AI 3D Models in Unreal Engine 5: Import and Nanite

In Unreal Engine 5, import an AI-generated model through the Content Browser as glTF/GLB or FBX, decide on Nanite and collision, and check scale. The documented route is simple; what the docs do not tell you is what a particular generator’s mesh will do. The FBX static mesh import page came back from our fetch tool as a table of contents with no body text, so this page keeps to what was returned and says what was not. Cross-generator results are planned, not done.

Formats and where to import

The Content Browser import page lists FBX, glTF, USD and OBJ. The glTF page says .gltf (text, with separate textures and a .bin) and .glb (a single file) are imported via Interchange. In the Content Browser use +Add > Import to, or drag and drop; to bring a whole scene in use File > Import Into Level. Either opens the “Interchange Pipeline Configuration (Import Content)” dialog. That page lists no supported glTF features or limits, and the Interchange import page does not give a list of supported formats. Epic’s Interchange page says “Support for the FBX file format is currently Experimental” and gives console variables to enable it (Interchange.FeatureFlags.Import.FBX, and .ToLevel for Import Into Level); the same page says Import Into Level “currently only works with glTF and MaterialX file formats.” The page’s version was not stated in the text we read, so confirm in your editor. For the format choice see GLB vs FBX vs OBJ.

FBX static mesh options (names only)

The FBX static meshes page returned only its table of contents. The option names it lists are: Auto Generate Collision, Generate Missing Collision, Build Nanite, Import Mesh LODs, Combine Meshes, Normal Import Method, Convert Scene Unit, Generate Lightmap UVs and Import Uniform Scale. We could not retrieve descriptions or defaults, so we make no statement about what each does beyond its name. The related FBX pipeline page says to make sure Import LODs is enabled in the import dialog when importing LODs; it does not say where the option sits, and it defers naming to the default naming rules on another page. Read the options in your own editor’s tooltips before changing them.

Collision naming

The FBX pipeline page says collision meshes are identified by name: UBX_[RenderMeshName]## (box), UCP (capsule), USP_ (sphere), UCX_ (convex hull). The name must match the render mesh: the page’s example pairs a mesh named Tree_01 with a collision mesh named UCX_Tree_01. Boxes and spheres do not work on non-uniformly scaled meshes. If a file has several meshes with custom collision, only the first mesh’s collision imports. The page says the import pipeline uses FBX 2020.2. How collision works on glTF imports is not documented on the pages we got. Methods for building proxies are in collision meshes for AI props.

Nanite

Epic’s Nanite page, as returned, says:

Enable it with Build Nanite in the import options, with Nanite > Enable on assets selected in the Content Browser, or per mesh via “Enable Nanite Support” under Nanite Settings in the Static Mesh (or Skeletal Mesh) Editor.
Platforms: current consoles and desktop with DirectX 12 and Shader Model 6.
Materials: Blend Mode Opaque or Masked only; unsupported materials are replaced with a default material and a warning appears in the Output Log. Mesh Decals (which need Translucent materials) and Morph Targets are not supported. Epic’s page also describes Nanite for skeletal meshes.
The instance cap is 16 million, counting all streamed-in instances, not only Nanite ones. Per-vertex tangents are not stored (tangent space is derived in the pixel shader).
Not supported: MSAA, Forward Rendering, stereo VR, Lighting Channels, and (by default) ray tracing against Nanite meshes, which use a Fallback Mesh; native support is described as experimental. View-specific filtering (Minimum Screen Radius, Distance culling) is also listed.
Lowering “Fallback Relative Error” in the Static Mesh Editor makes the ray-tracing Fallback Mesh keep more of the source mesh’s triangles.

The page sets no specific triangle cap and says triangles per mesh, instance counts and other factors have practical limits you should measure for your content and hardware. The page is titled “Unreal Engine 5.8 Documentation”; an earlier read found links to 5.5 pages, so treat it as possibly mixed-version. In plain terms for generated assets: a very dense AI mesh is the case Nanite is built for, which is why people ask whether retopology is still needed; see do you still need retopology?. But the page lists limits (materials, platforms, features above) and makes no claim that Nanite repairs mesh defects, so a generated mesh with translucent materials or a target outside the listed platforms is outside what it describes. How a given mesh performs is a measurement we have not made; use your own profiling.

Scale and units

Unreal’s FBX options include Convert Scene Unit and Import Uniform Scale (names only). We could not confirm the defaults or the unit conversion rule. glTF defines meters and +Y up in its spec; how Interchange maps that into Unreal’s axes is not stated on the pages we read. Test one known-size object first, and see scale, pivot and axis fixes.

Unconfirmed

Descriptions and defaults of the FBX static mesh options.
LOD naming rules beyond the _LOD0 suffix appearing in the table of contents.
glTF feature coverage in Interchange; whether FBX in Interchange is still experimental in your version.
Nanite recommended triangle ranges (none published on the page; the page tells you to measure).
Version differences between 5.5 and 5.8.

Compare with Unity for the other major engine, and size your assets with the polycount budget calculator. Hub: AI 3D models in game engines.