Merge pull request #1688 from sebavan/KHR_materials_sheen

KHR_materials_sheen

extensions/2.0/Khronos/KHR_materials_sheen/README.md

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+# KHR\_materials\_sheen
+
+## Contributors
+
+* Sebastien Vandenberghe, Microsoft, [@sebavan](https://github.com/sebavan)
+* Alexis Vaginay, Microsoft, [@Popov72](https://github.com/Popov72)
+* Tobias Häußler, Dassault System, [@proog128](https://github.com/proog128)
+* Ed Mackey, Analytical Graphics, Inc.
+* Romain Guy, Google, [@romainguy](https://github.com/romainguy)
+* Mike Bond, Adobe, [@MiiBond](https://github.com/MiiBond)
+* Don McCurdy, Google, [@donmccurdy](https://twitter.com/donrmccurdy)
+* Gary Hsu, Microsoft, [@bghgary](https://twitter.com/bghgary)
+* Jekfer Bichon, Dassault System, [@EliphasNUIT](https://github.com/EliphasNUIT)
+* Aidin Abedi, Epic Games, [@aidinabedi](https://github.com/aidinabedi)
+
+Copyright (C) 2018-2020 The Khronos Group Inc. All Rights Reserved. glTF is a trademark of The Khronos Group Inc.
+See [Appendix](#appendix-full-khronos-copyright-statement) for full Khronos Copyright Statement.
+
+## Status
+
+Draft
+
+## 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](#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.
+
+```json
+{
+    "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`](/specification/2.0/README.md#reference-textureInfo)             | The sheen color (RGB).<br> The sheen color is in sRGB transfer function | No               |
+|**sheenRoughnessFactor**               | `number`                                                                        | The sheen roughness.                   | No, default: `0.0`             |
+|**sheenRoughnessTexture**         | [`textureInfo`](/specification/2.0/README.md#reference-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)](#Westin1992)). 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)](#ContyKulla2017) 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](./figures/cushion.png)
+
+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](/specification/2.0/README.md#appendix-b-brdf-implementation) for more details on the BRDF calculations.
+
+The sheen formula `f_sheen` follows the common microfacet form:
+
+*f*<sub>*sheen*</sub> = *sheenColor* * *sheenFresnel* * *sheenDistribution* * *sheenVisibility* = *sheenColor* * *F*<sub>*S*</sub> * *G*<sub>*S*</sub> * *D*<sub>*S*</sub> / (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)](#ContyKulla2017):
+
+```glsl
+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:
+
+```glsl
+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)](#AshikhminPremoze2007) (but that will make the BRDF not energy conserving when using the albedo-scaling technique described below):
+```glsl
+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)](#ContyKulla2017). The base layer *f*<sub>*diffuse*</sub> + *f*<sub>*specular*</sub> from [Appendix B](/specification/2.0/README.md#appendix-b-brdf-implementation) is scaled with *sheenAlbedoScaling* to avoid energy gain.
+
+*f* = *f*<sub>*sheen*</sub> + (*f*<sub>*diffuse*</sub> + *f*<sub>*specular*</sub>) * *sheenAlbedoScaling* 
+
+```glsl
+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](#theory-documentation-and-implementations) 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](#theory-documentation-and-implementations), 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:
+```glsl
+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:
+```glsl
+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](http://www.graphics.cornell.edu/~westin/pubs/siggraph-rebuild.pdf)<a name="Westin1992"></a>
+
+[Alejandro Conty Estevez, Christopher Kulla – “Production Friendly Microfacet Sheen BRDF”, SIGGRAPH 2017](https://blog.selfshadow.com/publications/s2017-shading-course/imageworks/s2017_pbs_imageworks_sheen.pdf)<a name="ContyKulla2017"></a>
+
+[Michael Ashikhmin, Simon Premoze – “Distribution-based BRDFs”, 2007](http://www.cs.utah.edu/~premoze/dbrdf/dBRDF.pdf)<a name="AshikhminPremoze2007"></a>
+
+[Filament Material models - Cloth model](https://google.github.io/filament/Materials.md.html#materialmodels/clothmodel)
+
+[cmgen tool from Filament](https://github.com/google/filament)
+
+[David Neubelt, Matt Pettineo – “Crafting a Next-Gen Material Pipeline for The Order: 1886”, SIGGRAPH 2013](https://blog.selfshadow.com/publications/s2013-shading-course/rad/s2013_pbs_rad_notes.pdf)
+
+[cloth-shading](https://knarkowicz.wordpress.com/2018/01/04/cloth-shading/)
+
+[Enterprise PBR Shading Model - Sheen](https://dassaultsystemes-technology.github.io/EnterprisePBRShadingModel/spec-2021x.md.html#components/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
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extensions/2.0/Khronos/KHR_materials_sheen/schema/glTF.KHR_materials_sheen.schema.json

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+{
+    "$schema": "http://json-schema.org/draft-04/schema",
+    "title": "KHR_materials_sheen glTF extension",
+    "type": "object",
+    "description": "glTF extension that defines the sheen material model.",
+    "allOf": [ { "$ref": "glTFProperty.schema.json" } ],
+    "properties": {
+        "sheenColorFactor": {
+            "type": "array",
+            "items": {
+                "type": "number",
+                "minimum": 0.0,
+                "maximum": 1.0
+            },
+            "minItems": 3,
+            "maxItems": 3,
+            "description": "Color of the sheen layer (in linear space).",
+            "default": [0, 0, 0]
+        },
+        "sheenColorTexture": {
+            "allOf": [ { "$ref": "textureInfo.schema.json" } ],
+            "description": "The sheen color (RGB) texture.",
+            "gltf_detailedDescription": "The sheen color (RGB) texture. Stored in channel RGB, the sheen color is in sRGB transfer function."
+        },
+        "sheenRoughnessFactor": {
+            "type": "number",
+            "description": "The sheen layer roughness.",
+            "default": 0.0,
+            "minimum": 0.0,
+            "maximum": 1.0,
+            "gltf_detailedDescription": "The sheen layer roughness of the material."
+        },
+        "sheenRoughnessTexture": {
+            "allOf": [ { "$ref": "textureInfo.schema.json" } ],
+            "description": "The sheen roughness (Alpha) texture.",
+            "gltf_detailedDescription": "The sheen roughness (Alpha) texture. Stored in alpha channel, the roughness value is in linear space."
+        },
+        "extensions": { },
+        "extras": { }
+    }
+}