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KHR_materials_clearcoat

Contributors

Copyright 2018-2020 The Khronos Group Inc. All Rights Reserved. glTF is a trademark of The Khronos Group Inc. See Appendix for full Khronos Copyright Statement.

Status

Complete, Ratified by the Khronos Group

Dependencies

Written against the glTF 2.0 spec.

Exclusions

  • This extension must not be used on a material that also uses KHR_materials_pbrSpecularGlossiness.
  • This extension must not be used on a material that also uses KHR_materials_unlit.

Overview

This extension defines a clear coating that can be layered on top of an existing glTF material definition. A clear coat is a common technique used in Physically-Based Rendering to represent a protective layer applied to a base material. See Theory, Documentation and Implementations

Extending Materials

The PBR clearcoat materials are defined by adding the KHR_materials_clearcoat extension to any compatible glTF material (excluding those listed above). For example, the following defines a material like varnish using clearcoat parameters.

{
    "materials": [
        {
            "name": "varnish",
            "extensions": {
                "KHR_materials_clearcoat": {
                    "clearcoatFactor": 1.0
                }
            }
        }
    ]
}

Clearcoat

The following parameters are contributed by the KHR_materials_clearcoat extension:

Type Description Required
clearcoatFactor number The clearcoat layer intensity. No, default: 0.0
clearcoatTexture textureInfo The clearcoat layer intensity texture. No
clearcoatRoughnessFactor number The clearcoat layer roughness. No, default: 0.0
clearcoatRoughnessTexture textureInfo The clearcoat layer roughness texture. No
clearcoatNormalTexture normalTextureInfo The clearcoat normal map texture. No

If clearcoatFactor (in the extension) is zero, the whole clearcoat layer is disabled.

The values for clearcoat layer intensity and clearcoat roughness can be defined using factors, textures, or both. If the clearcoatTexture or clearcoatRoughnessTexture is not given, respective texture components are assumed to have a value of 1.0. All clearcoat textures contain RGB components in linear space. If both factors and textures are present, the factor value acts as a linear multiplier for the corresponding texture values.

clearcoat = clearcoatFactor * clearcoatTexture.r
clearcoatRoughness = clearcoatRoughnessFactor * clearcoatRoughnessTexture.g

If clearcoatNormalTexture is not given, no normal mapping is applied to the clear coat layer, even if normal mapping is applied to the base material. Otherwise, clearcoatNormalTexture may be a reference to the same normal map used by the base material, or any other compatible normal map.

A mesh primitive using a clearcoat material with a clearcoat normal texture MUST have a defined tangent space, i.e., it MUST have NORMAL and TANGENT attributes or its base material MUST have a normal texture. When the mesh primitive does not have NORMAL or TANGENT vectors, they are computed as defined in the glTF 2.0 specification.

Since the glTF 2.0 specification does not mandate any particular tangent space derivation algorithm, mesh primitives using clearcoat materials with clearcoat normal textures SHOULD always provide their NORMAL and TANGENT vectors.

When the material has both normalTexture and clearcoatNormalTexture properties defined, these textures SHOULD use the same texture coordinates because they operate in the same tangent space and their texel values are usually correlated to each other.

The clearcoat effect is modeled via a microfacet BRDF. The BRDF is layered on top of the glTF 2.0 Metallic-Roughness material, including emission and all extensions, using a new fresnel_coat function:

# from glTF 2.0 Metallic-Roughness material (core)
material = mix(dielectric_brdf, metal_brdf, metallic)

# clearcoat
clearcoat_brdf = specular_brdf(
  normal = clearcoatNormal,
  α = clearcoatRoughness^2)

# layering
coated_material =
  fresnel_coat(
    normal = clearcoatNormal,
    ior = 1.5,
    weight = clearcoat,
    base = material,
    layer = clearcoat_brdf)

The fresnel_coat function adds a BRDF as a layer on top of another BSDF according to weight and a Fresnel term. The layer is weighted with weight*fresnel(ior). The base is weighted with 1-(weight*fresnel(ior)). normal is a float3 vector that affects the top layer, but not the base.

Implementation

This section is non-normative.

Implementations of the BRDF or the layering operator can vary based on device performance and resource constraints.

