Substrate Materials

LLMMaterial supports UE5 Substrate layered material system. When a .llmmat file contains the substrate field, the system automatically uses the Substrate routing mode.

Routing Modes

Mode	Condition	Description
Traditional	Only nodes	Uses traditional Material Expression construction
Substrate	Only substrate.slabs	Full Substrate pipeline
Hybrid	nodes + substrate.slabs	Expression + Substrate mixed

Basic Structure

{
  "version": "1.0",
  "name": "MySubstrateMaterial",
  "domain": "Surface",
  "blendMode": "Opaque",
  "shadingModel": "DefaultLit",
  "substrate": {
    "slabs": [...],
    "compositions": [...],
    "root": {...}
  }
}

Slab Types

Type	Description	Key Inputs
SubstrateSlabBSDF	Basic surface BSDF (default)	DiffuseAlbedo, Roughness, F0, Metallic, Normal
SubstrateHairBSDF	Hair BSDF	BackboneRadius, Density, DirectVectorization
SubstrateUnlitBSDF	Unlit BSDF	EmissiveRadiance
SubstrateEyeBSDF	Eye BSDF	IrisDistance, PupilRadius
SubstrateSingleLayerWaterBSDF	Single-layer water surface	Extinction, Scattering

Composition Types

Type	Description	Required Fields
VerticalLayering	Vertical layering (top/bottom stack)	top slab ID
HorizontalMixing	Horizontal mixing (two slab blend)	slabA, slabB, MixRatio
Add	Add two slabs	slabA, slabB
Weight	Weight node	slab ID, weight
Select	Select node	slabA, slabB, Threshold

Slab Input Value Format

"inputs": {
  "DiffuseAlbedo": [0.8, 0.8, 0.8],   // RGB array = Constant3Vector
  "Roughness": 0.3,                      // Scalar = Constant
  "F0": [0.04, 0.04, 0.04],           // RGB array (default 0.04 Fresnel)
  "Metallic": 0.0,                      // Scalar
  "Normal": "/Game/Textures/Normal",     // Texture reference
  "BaseColor": { "node": "node_id" }    // Reference Expression node (Hybrid mode)
}

Example: Simple Substrate Material

{
  "version": "1.0",
  "name": "SimpleSubstrate",
  "domain": "Surface",
  "blendMode": "Opaque",
  "shadingModel": "DefaultLit",
  "substrate": {
    "slabs": [
      {
        "id": "main_slab",
        "type": "SubstrateSlabBSDF",
        "inputs": {
          "DiffuseAlbedo": [0.8, 0.8, 0.8],
          "Roughness": 0.3,
          "F0": [0.04, 0.04, 0.04],
          "Metallic": 0.0
        }
      }
    ],
    "root": {
      "type": "VerticalLayering",
      "top": "main_slab"
    }
  }
}

Example: Two-Layer Material

{
  "version": "1.0",
  "name": "TwoLayerMaterial",
  "substrate": {
    "slabs": [
      {
        "id": "surface_layer",
        "type": "SubstrateSlabBSDF",
        "inputs": {
          "DiffuseAlbedo": [0.9, 0.85, 0.8],
          "Roughness": 0.15
        }
      },
      {
        "id": "base_layer",
        "type": "SubstrateSlabBSDF",
        "inputs": {
          "DiffuseAlbedo": [0.7, 0.65, 0.6],
          "Roughness": 0.6
        }
      }
    ],
    "root": {
      "type": "VerticalLayering",
      "top": "surface_layer",
      "bottom": "base_layer",
      "topThickness": 0.01
    }
  }
}

Notes

• ShadingModel Auto-Switch: Substrate materials automatically set ShadingModel to MSM_Strata
• Hybrid Mode: Expression nodes in nodes can be referenced by { "node": "node_id" } in substrate.slabs[].inputs[]
• Texture Inputs: Textures use asset path strings, and the system automatically creates TextureObject or TextureSample nodes
• VerticalLayering without bottom: When root only specifies top, the Top slab becomes the root directly