glTF

KHR_materials_pbrSpecularGlossiness

Khronos 3D Formats Working Group

Saurabh Bhatia, Microsoft @iamsbtron
Gary Hsu, Microsoft @bghgary
Patrick Cozzi, Cesium, @pjcozzi

Acknowledgments

Max Limper, Fraunhofer IGD, @mlimper_cg
Timo Sturm, Fraunhofer IGD, @_tsturm_
Miguel Sousa, Fraunhofer IGD, @mfportela
Maik Thöner, Fraunhofer IGD, @mthoener
Eric Haines, Autodesk, @pointinpolygon
Cedric Pinson, Sketchfab, @trigrou

Status

Complete

Dependencies

Written against the glTF 2.0 spec.

Overview

This extension defines the specular-glossiness material model from Physically-Based Rendering (PBR). This extensions allows glTF to support this additional workflow.

The best practices section specifies what an implementation must to do when encountering this extension, and how the extension interacts with the materials defined in the base specification.

Extending Materials

The PBR specular-glossiness materials are defined by adding the KHR_materials_pbrSpecularGlossiness extension to any glTF material. For example, the following defines a material like gold using specular-glossiness parameters.

{
    "materials": [
        {
            "name": "gold",
            "extensions": {
                "KHR_materials_pbrSpecularGlossiness": {
                    "diffuseFactor": [ 0.0, 0.0, 0.0, 1.0 ],
                    "specularFactor": [ 1.0, 0.766, 0.336 ],
                    "glossinessFactor": 0.9
                }
            }
        }
    ]
}

Specular - Glossiness

The specular-glossiness material model is defined by the following properties:

diffuse - Reflected diffuse color of the material
specular - Specular color of the material
glossiness - Glossiness of the material

The diffuse value represents the reflected diffuse color of the material. The specular value defines the specific measured reflectance value at normal incidence (F0). The glossiness property is a factor between 0.0 (rough surface) and 1.0 (smooth surface) and represents the surface irregularities that cause light diffusion.

The specular property from specular-glossiness material model is the same as the base color value from the metallic-roughness material model for metals. The glossiness property from specular-glossiness material model is related with the roughness property from the metallic-roughness material model and is defined as glossiness = 1 - roughness. See appendix for more details on how you can convert between these two material models.

The value for each property (diffuse, specular, glossiness) can be defined using factors or textures. The specular and glossiness properties are packed together in a single texture called specularGlossinessTexture. If a texture is not given, all respective texture components within this material model are assumed to have a value of 1.0. The factors (diffuseFactor, specularFactor, glossinessFactor) scale, in linear space, the components given in the respective textures (diffuseTexture, specularGlossinessTexture). Texture content must be converted to linear space before it is used for any lighting computations.

The following equations show how to calculate bidirectional reflectance distribution function (BRDF) inputs (c_diff, F₀, α) from the metallic-roughness material properties. In addition to the material properties, if a primitive specifies a vertex color using the attribute semantic property COLOR_0, then this value acts as an additional linear multiplier to diffuse.

c_diff = diffuse.rgb * (1 - max(specular.r, specular.g, specular.b))
F₀ = specular
α = (1 - glossiness) ^ 2

All implementations should use the same calculations for the BRDF inputs. Implementations of the BRDF itself can vary based on device performance and resource constraints. See appendix for more details on the BRDF calculations.

The following table lists the allowed types and ranges for the specular-glossiness model:

	Type	Description	Required
diffuseFactor	`number[4]`	The reflected diffuse factor of the material.	No, default:`[1.0,1.0,1.0,1.0]`
diffuseTexture	`textureInfo`	The diffuse texture.	No
specularFactor	`number[3]`	The specular RGB color of the material.	No, default:`[1.0,1.0,1.0]`
glossinessFactor	`number`	The glossiness or smoothness of the material.	No, default:`1.0`
specularGlossinessTexture	`textureInfo`	The specular-glossiness texture.	No

Additional properties are allowed.

JSON schema: glTF.KHR_materials_pbrSpecularGlossiness.schema.json

diffuseFactor

The RGBA components of the reflected diffuse color of the material. Metals have a diffuse value of [0.0, 0.0, 0.0]. The fourth component (A) is the opacity of the material. The values are linear.

