# Substrate Production Guide > **Category:** guide · **Engine:** Unreal Engine 5.7 (Production-ready) · **Related:** [G36 Substrate Material System](G36_substrate_material_system.md), [G18 Material System & Shaders](G18_material_system_shaders.md), [G9 Rendering — Nanite & Lumen](G9_rendering_nanite_lumen.md) Substrate reached **production-ready** status in UE 5.7 and is **enabled by default** for new projects. This guide builds on the Substrate fundamentals in [G36](G36_substrate_material_system.md) with production-grade workflows: operator composition patterns, migration from legacy materials, automotive/archviz material recipes, and performance tuning. For the conceptual foundation (Slab BSDF, operator model, why Substrate exists), start with G36. --- ## Production Status Summary | Version | Status | Notes | |---------|--------|-------| | UE 5.3 | Experimental | Initial Substrate (then called "Strata") | | UE 5.5 | Beta | Renamed to Substrate; major API stabilization | | UE 5.7 | **Production-ready** | Enabled by default; 280+ free automotive materials on Fab | **Hybrid workflows are supported** — legacy materials and Substrate materials coexist in the same project without conflicts. You can migrate surfaces incrementally. --- ## Operator Reference Substrate materials are built by composing Slabs (layers of matter) with Operators. Understanding the operator stack is essential for production work. ### Substrate Slab BSDF The foundational building block. Each Slab represents a layer of physical material: ``` ┌─────────────────────────────────────────┐ │ Substrate Slab BSDF │ │ │ │ Base Color ──────────▶ Diffuse albedo │ │ Metallic ────────────▶ 0=dielectric │ │ Specular ────────────▶ F0 adjustment │ │ Roughness ───────────▶ Microfacet rough │ │ Normal ──────────────▶ Surface normal │ │ Emissive ────────────▶ Self-illumination│ │ Subsurface Color ────▶ SSS profile │ │ Mean Free Path ──────▶ SSS depth │ │ Fuzz Amount ─────────▶ Cloth/velvet │ │ Fuzz Color ──────────▶ Fuzz tint │ │ Second Roughness ────▶ Dual-lobe spec │ │ Glint ───────────────▶ Sparkle effect │ │ Thin Film ───────────▶ Iridescence │ └─────────────────────────────────────────┘ ``` A single Slab can represent metal, dielectric, cloth, subsurface skin, or any blend — no more switching between discrete shading models. ### Vertical Layer Operator Stacks one Slab on top of another with **physically correct transmittance**: ``` ┌──────────┐ │ Top │ ← Clear coat, varnish, water film │ Slab │ └────┬─────┘ │ light transmits through top thickness ┌────▼─────┐ │ Bottom │ ← Base paint, wood, metal │ Slab │ └──────────┘ [Substrate Vertical Layer] Top Input ──── clear coat Slab (roughness 0.05, IOR 1.5) Bottom Input ─ base paint Slab (metallic flake) Thickness ──── controls transmittance depth ``` **Use cases:** Car paint with clear coat, varnished wood, wet surfaces, skin with oily sheen. **Key insight:** The Top Slab's thickness drives how much light reaches the Bottom. Thicker top = more absorption = darker base appearance. Use the `Substrate Transmittance-To-MeanFreePath` helper to convert an intuitive transmittance color into the correct Mean Free Path value. ### Horizontal Blend Operator Mixes two Slabs spatially using a blend mask: ``` [Substrate Horizontal Blend] Foreground ── rust Slab Background ── clean metal Slab Mix ───────── rust mask texture (0=metal, 1=rust) ``` **Use cases:** Rust on metal, dirt on paint, moss on stone, damage layers. ### Coverage Weight Operator Controls the opacity ratio of a Slab in a vertical layer stack: ``` [Substrate Coverage Weight] Slab ──────── scratches Slab Weight ─────── scratch mask (0=no scratch, 1=full scratch) │ ▼ [Substrate Vertical Layer] Top Input ──── (from Coverage Weight) Bottom Input ─ base surface ``` **Use cases:** Partial coverage effects — scratches that reveal base metal, chipped paint, worn edges. ### Add Operator Combines Slab outputs additively — each Slab is evaluated independently and the results are summed. This is cheaper than vertical layering but less physically accurate. **Use cases:** Performance-critical materials where transmittance simulation is unnecessary. --- ## Production Material Recipes ### Recipe 1 — Automotive Paint (3-Layer) The classic car paint stack: base metallic flake → color tint layer → clear coat. ``` Layer 3 (Top): Clear Coat Slab: Roughness 0.02–0.08, no color, IOR 1.5 │ ▼ Layer 2 (Mid): [Vertical Layer] Color Tint Slab: Subsurface color = paint hue, MFP from Transmittance helper │ ▼ Layer 1 (Base): [Vertical Layer] Metallic Flake Slab: Metallic 0.8–1.0, high roughness variation, Glint enabled for sparkle ``` **Tips:** - Use `Thin Film` on the base flake for pearlescent/chameleon effects. - Modulate clear coat roughness with a subtle noise for orange-peel realism. - The 280+ free automotive Substrate materials on Fab implement this pattern — use them as starting points. ### Recipe 2 — Weathered Wood ``` [Horizontal Blend] ├── Foreground: aged wood Slab │ Base Color: desaturated, lighter │ Roughness: 0.8–0.95 │ └── Background: fresh wood Slab Base Color: warm brown Roughness: 0.4–0.6 Subsurface: slight warm tint (wood SSS) Mix: edge-wear mask + noise │ ▼ [Vertical Layer] Top: varnish Slab (optional) Roughness: 0.05–0.15 Thickness: modulated by wear mask Bottom: blended wood from above ``` ### Recipe 3 — Character Skin ``` [Vertical Layer] Top: oil/sweat film Slab: Roughness 0.1–0.3, very thin Coverage Weight: mask for forehead, nose, chin Bottom: skin base Slab: Subsurface profile enabled Base Color: diffuse skin texture Subsurface Color: warm red (blood layer) Mean Free Path: from TransmittanceToMFP(skin scatter color) Fuzz Amount: 0.1–0.3 (peach fuzz) Fuzz Color: blonde/light ``` --- ## Migration from Legacy Materials ### When to Migrate Migrate when you need: - **Blended shading models** — a surface that transitions between metal, cloth, and subsurface. - **Physically layered finishes** — clear coat over paint over flake. - **New Substrate-only features** — Glint, Thin Film interference, Coverage Weight. **Don't force-migrate** materials that work fine as legacy — the two systems render side by side. Prioritize hero assets, vehicles, and characters. ### Migration Steps 1. **Open the legacy material** in the Material Editor. 