PBR materials

PBR materials are one of the supported material types in Azure Remote Rendering. They are used for meshes that should receive realistic lighting.

PBR stands for Physically Based Rendering and means that the material describes the visual properties of a surface in a physically plausible way, such that realistic results are possible under all lighting conditions. Most modern game engines and content creation tools support PBR materials because they are considered the best approximation of real world scenarios for real-time rendering.

Helmet glTF sample model rendered by ARR

PBR materials are not a universal solution, though. There are materials that reflect color differently depending on the viewing angle. For example, some fabrics or car paints. These kinds of materials are not handled by the standard PBR model, and are currently not supported by Azure Remote Rendering. This includes PBR extensions, such as Thin-Film (multi-layered surfaces) and Clear-Coat (for car paints).

Common material properties

These properties are common to all materials:

  • albedoColor: This color is multiplied with other colors, such as the albedoMap or vertex colors. If transparency is enabled on a material, the alpha channel is used to adjust the opacity, with 1 meaning fully opaque and 0 meaning fully transparent. Default is white.

    Note

    When a PBR material is fully transparent, like a perfectly clean piece of glass, it still reflects the environment. Bright spots like the sun are still visible in the reflection. This is different for color materials.

  • albedoMap: A 2D texture for per-pixel albedo values.

  • alphaClipEnabled and alphaClipThreshold: If alphaClipEnabled is true, all pixels where the albedo alpha value is lower than alphaClipThreshold won't be drawn. Alpha clipping can be used even without enabling transparency and is much faster to render. Alpha clipped materials are still slower to render than fully opaque materials, though. By default alpha clipping is disabled.

  • textureCoordinateScale and textureCoordinateOffset: The scale is multiplied into the UV texture coordinates, the offset is added to it. Can be used to stretch and shift the textures. The default scale is (1, 1) and offset is (0, 0).

  • useVertexColor: If the mesh contains vertex colors and this option is enabled, the meshes' vertex color is multiplied into the albedoColor and albedoMap. By default useVertexColor is disabled.

  • isDoubleSided: If double-sidedness is set to true, triangles with this material are rendered even if the camera is looking at their back faces. For PBR materials lighting is also computed properly for back faces. By default this option is disabled. See also Single-sided rendering.

  • TransparencyWritesDepth: If the TransparencyWritesDepth flag is set on the material and the material is transparent, objects using this material will also contribute to the final depth buffer. See the PBR material flag transparent in the next section. Enabling this feature is recommended if your use case needs a more plausible late stage reprojection of fully transparent scenes. For mixed opaque/transparent scenes, this setting may introduce implausible reprojection behavior or reprojection artifacts. For this reason, the default and recommended setting for the general use case is to disable this flag. The written depth values are taken from the per-pixel depth layer of the object that is closest to the camera.

  • FresnelEffect: This material flag enables the additive fresnel effect on the respective material. The appearance of the effect is governed by the other fresnel parameters explained in the following.

  • FresnelEffectColor: The fresnel color used for this material. Only important when the fresnel effect bit has been set on this material (see above). This property controls the base color of the fresnel shine (see fresnel effect for a full explanation). Currently only the rgb-channel values are important and the alpha value will be ignored.

  • FresnelEffectExponent: The fresnel exponent used for this material. Only important when the fresnel effect bit has been set on this material (see above). This property controls the spread of the fresnel shine. The minimum value 0.01 causes a spread across the whole object. The maximum value 10.0 constricts the shine to only the most gracing edges visible.

PBR material properties

The core idea of physically based rendering is to use BaseColor, Metalness, and Roughness properties to emulate a wide range of real-world materials. A detailed description of PBR is beyond the scope of this article. For more information about PBR, see other sources. The following properties are specific to PBR materials:

  • baseColor: In PBR materials, the albedo color is referred to as the base color. In Azure Remote Rendering the albedo color property is already present through the common material properties, so there is no additional base color property.

  • roughness and roughnessMap: Roughness defines how rough or smooth the surface is. Rough surfaces scatter the light in more directions than smooth surfaces, which make reflections blurry rather than sharp. The value range is from 0.0 to 1.0. When roughness equals 0.0, reflections will be sharp. When roughness equals 0.5, reflections will become blurry.

    If both a roughness value and a roughness map are supplied, the final value will be the product of the two.

  • metalness and metalnessMap: In physics, this property corresponds to whether a surface is conductive or dielectric. Conductive materials have different reflective properties, and they tend to be reflective with no albedo color. In PBR materials, this property affects how much a surface reflects the surrounding environment. Values range from 0.0 to 1.0. When metalness is 0.0, the albedo color is fully visible and the material looks like plastic or ceramics. When metalness is 0.5, it looks like painted metal. When metalness is 1.0, the surface almost completely loses its albedo color and only reflects the surroundings. For instance, if metalness is 1.0 and roughness is 0.0 then a surface looks like real-world mirror.

    If both a metalness value and a metalness map are supplied, the final value will be the product of the two.

    Spheres rendered with different metalness and roughness values

    In the picture above, the sphere in the bottom-right corner looks like a real metal material, the bottom-left looks like ceramic or plastic. The albedo color is also changing according to physical properties. With increasing roughness, the material loses reflection sharpness.

  • normalMap: To simulate fine grained detail, a normal map can be provided.

  • occlusionMap and aoScale: Ambient occlusion makes objects with crevices look more realistic by adding shadows to occluded areas. Occlusion value range from 0.0 to 1.0, where 0.0 means darkness (occluded) and 1.0 means no occlusions. If a 2D texture is provided as an occlusion map, the effect is enabled and aoScale acts as a multiplier.

    An object rendered with and without ambient occlusion

  • transparent: For PBR materials, there is only one transparency setting: it is enabled or not. The opacity is defined by the albedo color's alpha channel. When enabled, a more complex rendering pipeline is invoked to draw semi-transparent surfaces. Azure Remote Rendering implements true order independent transparency (OIT).

    Transparent geometry is expensive to render. If you only need holes in a surface, for example for the leaves of a tree, it is better to use alpha clipping instead.

    Spheres rendered with zero to full transparency Notice in the image above, how the right-most sphere is fully transparent, but the reflection is still visible.

    Important

    If any material is supposed to be switched from opaque to transparent at runtime, the renderer must use the TileBasedComposition rendering mode. This limitation does not apply to materials that are converted as transparent materials to begin with.

Technical details

Azure Remote Rendering uses the Cook-Torrance micro-facet BRDF with GGX NDF, Schlick Fresnel, and a GGX Smith correlated visibility term with a Lambert diffuse term. This model is the de facto industry standard at the moment. For more in-depth details, refer to this article: Physically based Rendering - Cook Torrance

An alternative to the Metalness-Roughness PBR model used in Azure Remote Rendering is the Specular-Glossiness PBR model. This model can represent a broader range of materials. However, it is more expensive, and usually does not work well for real-time cases. It is not always possible to convert from Specular-Glossiness to Metalness-Roughness as there are (Diffuse, Specular) value pairs that cannot be converted to (BaseColor, Metalness). The conversion in the other direction is simpler and more precise, since all (BaseColor, Metalness) pairs correspond to well-defined (Diffuse, Specular) pairs.

API documentation

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