Substrate Materials Overview

Substrate is Unreal Engine 5's approach to authoring materials, which replaces the fixed suite of shading models and blend modes, such as Default Lit and Clear Coat, with a more expressive and modular framework.

Certain abstractions of the non-Substrate (or legacy) material system are done away with by Substrate — it replaces them with measured properties of matter. This creates wider parameter space from which to work and makes it possible to more accurately blend between discrete surface types like metal, glass, and plastic. Substrate also streamlines the process of layering materials, making it easier to represent surfaces like liquid on metal, or a clear coat over subsurface scattering.

Materials in Substrate are conceptualized as "slabs of matter." These slabs are a principled BSDF (bidirectional scattering distribution function) representation, parameterized by physical quantities with well-defined units. Materials are expressed as a graph of slabs on which operations are performed (like mixing and layering). Because of their principled representation, Substrate Materials can be simplified according to the capacity of a platform in order to trade visual quality for performance.

Enabling Substrate

To enable Substrate Materials in your project:

1. Open the Project Settings and go to Engine > Rendering.
2. Under the Substrate section, enable Substrate materials (Experimental).

3. Follow the prompts and messages to Restart the project.

   

   We recommend using DirectX 12 (DX12) on Windows. Substrate works with DirectX 11 (DX11) but has several issues and requires further testing. For more information, see the Limitations and Known Issues section of this page.

Optional Project Settings and Console Variables

Substrate includes these optional project settings and console variables:

Converting Legacy Materials

Substrate materials cannot be reverted to a non-Substrate Material. Conversions are permanent once opened and saved. We advise you to save a backup copy of your project before enabling Substrate.

Existing materials are automatically converted to a Substrate Material when opening them for editing in a Substrate-enabled project. These are converted to a Substrate-style shading network using the Substrate Shading Models node.

The example above shows a "legacy" material that has been opened with Substrate enabled for the project and the resulting material graph with nodes now plugged into the Substrate Shading Models node.

Follow these guidelines when enabling Substrate in an existing project:

• Opening an existing non-Substrate material in a Substrate-enabled project automatically converts it to a "legacy" Substrate material when you save the material. The conversion is permanent and cannot be reverted to a non-Substrate material.
• Substrate Materials renders black if Substrate is disabled for the project. This includes any legacy Substrate materials that were created from converted materials. You can manually rewire any converted Substrate material to a legacy-style material but this will not get rid of the Substrate nodes that are now in the material graph.

Substrate's Relationship to Material Layers

Traditional Material Layering in Unreal Engine (both graph-based and in the custom layers GUI) are based on the concept of parameter blending. Each layer defines a pattern graph of parameters which are blended and fed into the final shading model.

There is nothing preventing material layer-driven parameters from feeding into a Substrate-defined shading model. However, you would need to set this logic up manually using the output of a Material Attribute node in the parent material. A limitation of this approach is that the Material Attributes systems has a fixed list of parameters, but there might not be enough slots to feed into a multi-slab setup with Substrate — it might require using pins arbitrarily and unrelated to their true meaning.

Substrate can natively use parameter blending, as described later in this page, but there is no way to access that functionality from the Material Layers interface. Unifying Substrate and Material Layers is a strong area of interest for future development.

Working with Substrate Materials

Substrate Materials are authored similarly to legacy materials. This section covers the key elements that make up Substrate Materials, including its nodes, blend modes, and details on the types of materials you can create.

Substrate Material Root Node

Like legacy materials, the Material Root node is where Substrate slabs and other Substrate nodes, like operators and building blocks, are fed into.

Also like legacy materials, you'll use the Details panel when the Material Root node is selected to set the Blend Mode and other properties that define the look of the material.

All Substrate Material graphs must be connected to the Front Material input on the Material Root node. This input is the end-point of every Substrate graph.

Substrate Blend Modes

Substrate uses its own set of Blend Modes to define how the material's color is blended with the background. Legacy material blend modes are limited in how they can mix and blend with one another, thus limiting the types of materials they can create. Substrate offers a broader selection of blend modes that can blend together to create all sorts of materials. This is especially important for achieving physically correct translucent surface shading.

Substrate includes the following Blend Modes:

Substrate makes working with translucency more straightforward than traditional materials do — translucent Blend Modes are better defined by their intent. One aspect that remains unchanged between the two is that any translucent Blend Mode must also set a Lighting Mode to define how lighting is calculated for its surface. This is important to achieve the correct look for translucent materials.

