USD asset creation pipeline

The following details how to prepare a vehicle asset for export into the Universal Scene Description (USD) format and import into AeroSim.

3D modelling a complex object such as an aircraft requires a significant degree of knowledge about modelling techniques and software, therefore, if you do not have previous experience in using these applications, we suggest you refer to educational or training materials for 3D modelling prior to attempting this procedure.

In this guide, we will use Blender, since it is a free, open-source 3D modelling tool with all the functionality needed to prepare a model for import into AeroSim. For preparing USD assets, it is also useful to install the Omniverse USD composer from NVIDIA. Download the Omniverse Launcher for your OS and then execute the launcher and follow the installation instructions. You will need an Omniverse account, if you don't already have one, follow the link in the Omniverse Launcher to create a new account. After launching Omniverse go to the exchange tab and search for the Omniverse USD Composer.

omniverse_exchange

Model organization

The model should be organized in such a way that all large components of the vehicle are separated into different objects. A rule of thumb would be to separate any large component you might expect to move significantly during normal operation of the vehicle and separate components that are likely to have very different material properties e.g. metalwork vs glass. With the components as separate objects

For example, the following is an example of an appropriate hierarchy for a helicopter model:

Body:
Metal fuselage and bodywork
Interior: seats, fittings and flight control panel
Left front door
Right front door
Left rear door
Right rear door
Glass
Main rotor
Rear rotor
Undercarriage

In order to comply with the standard USD model organization, each separate mesh component of the model should be parented to an empty in Blender. The entire model hierarchy should also grow from a root empty placed at the root of the model hierarchy. Each sub-component of the model (e.g. fuselage, doors, rotors) should have its own empty which is a child of the root empty. Each sub-component's mesh object should be a child of its corresponding empty object.

For non-rotating components, the empty should be placed at a sensible location, for example, where you might expect the center of mass of the object to be. For rotating components, the empty should be placed at the expected rotation center of an object. For example, for a rotor, the empty should be placed along the axis of rotation, close to the geometric center of the object. This is an important procedure to follow for preparing SimReady content, since semantic labels will be applied to the empty components, not to meshes.

To parent a mesh object to an empty in Blender, select the mesh object in the 3D vieport or outliner panel. Holding Shift, select the target empty, it is important that the intended parent is the last object selected. Then, press Ctrl + P and choose object from the dropdown menu. You can also establish parent relationships directly in the outliner. To do so, click and hold the child item (the mesh) in the outliner panel and drag it over the parent item (the target empty), ensuring that the parent item is highlighted. Release the mouse button and you will see the child mesh get nested under the parent empty in the outliner view.

The following image shows an example hierarchy of a helicopter model organized in Blender:

usd_hierarchy

Materials

USD supports a subset of the material functionality Blender provides. To ensure Blender model materials are compatible with the USD standard, use the Principled BSDF for all materials. For transparent materials like glass and perspex, adjust the alpha parameter and transmission parameter to achieve the desired appearance. USD supports the standard texture node with UV mapping in Blender. It is recommended to limit materials to this base set of material node types. Other node types may not be supported. If you require more material node types in Blender for your aims, ensure to inspect the model in the Omniverse USD Composer to confirm the node's effect can be observed after export.

The following material node types are known to be supported by Blender's USD export:

Principled BSDF
Image Texture
Combine Color
Separate Color
Normal/Map
Multiply

Materials can be adjusted or replaced in the Omniverse USD Composer if needed. The USD composer also provides a comprehensive library of USD compatible materials to use.

Semantic labelling and Q codes

To comply with the SimReady content standard and facilitate proper semantic labelling in simulation, the vehicle model and/or its subcomponents should have custom properties that identify the proper semantic label and Q-code as defined in the Wikidata database. The Wikidata database is a structured knowledge base that standardizes semantic labels and Q-codes and serves as a universally recognized reference for semantic identification.