Type: number[4]
Required: No, default:[1.0,1.0,1.0,1.0]
Range: [0,1] for all components

diffuseTexture

The diffuse texture. This texture contains RGB(A) components of the reflected diffuse color of the material in sRGB color space. If the fourth component (A) is present, it represents the alpha coverage of the material. Otherwise, an alpha of 1.0 is assumed. The alphaMode property specifies how alpha is interpreted. The stored texels must not be premultiplied.

Type: textureInfo
Required: No

specularFactor

The specular RGB color of the material. This value is linear.

Type: number[3]
Required: No, default:[1.0,1.0,1.0]
Range: [0,1] for all components

glossinessFactor

The glossiness or smoothness of the material. A value of 1.0 means the material has full glossiness or is perfectly smooth. A value of 0.0 means the material has no glossiness or is perfectly rough. This value is linear.

Type: number
Required: No, default:1.0
Range: [0,1]

specularGlossinessTexture

The specular-glossiness texture is a RGBA texture, containing the specular color (RGB) in sRGB space and the glossiness value (A) in linear space.

Type: textureInfo
Required: No

Additional Maps

The additional maps defined in glTF materials node can also be used with the PBR specular-glossiness material model parameters defined in this extension.

{
    "materials": [
        {
            "extensions": {
                "KHR_materials_pbrSpecularGlossiness": {
                    "diffuseTexture": {
                        "index": 0
                    },
                    "glossinessFactor": 0.5,
                    "specularGlossinessTexture": {
                        "index": 1
                    }
                }
            },
            "normalTexture": {
                "index": 2
            },
            "emissiveFactor": [ 0.2, 0.1, 0.0 ]
        }
    ]
}

Best practices

The PBR specular-glossiness extension can be used along with PBR metallic-roughness material model in glTF to enable support for both PBR workflows. Specular-glossiness can represent a broader range of materials compared to metallic-roughness. However, supporting specular-glossiness on low-resource devices may not be possible as it is more resource heavy than the metallic-roughness model. To get the best of both worlds a glTF asset can include both metallic-roughness and specular-glossiness in a single glTF asset. This allows the asset to take advantage of richer specular-glossiness materials where possible and still have a fall back with metallic-roughness to ensure that the asset can be rendered everywhere. Since such an approach requires including both material models it is best suited for a web scenario where a client can choose to download the appropriate material model from a server hosting the glTF asset.

The following example shows how the same material can be defined using both metallic-roughness as well as specular-glossiness material models:

{
    "materials": [
        {
            "name": "gold",
            "pbrMetallicRoughness": {
                "baseColorfactor": [ 1.0, 0.766, 0.336, 1.0 ],
                "roughnessFactor": 0.1
            },
            "extensions": {
                "KHR_materials_pbrSpecularGlossiness": {
                    "diffuseFactor": [ 0.0, 0.0, 0.0, 1.0 ],
                    "specularFactor": [ 1.0, 0.766, 0.336 ],
                    "glossinessFactor": [ 0.9 ]
                }
            }
        }
    ]
}

If the specular-glossiness extension is included in an asset, then any client implementation that supports the extension should always render the asset using the specular-glossiness material properties.

A material may specify the specular-glossiness extension without including any metallic-roughness properties. For such an asset, the best practice is to include the specular-glossiness extension using the extensionRequired property so that the asset is only loaded by clients that can render the specular-glossiness material. This approach trades asset portability for a smaller file size. If extensionsUsed property is specified instead of extensionsRequired, then a client that does not support the specular-glossiness extension will render the asset as if no material was specified.

The following table describes the expected rendering behavior based on the material definitions included in the asset:

	Client supports metallic-roughness	Client supports metallic-roughness and specular-glossiness
Asset has metallic-roughness	Render metallic-roughness	Render metallic-roughness
Asset has metallic-roughness and specular-glossiness	Render metallic-roughness	Render specular-glossiness
Asset has specular-glossiness with `extensionsRequired`	Fail to load	Render specular-glossiness
Asset has specular-glossiness with `extensionsUsed`	Render as if no material	Render specular-glossiness

Appendix

Conversion between the two PBR material models