The vast majority of translucent materials you create will use Surface Translucency Volume or Surface Forward Shading.

The following Lighting Modes are available to choose from:

For some examples of setting up and using translucency in a Substrate Material, see the Translucency section of this page.

Substrate Slab

The Substrate Slab is the elementary building block from which one assembles a Substrate Material. It is designed to be the minimum necessary set of parameters with which the vast majority of material appearances can be achieved. As such, it is the basis of a much more expressive type of appearance authoring.

A slab is a principled representation of a slab of matter that is composed of an interface and a medium.

The composition of a Substrate Slab: The interface (1) and the medium (2).

1. The interface is the boundary where light interacts with the material's surface. The properties of the interface are primarily defined by the Roughness, Normal, Diffuse Albedo, F0 and F90 values fed into them.
2. The medium is the volume of matter beneath the interface where light is scattered, transmitted, and absorbed. The properties of the medium are primarily defined by the Mean Free Path (or MFP) inputs.

The Substrate Slab is a modular replacement for the monolithic Material root node in non-Substrate Materials. It is made up of multiple surface attributes, such as Diffuse, Specular, Roughness, Emissive, Cloth, Anisotropy, and many more. All Substrate BSDF nodes contain properties that are relevant to their named output for the type of material they produce, such as eye, hair, simple clear coat, and so on.

Traditional Materials relied on their Blend Mode to present inputs that can be used. Substrate uses its different BSDF slabs to define the type of material. And because these are not tied directly to Blend Modes any longer they can be layered and mixed to produce varying types of materials.

The primary Substrate Slab BSDF node used includes the following inputs:

Substrate Material Nodes

You can use the following types of nodes to author a Substrate material:

Substrate BSDF Nodes

Substrate BSDF (Bidirectional Scattering Distribution Function) nodes are used to represent most types of surfaces, they control the visual look of materials you author and automatically set their domain and shading model accordingly. The goal is to remove these aspects from being manually set on the material root node through the Details panel.

Substrate includes the following BSDFs:

The Slab BSDF is the primary node for authoring in Substrate and can be layered with other slabs. The other BSDFs are for specialized use cases and must be used on their own without mixing with other BSDFs.

Substrate Operator Nodes

Substrate Operator nodes mix or layer multiple Substrate Slabs to create complex and varied surfaces. If the Substrate Slab represents a piece of matter, Operators present ways to combine those pieces.

The following Substrate Operators are available to choose from:

Substrate Operators do not work with all Substrate BSDFs. Only Substrate Slab BSDF and Substrate Simple Clear Coat can use these Operator nodes.

Operator nodes include an option to blend their background and foreground into a single material when toggling on Use Parameter Blending. Because Substrate Operators can create complex material appearances by mixing and layering slabs together, their expense at runtime (primarily due to lighting evaluation) can be costly to performance. Parameter blending is an optimization that trades expensive lighting evaluation for runtime performance and less costly lighting evaluation.

For more information on this parameter blending optimization, see the Parameter Blending section of this page.

Substrate Coverage Weight

The Substrate Coverage Weight Operator controls the ratio of two slabs in a vertical layering operation. The Weight input drives the coverage of this material when layering together with a Substrate Vertical Layer Operator (like the example below). You can also use the operator to achieve translucent surfaces when using alpha-as-coverage or alpha-as-opacity, similar to how the Translucent Blend Mode uses opacity.

The graph above uses the Substrate Coverage Weight Operator, where Weight drives the amount of coverage applied over the bottom slab. A weight of 1 is fully opaque, blocking the green textured pattern. 0.5 is 50% transparent, mixing the two materials' colors and showing the texture pattern. 0 is fully transparent and shows only the green textured pattern.

Because the Weight input takes a Float3 value, you can also use a texture to drive the amount of coverage applied to the slab when layering materials.

Substrate Horizontal Layer

The Substrate Horizontal Layer Operator mixes two slabs together, one representing the background and the other the foreground. The Mix input controls their mixture ratio using linear interpolation.

The Background input is fully visible when it is 0, and the Foreground is fully visible when it equals 1. When the mix ratio is 0.5, the slabs are mixed together, and the mix is evaluated per-pixel. The Mix input can use textures to control the mix ratios, like the example below.

