Physically Based Rendering in Materials

Physically based rendering (PBR) is rendering in a photorealistic way by simulating the physics of real-world lighting models. The following sections describe how the concepts of PBR rendering are represented in Atom’s core material types.

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Core PBR Material Types

The following core material types are included in Atom:

• StandardPBR
  A full-featured PBR material type that provides as much functionality as possible with a limited number of render targets.

  This file is located in Gems/Atom/Feature/Common/Assets/Materials/Types/StandardPBR.materialtype .

• EnhancedPBR
  An enhanced version of StandardPBR that includes additional features, but comes at a higher performance cost. It supports more advanced features that require additional render targets (g-buffers).

  This file is located in Gems/Atom/Feature/Common/Assets/Materials/Types/EnhancedPBR.materialtype .

• BasePBR
  A simplified alternative to StandardPBR that is limited to only the most common PBR features such as base color, metallic, roughness, and normal. This is especially helpful for testing and debugging, because it eliminates some features from StandardPBR that add considerable complexity.

  This file is located in Gems/Atom/Feature/Common/Assets/Materials/Types/BasePBR.materialtype .

• StandardMultilayerPBR
  Combines up to three layers of StandardPBR, blended together. This is especially useful for breaking up repeated patterns on large surfaces.

  This file is located in Gems/Atom/Feature/Common/Assets/Materials/Types/StandardMultilayerPBR.materialtype .

More types are available or may be added beyond those listed above, especially for specialized use cases like skin and eyes. To find the full list of available material types check the Gems/Atom/Feature/Common/Assets/Materials/Types folder.

PBR Shading Model

The PBR shading model is composed of properties that describe how a material interacts in the physical world. At the basic level, the PBR shading model requires the following properties: base color, metallic, roughness, and specular reflectivity. These properties are enough to define materials such as wood, metal, concrete, and other raw materials. However, materials can become much more complex with properties such as clear coat, subsurface scattering, and more. For example, varnished wood is made from a basic wood material type with an additional clear coat layer on top.

The following list of properties are used to define PBR materials in Atom. An overview of which properties are included in which material type is shown in the Properties in PBR Material Types table.

Base Color

The base color defines the diffuse albedo for non-metals, and the specular color for metals.

When configuring the Base Color property group in a PBR material type, you can set a linear sRGB color in the Color property. This color can be combined with a texture by assigning an image to the Texture Map property. The Texture Blend Mode dictates how the Color and Texture Map are combined. And the Factor value can be used to adjust the strength of the blend. Colors are stored as linear sRGB on disk and later converted to ACEScg before passing to the shaders and the GPU.

Metallic

The Metallic properties determine whether a material behaves as a dielectric (non-metal) surface, or as a conductor (metal) surface. The metallic value is usually either a completely metal (1) or completely non-metal (0), though in between values will occur when transitioning between the two using a texture map.

When configuring the Metallic property group in a PBR material type, the metallic value is defined using the Factor property for uniform surfaces. For surfaces with varying levels of metalness, you can assign an image in the Texture Map property to indicate values between 0 and 1 per pixel.

Roughness

The Roughness properties determine the roughness or glossiness of a surface. The rougher a surface is, the more blurred its reflections appear to be.

When configuring the Roughness property group of a PBR material, the Factor property defines how rough (factor = 1) or smooth (factor = 0) a material is, for uniform surfaces. For surfaces with varying levels of roughness, you can assign an image in the Texture Map property to indicate roughness values per pixel. You can adjust how the values of the texture map translate to roughness level using the Upper Bound and Lower Bound properties.

Specular Reflectance f0

The Specular Reflectance f0 properties determine how much light reflects from dielectric (non-metal) surfaces. (Specular reflectivity on conductor (metal) surfaces are controlled by the Base Color). Specular reflection is based on the Fresnel effect, a model which describes how the amount of light that reflects from a surface depends on the viewing angle and the index of refraction (IOR). When looking straight ahead at a surface, the view is at a 0-degree angle, also known as a normal incidence. From this viewing angle, the amount of light reflected is denoted by f0. The f0 values lie in the range 0 to 0.08, meaning the amount of light reflected can be between 0% to 8%.

When configuring the Specular Reflectivity f0 property group in a PBR material type, you can specify a specular reflectance value between 0 and 1 in the Factor property. This value is mapped to the [0,0.08] range, representing an f0 value between 0% and 8%. For surfaces with varying levels of specular reflectivity, you can assign an image to the Texture Map property to indicate values between 0 and 1 per pixel. A texture map is most useful for composite materials, or materials with a significant variation of material types (for example, material for a character with skin, a metal belt, and a leather watch).

Multi-scattering Compensation

By default, Atom uses a lighting model that assumes light only bounces once (single-scattering); but in reality, light may bounce multiple times (multi-scattering). With multi-scattering compensation, you can configure the lighting model to perform additional calculations to produce more accurate surface lighting.

You can take multi-scattering into account in your materials by toggling Multiscattering Compensation. For a bit extra performance cost, this simulates the fact that light may bounce off a rough surface multiple times at the microscopic level. This feature makes the lighting on certain materials appear more realistic (brighter). The impact is most noticeable on rough metallic surfaces. For smooth surfaces and non-metal surfaces, the impact likely won’t be noticed and should be disabled.

Properties in Material Types

The following table lists which properties are included in which core PBR material types.