2. **Change shading model** — in Material Details, set `Material Domain` to `Surface` and enable `Use Substrate`. 3. **Replace the output node** — the legacy output node is replaced by a Substrate output that accepts a single Slab (or operator chain). 4. **Convert inputs:** | Legacy Input | Substrate Equivalent | |-------------|---------------------| | Base Color | Slab → Base Color | | Metallic | Slab → Metallic | | Roughness | Slab → Roughness | | Normal | Slab → Normal | | Emissive Color | Slab → Emissive Color | | Opacity (translucent) | Coverage Weight Operator | | Subsurface Color | Slab → Subsurface Color + Mean Free Path | | Clear Coat | Vertical Layer with clear coat Slab on top | | Cloth | Slab → Fuzz Amount + Fuzz Color | 5. **Test with Lumen** — Substrate materials integrate with Lumen GI. Verify that indirect lighting looks correct after migration. ### Automated Conversion UE 5.7 includes an experimental **Substrate Auto-Convert** option (right-click material → Convert to Substrate). This handles simple materials (Default Lit, basic Clear Coat) automatically. Complex materials with custom shading models require manual conversion. --- ## Performance Considerations ### Cost Model Substrate materials are slightly more expensive than legacy equivalents due to the composable evaluation: | Configuration | Relative Cost | |--------------|---------------| | Single Slab (equivalent to Default Lit) | ~1.0x legacy | | Two Slabs + Horizontal Blend | ~1.2x | | Two Slabs + Vertical Layer | ~1.3x | | Three Slabs + nested Vertical Layers | ~1.6x | | Four+ Slabs | ~2.0x+ | ### Optimization Strategies - **Limit layer count** — 2–3 Slabs covers most real-world surfaces. Reserve 4+ for hero close-up assets. - **Use Horizontal Blend over Vertical Layer when physically accurate transmittance isn't needed** — it's cheaper. - **Leverage material instances** — parameterize your Substrate master material and swap parameters per instance rather than duplicating graphs. - **Shader complexity view** — use `View Mode → Shader Complexity` to identify expensive materials. Substrate shows accurate per-pixel cost. - **Nanite material slots** — Nanite meshes with Substrate materials benefit from the same per-cluster material evaluation. Keep material slot count per mesh low. ### Console Variables | CVar | Default | Description | |------|---------|-------------| | `r.Substrate` | 1 | Enable Substrate rendering (project-wide) | | `r.Substrate.Debug` | 0 | Debug visualization for Substrate layers | | `r.Substrate.MaxClosureCount` | 4 | Maximum evaluated Slab closures per pixel | | `r.Substrate.OpaqueMaterialMode` | 0 | 0=auto, 1=force single-slab fast path | --- ## Substrate + Lumen Integration Substrate materials fully integrate with Lumen's global illumination and reflections: - **Diffuse GI** — Slab albedo contributes to Lumen's surface cache for indirect bounce lighting. - **Specular reflections** — each Slab's roughness is evaluated independently for Lumen screen traces and ray-traced reflections. - **Emissive** — Substrate emissive surfaces are Lumen light sources. Use for neon signs, screens, glowing materials. - **Subsurface** — Lumen correctly handles SSS profiles in Substrate Slabs for skin and translucent materials. **Gotcha:** Very complex layer stacks (4+ Slabs) may produce slightly different indirect lighting than expected because Lumen's surface cache simplifies the material evaluation. Test with `r.Lumen.Reflections.Quality` at 4 for validation, then dial back for shipping. --- ## Common Pitfalls 1. **MFP confusion** — Mean Free Path is not transmittance color. Use `Substrate Transmittance-To-MeanFreePath` to convert. Plugging a color directly into MFP produces incorrect results. 2. **Over-layering** — more layers ≠ better. Each vertical layer adds a transmittance evaluation. Start simple; add layers only when the visual difference justifies the cost. 3. **Thickness of zero** — a Vertical Layer with thickness 0 makes the top Slab invisible. Always set a non-zero thickness. 4. **Legacy material instances** — material instances of a legacy parent cannot use Substrate operators. The parent material must be converted first. 5. **Mobile support** — Substrate is desktop/console only as of UE 5.7. Mobile projects should continue using legacy shading models. --- ## Resources - **Fab Automotive Materials** — 280+ free Substrate materials from Epic: search "Automotive Substrate Materials" on [Fab](https://www.fab.com). - **SIGGRAPH 2023 talk** — "Substrate: Authoring Materials That Matter" covers the rendering architecture in depth. - **Unreal Fest Bali 2025** — "Exploring Substrate Materials: Basic to Advanced Techniques" demonstrates production patterns.