To add semantic labels to the model for SimReady preparation, first, look up the appropriate item in the Wikidata database search bar. In this example case, we will look for the label and Q-code for helicopter.

helicopter_wikidata

Wikidata shows us that the appropriate semantic label is helicopter in lower case and the corresponding Q-code is Q34486. We now add these labels to custom properties of the root empty object in the example helicopter model. With the root empty of the model selected open the Properties panel in Blender.

blender_properties_panel

Add a new custom property, select the String type, name the property as omni:simready:documnetation and set the value as AeroSim helicopter. The specific entry for this variable is optional and can be set to suit your naming preference.

blender_custom_property

After this, add three more custom properties with the appropriate semantic label and Q-code derived from Wikidata:

Name: omni:simready:vehicle, value: vehicle
Name: semantic:wikidata_class:params:semanticData, value: helicopter
Name: semantic:wikidata_qcode:params:semanticData, value: Q34486

custom_properties_helicopter

Adding these properties to the model ensures that the model will be correctly semantically labelled within a simulation.

Exporting the model to USD

To export the finished model from Blender, go to File > Export. From the drop-down menu choose Universal Scene Description (.usd*) to open the USD export dialogue.

blender_usd_export

Ensure that Selection Only, Visible Only and Animation are unchecked. In the Root Prim field put the name of the root empty of your model with a leading forward slash e.g. /Helicopter.

You may also find it convenient to use the script editor to export. Oment the script editor and add the following code, adjusting the filepath and root_prim_path parameters accordingly:

bpy.ops.wm.usd_export(
    filepath="/path/to/output/file",
    selected_objects_only=False,
    visible_objects_only=False,
    convert_world_material=False,
    root_prim_path="/Helicopter",
    author_blender_name=False,
)

Inspecting the model in Omniverse USD composer

Open the Omniverse USD Composer using the Omniverse launcher and open the USD file exported from Blender.

blender_usd_export

There are several things to check about the model in USD composer:

Hierarchy: on the right hand side in the Stage panel, check that the model hierarchy is correct.
Labels:
Select the root prim and inspect the Raw USD Properties section in the Property panel
Check that the Wikidata class and Q-code are correct
Root transform: with the root prim selected look at the Transform in the Property panel:
If there is no transform, add the default one
If there is a transform, ensure that Translate and Rotate are all zeroed and that all Scale is 1.0 on all axes
If the transform is incorrect, check the model in Blender and export again
Sub-component transforms:
Click on a sub-component in the Stage panel
Reference the blender model to ensure the sub-component transform is as expected
Visualy inspect the sub-component root in the 3D view
Ensure that the model has the correct dimensions:
Choose the measure tool from the Tools > Measure dropdown
Click on two vertices on opposing sides of the model and ensure they show the expected distance
Inspect the material properties in the 3D view for anomalies or deviations
If a material looks odd, click on the component in the 3D view and inspect the Material section of the Property panel
Adjust the parameters accordingly
Transparent components like glass may need opacity adjustment

blender_usd_export

You may also wish to use materials from the Omniverse library. Drag and drop the material onto a part of the model in the 3D view from the Materials panel of the content browser:

blender_usd_export

Exporting the model from USD composer

Once you are satisfied with the appearance of the model, export it from the file menu with File > Collect as. Check Flat Collection and choose a directory location for the files. This will export a directory, inside of which will be the root USD file, acompanied by a directory named SubUSDs. Move this entire directory and its contents into the appropriate location in the aerosim-assets repository, e.g. the vehicle directory.

Once placed into the aerosim-assets repository directory, the model will be accessible in the AeroSim config using the actor_asset field of the actor section of the simulation configuration JSON file:


    ...

    "actors": [
    {
        "actor_name": "actor1",
        "actor_asset": "vehicles/generic_helicopter/generic_helicopter",
        "parent": "",
        "description": "Generic helicopter",

    ...

    }
}

Any effectors that are part of the model as sub-components can be accessec through their relative position in the hierarchy:


    ...

        "effectors": [
            {
                "id": "rotor_main",
                "relative_path": "generic_helicopter/rotor_main",

    ...

    }
}