhysicsModule.dll	FUMES/BepInEx/interop/

FUMES/ is the Steam game directory, typically C:\Program Files (x86)\Steam\steamapps\common\FUMES\.

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Releases

Pre-built DLLs are published on the Releases page. Each release is built automatically from the tagged commit via GitHub Actions.

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License

MIT