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KHR_materials_sheen

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 sheen that can be layered on top of an existing glTF material definition. A sheen layer is a common technique used in Physically-Based Rendering to represent cloth and fabric materials, for example. See Theory, Documentation and Implementations

Extending Materials

The PBR sheen materials are defined by adding the KHR_materials_sheen extension to any compatible glTF material (excluding those listed above). For example, the following defines a material like velvet.

{
    "materials": [
        {
            "name": "velvet",
            "extensions": {
                "KHR_materials_sheen": {
                    "sheenColorFactor": [0.9, 0.9, 0.9]
                }
            }
        }
    ]
}

Sheen

The following parameters are contributed by the KHR_materials_sheen extension:

Type Description Required
sheenColorFactor array The sheen color in linear space No, default: [0.0, 0.0, 0.0]
sheenColorTexture textureInfo The sheen color (RGB).
The sheen color is in sRGB transfer function		 No
sheenRoughnessFactor number The sheen roughness. No, default: 0.0
sheenRoughnessTexture textureInfo The sheen roughness (Alpha) texture. No

The sheen BRDF is layered on top of the glTF 2.0 Metallic-Roughness material. If clearcoat (KHR_materials_clearcoat) is active at the same time, clearcoat is layered on top of sheen. The sheenColorFactor determines the view-independent intensity of the sheen BRDF. If sheenColorFactor is zero, the whole sheen layer is disabled. Implementations of the BRDF itself can vary based on device performance and resource constraints.

If a texture is defined:

  • The sheen color is computed with : sheenColor = sheenColorFactor * sampleLinear(sheenColorTexture).rgb.
  • The sheen roughness is computed with : sheenRoughness = sheenRoughnessFactor * sample(sheenRoughnessTexture).a.

Otherwise, sheenColor = sheenColorFactor and sheenRoughness = sheenRoughnessFactor

The sheen BRDF simulates the back-scattering of velvet-like materials. The specular response from velvet-like materials can be modeled as the reflection from specular, cylindrical micro-fibers mainly oriented in the normal direction (Westin et al. (1992)). This results in a forest of narrow cylinders extending from a fabric base. The sheen roughness determines how much the micro-fibers diverge from this direction. A small roughness means that the random perturbation around the normal direction is small, resulting in a sharp specular response around grazing angles. A large roughness on the other hand results in a smooth specular response around grazing angles. To model the specular response from micro-fibers, we use the exponentiated sinusoidal distribution term introduced by Conty and Kulla (2017) which is based on microfacet theory. We use the mapping r = sheenRoughness^2 which results in more perceptually linear changes in the roughness.

The sheen roughness is independent from the material roughness to allow materials like this one, with high material roughness and small sheen roughness:

Cushion

Not all incoming light is reflected at a micro-fiber. Some of the light may hit the base layer, either directly or as a result of multiple bounces on the micro-geometry. Light that hits the base layer behaves as specified in the glTF 2.0 Metallic-Roughness material.

Implementation Notes

This section is non-normative.

All implementations should use the same calculations for the BRDF inputs. See Appendix B for more details on the BRDF calculations.

The sheen formula f_sheen follows the common microfacet form:

fsheen = sheenColor * sheenFresnel * sheenDistribution * sheenVisibility = sheenColor * FS * GS * DS / (4 * abs(dot(N, L)) * abs(dot(N, V)))

Sheen distribution

The sheen distribution follows the "Charlie" sheen definition from ImageWorks Conty and Kulla (2017):

alphaG = sheenRoughness * sheenRoughness
invR = 1 / alphaG
cos2h = NdotH * NdotH
sin2h = 1 - cos2h
sheenDistribution = (2 + invR) * pow(sin2h, invR * 0.5) / (2 * PI);

Sheen visibility

The "Charlie" sheen visibility is also defined in the same document:

float l(float x, float alphaG)
{
    float oneMinusAlphaSq = (1.0 - alphaG) * (1.0 - alphaG);
    float a = mix(21.5473, 25.3245, oneMinusAlphaSq);
    float b = mix(3.82987, 3.32435, oneMinusAlphaSq);
    float c = mix(0.19823, 0.16801, oneMinusAlphaSq);
    float d = mix(-1.97760, -1.27393, oneMinusAlphaSq);
    float e = mix(-4.32054, -4.85967, oneMinusAlphaSq);
    return a / (1.0 + b * pow(x, c)) + d * x + e;
}

float lambdaSheen(float cosTheta, float alphaG)
{
    return abs(cosTheta) < 0.5 ? exp(l(cosTheta, alphaG)) : exp(2.0 * l(0.5, alphaG) - l(1.0 - cosTheta, alphaG));
}

sheenVisibility = 1.0 / ((1.0 + lambdaSheen(NdotV, alphaG) + lambdaSheen(NdotL, alphaG)) * (4.0 * NdotV * NdotL));

However, depending on device performance and resource constraints, one can use a simpler visibility term, like the one defined by Ashikhmin and Premoze (2007) (but that will make the BRDF not energy conserving when using the albedo-scaling technique described below):

sheenVisibility = 1 / (4 * (NdotL + NdotV - NdotL * NdotV))

Sheen Fresnel

The Fresnel term may be omitted, i.e., F = 1.

Sheen layering

Albedo-scaling technique

The sheen layer can be combined with the base layer with an albedo-scaling technique described in Conty and Kulla (2017). The base layer fdiffuse + fspecular from Appendix B is scaled with sheenAlbedoScaling to avoid energy gain.

f = fsheen + (fdiffuse + fspecular) * sheenAlbedoScaling

float max3(vec3 v) { return max(max(v.x, v.y), v.z); }

sheenAlbedoScaling = min(1.0 - max3(sheenColor) * E(VdotN), 1.0 - max3(sheenColor) * E(LdotN))

The values E(x) can be looked up in a table which can be found in section 6.2.3 of Enterprise PBR Shading Model if you use the "Charlie" visibility term. If you use Ashikhmin instead, you can get the lookup table by using the cmgen tool from Filament, with the --ibl-dfg and --ibl-dfg-cloth flags: the table is in the blue channel of the generated picture. The lookup must be done with x = VdotN and y = sheenRoughness.

If you want to trade a bit of accuracy for more performance, you can use the VdotN term only and thus avoid doing multiple lookups for LdotN. The albedo scaling term is simplified to:

sheenAlbedoScaling = 1.0 - max3(sheenColor) * E(VdotN)

In this simplified form, it can be used to scale the base layer for both direct and indirect lights:

specular_direct *= sheenAlbedoScaling;
diffuse_direct *= sheenAlbedoScaling;
environmentIrradiance_indirect *= sheenAlbedoScaling
specularEnvironmentReflectance_indirect *= sheenAlbedoScaling

Reference

Theory, Documentation and Implementations

Stephen H. Westin, James R. Arvo, Kenneth E. Torrance – “Predicting Reflectance Functions from Complex Surfaces”, SIGGRAPH 1992

Alejandro Conty Estevez, Christopher Kulla – “Production Friendly Microfacet Sheen BRDF”, SIGGRAPH 2017

Michael Ashikhmin, Simon Premoze – “Distribution-based BRDFs”, 2007

Filament Material models - Cloth model

cmgen tool from Filament

David Neubelt, Matt Pettineo – “Crafting a Next-Gen Material Pipeline for The Order: 1886”, SIGGRAPH 2013

cloth-shading

Enterprise PBR Shading Model - Sheen

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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