Substrate Vertical Layer

The Substrate Vertical Layer Operator takes a slab on its Top and Bottom inputs and layers them together. This node takes into account the thickness of the top layer to apply physically correct transmittance and scattering, which is similar to a coating operation, with the top layer covering the bottom layer. The appearance of the Bottom slab is dependent on the properties of the Top slab. If the BSDF passed into the Top input is fully opaque, the bottom slab is not seen at all.

Vertical layering is particularly useful in situations that call for a transparent, or semi-transparent, top coat over an opaque bottom layer. Examples include car paint, varnished wood surfaces, or wetness on a surface, such as puddles of water.

Substrate Add

The Substrate Add Operator combines two slabs and outputs their result. This operator is not considered physically plausible because it can produce a material where the outgoing energy from the surface exceeds the incoming energy. It is useful when art direction is more important than being physically plausible. However, to maintain physically accurate surfaces, this operator should be avoided.

Substrate Building Block Nodes

The Substrate Building Block nodes are a set of Material Functions that provide translations for some common use cases. Because these are material functions, you can open and inspect them directly.

The following Substrate Building Blocks are available to choose from:

Substrate Extras Nodes

The Substrate Extras nodes specify the type of material and function it serves, such as setting the Substrate material to be used as a decal or light function. These nodes are directly analogous to non-Substrate Materials that were assigned them as part of their Material Domain.

The following Substrate Extras are available to choose from:

These nodes are monolithic and must be used in isolation. They are not compatible with Substrate Operators.

As a good habit, we recommend placing these nodes at the end of the material graph, right before plugging into the Front Material input.

For example, when using the Substrate Convert To Decal node, any Substrate Material can be used as a decal material applied to Mesh Decals and Decal Actors in the scene.

Extras nodes automatically set the Material Domain when connected to the Front Material input of the Material Root node. Some Extras nodes require the Blend Mode to be changed to support the output.

When using the Substrate Convert To Decal node, you must set the Blend Mode to TranslucentGrey Transmittance, Colored Transmittance, TranslucentColorTransmittance, or AlphaComposite (Premultiplied Alpha). Otherwise, an error is displayed in the Stats panel of the Material Editor.

Substrate Helper Nodes

The Substrate Helper nodes are a set of nodes and material functions to perform some conversion or achieve something that legacy materials could do.

Additional Notes about Substrate Nodes

• Substrate Decal Materials
  • Substrate Decals currently use the same features as the legacy Decals blend mode path.
  • Future versions of Substrate Decals look to offer a more robust feature set similar to other features already available with Substrate, like Layer Translucent Slabs for things like water, blood, and goo. For example, having layers that can erode according to thickness like car paint scratches, ground steps, and tire marks.
• Substrate Shading Models Node
  • Opening any prior created material once Substrate is enabled for your project automatically converts the material to use its slabs. All existing inputs are fed into a Substrate Shading Model node.
  

  Avoid manually adding or using this node when creating new Substrate materials.

Substrate Stats Panel

The Substrate stats panel is available in the Material Editor below the Material Graph.

The Substrate panel displays stats about the material, simplification, and topology.

Parameter Blending with Operators

Using multiple BSDFs (Bidirectional Scattering Distribution Functions) per pixel slows the rendering process proportionally to their count in the material graph. It is twice as slow to evaluate the lighting for two BSDFs as it is a single one. This is true for both opaque and translucent surfaces.

Operator nodes include a Use Parameter Blending checkbox that attempts to optimize the performance and memory footprint of the material while maintaining the appearance of all the mixing and layer operations in the graph. Only the rightmost Operator node before the Material Root node must have the setting enabled. All other nodes in the graph automatically apply parameter blending.

Parameter blending is a good fallback option when the performance of multiple Slabs in a material is a concern. When enabled, two Slabs are merged into a single Slab that needs only a single lighting evaluation. This merge uses a lot less memory than two individual slabs.

The example materials below are taken from Content Examples Substrate Level without and with Use Parameter Blending enabled.

This material (M_Substrate_ShaderBall_IceRocks) uses two BSDFs. The left is without any blending and the right is using parameter blending.

This is a more complex material (M_Substrate_ShaderBall_AnisoOverSSS) that blends four slabs using two Vertical Layer Operators and a single Coverage Weight Operator. The material has a memory cost of 108 bytes per pixel. With Use Parameter Blending enabled, blending on all operators comes down to 28 bytes per pixel. The left material is without any blending and the right uses parameter blending.

