Creating standalone asset packages for distribution

Please note that we will use the term assets for referring to props and also maps.

The main objective for importing and exporting assets is to reduce the size of the distribution build. This is possible since these assets will be imported as independent packages that can be plugged in anytime inside Carla and also exported.

How to import assets inside Unreal Engine

The first step is to create an empty folder inside the Carla Import folder and rename it with any folder name desired. For simplifying this newly created folder structure, we recommend having as many subfolders as maps to import and one single subfolder containing all the props to import. Inside each subfolder, we will place all the files needed for importing.
So basically, for a map subfolder, we will need to place the following files:

• The map itself in .FBX format.
• Optionally, the textures required by the map.
• Optionally, the .xodr OpenDrive file corresponding to that map.

And for the props folder, we will need the following files:

• The prop itself in .FBX format.
• Optionally, the textures required by the prop.

Additionally, we have to create a JSON file inside the package that will contain information about its assets. The file extension must be .json. We recommend the JSON file to have the same name as the package name in order to keep it organized. Please, keep in mind that the name of this file will used as the name of the distribution package.

The content of this JSON file should be similar to the following:

{
  "maps": [
    {
      "name": "MyTown01",
      "source": "./MapToImport01/MapToImport01.fbx",
      "use_carla_materials": true,
      "xodr": "./MapToImport01/MapToImport01.xodr"
    },
    {
      "name": "MyTown02",
      "source": "./MapToImport02/MapToImport02.fbx",
      "use_carla_materials": false,
      "xodr": "./MapToImport02/MapToImport02.xodr"
    }
  ],
  "props": [
    {
      "name": "MyProp01",
      "size": "medium",
      "source": "./AssetsToImport/PropToImport01/PropToImport01.fbx",
      "tag": "SemanticSegmentationTag01"
    },
    {
      "name": "MyProp02",
      "size": "small",
      "source": "./AssetsToImport/PropToImport02/PropToImport02.fbx",
      "tag": "SemanticSegmentationTag02"
    }
  ]
}

As you can observe in the JSON file content, we have defined a JSON array of maps and props.

Each item inside the maps array has the following parameters:

• name: The name of the map. It is possible to import the same map mesh but with a different name and, if so, it will create separate mesh files inside Unreal for each different name.
• source: Source path of the map inside the package folder.
• use_carla_materials: If true, we will use Carla materials, otherwise, we will use RoadRunner materials.
• xodr: Path to the .xodr Opendrive file for that map.

And each item inside the props array has the following parameters:

• name: The name of the prop. It is possible to import the same prop mesh but with different name and, if so, it will create separate mesh files inside Unreal for each different name.
• source: Source path of the prop inside the package folder.

• size: Size of the prop, possible values are:

  • tiny
  • small
  • medium
  • big
  • huge

• tag: Semantic segmentation tag. Possible values are:

  • None
  • Buildings
  • Fences
  • Other
  • Pedestrians
  • Poles
  • RoadLines
  • Roads
  • Sidewalks
  • TrafficSigns
  • Vegetation
  • Vehicles
  • Walls

  Note that if the tag is not spelled correctly, it will interpret it as None value by default.

To sum up, the Import folder should have this similar structure:

Import
|
├── Folder01
|   ├── MapToImport01
│   │   ├── Asphalt1_Diff.png
│   │   ├── Asphalt1_Norm.png
│   │   ├── Asphalt1_Spec.png
│   │   ├── Grass1_Diff.png
│   │   ├── Grass1_Norm.png
│   │   ├── Grass1_Spec.png
│   │   ├── LaneMarking1_Diff.png
│   │   ├── LaneMarking1_Norm.png
│   │   ├── LaneMarking1_Spec.png
│   │   ├── MapToImport01.fbx
│   │   └── MapToImport01.xodr
|   ├── MapToImport02
│   │   ├── MapToImport02.fbx
│   │   └── MapToImport02.xodr
|   ├── AssetsToImport
|   │   ├── PropToImport01
│   │   │   ├── PropToImport01_Diff.png
│   │   │   ├── PropToImport01_Norm.png
│   │   │   ├── PropToImport01_Spec.png
│   │   │   └── PropToImport01.fbx
│   │   ├── PropToImport02
│   │   │   └── PropToImport02.fbx
|   │   └── PropToImport03
│   │       └── PropToImport03.fbx
|   └── Package1.json
└── Folder02
    ├── AssetsToImport
    |   └── PropToImport04
    |       └── PropToImport04.fbx
    └── Package2.json

Maps exported from RoadRunner are also supported for importing them inside Carla. So, basically, once created your RoadRunner map, you just need to export it, take the required files and place them following the structure listed above.

Now we have everything ready for importing assets. To do so, you just need to run the command:

make import

This command will read the JSON file and take each asset and place it inside the Content in Unreal Engine. Furthermore, it will create a Package1.Package.json file inside the package's Config folder that will be used for defining its props in the Carla blueprint library, exposing them in the PythonAPI and also for exporting those assets if needed.

Packages with the same name will produce an error. Delete or rename the package before importing a new one with the same name.

How to export assets

Once imported all the packages inside Unreal, users could also generate a cooked package for each of them. This last step is important in order to have all packages ready to add for distribution versions of Carla and for any supported platform. To export the packages, simply run the command:

make package ARGS="--packages=Package1,Package2"

This command will create the distribution package compressed in a .tar.gz file for each cooked package and place it in the Carla Dist folder. Finally, you could import these packages in a Carla distribution by simply moving them in the Carla Import folder and executing the ImportAssets.sh script.