Clearcoat BRDF

The specular BRDF for the clearcoat layer clearcoat_brdf is computed using the specular term from the glTF 2.0 Metallic-Roughness material, defined in Appendix B. Roughness and normal are taken from clearcoatNormal and clearcoatRoughness.

Layering

The fresnel_coat function is computed using the Schlick Fresnel term from the glTF 2.0 Metallic-Roughness material, defined in Appendix B.

function fresnel_coat(normal, ior, weight, base, layer) {
  f0 = ((1-ior)/(1+ior))^2
  fr = f0 + (1 - f0)*(1 - abs(dot(V, normal)))^5
  return mix(base, layer, weight * fr)
}

Applying the functions we arrive at the coated material

coated_material = mix(material, clearcoat_brdf(clearcoatRoughness^2), clearcoat * (0.04 + (1 - 0.04) * (1 - VdotNc)^5))

and finally, substituting and simplifying, using some symbols from Appendix B and Nc for the clearcoat normal:

clearcoat_fresnel = 0.04 + (1 - 0.04) * (1 - abs(VdotNc))^5
clearcoat_alpha = clearcoatRoughness^2
clearcoat_brdf = D(clearcoat_alpha) * G(clearcoat_alpha) / (4 * abs(VdotNc) * abs(LdotNc))

coated_material = mix(material, clearcoat_brdf, clearcoat * clearcoat_fresnel)

Emission

The clearcoat layer is on top of emission in the layering stack. Consequently, the emission is darkened by the Fresnel term.

coated_emission = emission * (1 - clearcoat * clearcoat_fresnel)

Discussion

In order to make the material energy conserving with a simple layering function, we compute the microfacet Fresnel term with NdotV instead of VdotH. That means that we ignore the orientation of the microsurface. As the clearcoat roughness is usually very low the microfacets orientation is very close to the normal direction, and NdotV ≈ NdotL.

The simple layering function ignores many effects that occur between clearcoat and base layer. For example:

  • The clearcoat layer is assumed to be infinitely thin. There is no refraction.
  • The index of refraction of clearcoat and base layer do not influence each other. The Fresnel terms are computed independently.
  • There is no scattering between layers.
  • There is no diffraction.

More sophisticated layering techniques that improve the accuracy of the renderings are described in Appendix B.

Schema

Reference

Theory, Documentation and Implementations

Appendix: Full Khronos Copyright Statement

Copyright 2018-2020 The Khronos Group Inc.

Some parts of this Specification are purely informative and do not define requirements necessary for compliance and so are outside the Scope of this Specification. These parts of the Specification are marked as being non-normative, or identified as Implementation Notes.

Where this Specification includes normative references to external documents, only the specifically identified sections and functionality of those external documents are in Scope. Requirements defined by external documents not created by Khronos may contain contributions from non-members of Khronos not covered by the Khronos Intellectual Property Rights Policy.

This specification is protected by copyright laws and contains material proprietary to Khronos. Except as described by these terms, it or any components may not be reproduced, republished, distributed, transmitted, displayed, broadcast or otherwise exploited in any manner without the express prior written permission of Khronos.

This specification has been created under the Khronos Intellectual Property Rights Policy, which is Attachment A of the Khronos Group Membership Agreement available at www.khronos.org/files/member_agreement.pdf. Khronos grants a conditional copyright license to use and reproduce the unmodified specification for any purpose, without fee or royalty, EXCEPT no licenses to any patent, trademark or other intellectual property rights are granted under these terms. Parties desiring to implement the specification and make use of Khronos trademarks in relation to that implementation, and receive reciprocal patent license protection under the Khronos IP Policy must become Adopters and confirm the implementation as conformant under the process defined by Khronos for this specification; see https://www.khronos.org/adopters.

Khronos makes no, and expressly disclaims any, representations or warranties, express or implied, regarding this specification, including, without limitation: merchantability, fitness for a particular purpose, non-infringement of any intellectual property, correctness, accuracy, completeness, timeliness, and reliability. Under no circumstances will Khronos, or any of its Promoters, Contributors or Members, or their respective partners, officers, directors, employees, agents or representatives be liable for any damages, whether direct, indirect, special or consequential damages for lost revenues, lost profits, or otherwise, arising from or in connection with these materials.

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