Aside from Parameter Blending on Operators nodes, you can use one of the following workflows to achieve similar results:

• Blend the DiffuseAlbedo, F0, F90, Roughness, and other attributes manually in the graph. Pass all the attributes into a single Slab connected to the Front Material input. This approach can work well for isolated cases but may become unmanageable for a large library of complex materials.
• Use the graph-based Layered Materials workflow. Since it employs material functions to reuse work, this can scale better than the first option.

On lower end platforms, such as mobile, the compiler automatically enables parameter blending for the sake of performance. On intermediate platforms, parameter blending is progressively introduced on the bottom layers of a material in order to remain within the target performance and memory constraints.

Metalness and Specular Response

The parameterization used by Substrate differs from the DefaultLit Shading Model in non-Substrate (or legacy) materials — there is no longer a Metallic input. This parameterization attempts to do away with abstracted values (like metallic and specular) and moves toward physical properties with real-world units.

The reflective properties and specular response of Substrate Materials are defined with three attributes: DiffuseAlbedo, F0, and F90. Substrate enforces energy conservation automatically, ensuring the specular interface and medium do not add any energy. Therefore, the higher F0 gets, the less visible the diffuse contribution will be.

Metalness is emulated using the Substrate Metalness-To-DiffuseAlbedo-F0 Helper node. It takes BaseColor, Specular, and Metallic values as inputs and converts them to values which map to Diffuse Albedo and F0 on the Substrate Slab.

It's possible to achieve a wide variety of complex material diffuse and specular response to light using the EdgeColor or F90 inputs. For example, a red sphere with cyan-to-yellow perpendicular-to-tangent specular reflections.

Substrate FlipFlop Helper nodes are useful for achieving normal-based specular colorization. It controls the specular color of F0 and F90 as a function of NoV with adjustable falloff transition.

Rough Refraction

Substrate supports rough refraction through translucent objects and layered opaque materials with a translucent top layer. The blurriness of the scene background, as well as the distance to the refracted object, is computed from the primary material roughness when you use distortion / refraction.

Translucent Rough Refraction

To create a translucent material with rough refraction, set the following properties in the Details panel.

• Blend Mode: TranslucentColoredTransmittance, TranslucentGreyTransmittance, or ColoredTransmittanceOnly.
• Refraction Method: Index of Refraction (IOR), Pixel Normal Offset, or 2D Offset.

Pass values into Refraction, Roughness, and SSS MFP. The graph below produces a simplistic frosted glass result when roughness is greater than 0. A high SSS MFP value is used to create a fully transparent material, while the IOR of 1.514 approximates that of glass.

In the examples below, as the value of Roughness increases (0, 0.2, and 0.6 left to right), objects behind the glass become blurrier.

The blurring from rough refraction uses an approximation for accounting for depth behind translucent elements in the scene.

Opaque Rough Refraction

Substrate coating layers can blur the layers beneath them based on the roughness and thickness of the top coating layer. This type of refraction is more costly to performance and must be enabled for the project in the Project Settings under the Engine > Rendering category. Check the box for Substrate opaque material rough refraction to enable this feature.

The graph below shows an example using opaque material rough refraction using a vertically layered material with a clear coat on top of an opaque checkerboard.

The Roughness and Thickness parameters determine the strength of the blur being applied to the bottom material layer. Increasing either value increases how blurred the refraction becomes.

You can see this in the examples below, where the clear coat top layer on the left has a roughness and thickness of 0.1. The example on the right has a roughness of 0.8 and a thickness of 6, causing the bottom layer to become blurred.

Subsurface Scattering and Participating Media

A Substrate Slab contains participating media and can be used to simulate various volumetric appearances.

For instance, if you render an opaque material only, when a slab is at the bottom of the material topology, it is considered for subsurface scattering. There are two things to consider:

• If a Subsurface Profile is assigned to a Slab in the Details panel of the Material, the profile is used per pixel. Keep in mind that Subsurface Profiles are not blendable.
• If there is no Subsurface Profile assigned, scattering is determined from the DiffuseAlbedo and SSS MPF properties of the Slab. These properties are blendable.

The MFP (or Mean Free Path) of Subsurface Scattering is the distance (in centimeters) that different wavelengths of light will penetrate through a medium before encountering a collision. The example below shows a DiffuseAlbedo (colored white) and a SSS MFP (colored red) scaled from 0 to 1, left to right.

Any slab not at the bottom of an opaque material or used in a translucent one, is considered for a volumetric representation, which also relies on DiffuseAlbedo and SSS MFP attributes. The DiffuseAlbedo represents the medium's base color, accounting for single and multiple scattering.

The SSS MFP attribute is a way to control the medium's transmittance for a view perpendicular to a surface — it represents how visible the surface below is.

Examples of a material with transmittance color ranging from black to blue from left to right, and DiffuseAlbedo ranging from black to white from bottom to top.

Vertically layering a Slab on top of another is similar to a coating operation. The visibility of the bottom Slab is dependent on the transmittance of the top Slab. It is possible to reduce the coverage of the top Slab (like at the edge of a puddle of water on a surface) to make it progressively disappear. You can achieved this using a Coverage Weight operator node, which is analogous to alpha blending.

Examples of a material with Transmittance ranging from black to blue from left to right, and coverage ranging from 0 to 1 from bottom to top.

To achieve a specific transmittance or scattering color, you should use the Substrate Transmittance-To-MeanFreePath Helper node. The MFP is derived for the TransmittanceColor to match at normal incidence — when the surface is viewed perpendicularly, along the normal.

The example below shows a blue subsurface scattering on an opaque material that is pink, where the SSS MFP is derived from Transmittance Color.

It is not recommended to directly control MFP when trying to achieve a particular transmittance color because MFP is not a color, it's a measurement of light transport. The relationship between MFP and subsurface scattering color is non-intuitive and non-linear from an artist's perspective.

Translucency and Blend Modes

Substrate offers more robust options for translucent surface shading than is possible with traditional non-Substrate materials. The list of Substrate Blend Modes now makes more sense when considering a surface is made of matter (a Substrate Slab).

To create a Translucent Material:

1. Choose a Blend Mode that supports translucency.
   • Opaque
   • Masked
   • TranslucentColoredTransmittance
   • TranslucentGreyTransmittance
   • ColoredTransmittanceOnly
   • AlphaHoldout
2. With the Material Root node selected, use the Details panel to select a Lighting Mode. Choose between:
   • Surface Forward Shading
   • Surface Translucency Volume — this option supports reflections on the surface.
   • Volumetric NonDirectional — cheaper to use but does not reflect light.

Below is an example setup of a translucent Substrate material. Its Blend Mode is set to TranslucentColoredTransmittance and uses the Surface ForwardShading Lighting Mode. It uses a single Slab passed into the Front Material pin of the Material Root node to produce a translucent material that appears opaque.

Using the Substrate Coverage Weight Operator between the Slab and the Front Material input controls the transmittance of the material. Use the Weight input on the Substrate Coverage Weight node to control how transparent the material is.

You can use a constant value of 0 to 1 to control the opacity of the entire material (like above) or use a texture (like below) to control parts of the material.

You can go a step further to create a slab of matter that resembles colored glass by specifying the MFP of the participating medium. This is set up using the Transmittance-To-MeanFreePath Helper node, like in the example below, using an orange TransmittanceColor connected to SSS MFP to tint the material orange only in regions where it transmits light. The specified TransmittanceColor is the "target" color, which is reached at the provided thickness input (default is 0.01 centimeters).

Additional Notes about Substrate Translucency

• Translucent materials do not support Screen Space Subsurface Scattering, even though the Slab is considered a volume of participating medium.

Substrate Debug View Modes

When using Substrate, it's useful to see how its materials are performing and which would benefit from more attention. Substrate's debug visualization modes are in the View Modes dropdown list under the Substrate category.

Substrate includes the following visualization modes for debugging:

Click images in the table to enlarge them.

Limitations and Known Issues

• Substrates is experimental, so we recommend not using it for any production work.
• Platform support and testing is currently incomplete. As it moves into Beta and then Production Ready states, more testing coverage will happen.
• Features and UX are subject to change such that existing assets may behave differently or be invalidated altogether.
• Mobile is supported when using Mobile Forward Shading.
• Experimental support for Path Tracer.
• Some platforms and rendering paths, such as DirectX 11 (DX11) and Mac, exhibit issues and may not work entirely.

Additional Resources

• The Future of Materials in Unreal Engine - GDC 2023
• The State of Unreal Livestream — Timestamp: 02:29:42

• The Content Examples sample project includes a Level named "SubstrateMaterials" where you can explore different examples and demonstrations of how Substrate Materials function.

  Using Substrate with the Content Examples project requires you to enable Substrate for the project. Only this map has been validated for use with Substrate enabled. If you are using only a single instance of the Content Examples project, we recommend only enabling Substrate for this level and disabling it anytime you're using the rest of the project.