Add KHR_materials_volume_scatter

extensions/2.0/Khronos/KHR_materials_volume_scatter/README.md

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+# KHR_materials_volume_scatter
+## Contributors
+
+* TODO: Name, affiliation, and contact info for each contributor
+
+## Status
+
+Draft
+
+## Dependencies
+
+Written against the glTF 2.0 spec. Needs to be combined with `KHR_materials_volume`
+
+## 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`.
+
+## Table of Contents
+
+- [Overview](#overview)
+- [Extending Materials](#extending-materials)
+- [Properties](#properties)
+- [Scattering](#scattering)
+    - [Attenuation](#attenuation)
+    - [Single-Scatter Albedo](#single-scatter-albedo)
+    - [Multi-Scatter Albedo](#multi-scatter-albedo)
+    - [Phase Function](#phase-function)
+- [Scattering & (Diffuse-)Transmission](#scattering--diffuse-transmission)
+- [Schema](#schema)
+- [References](#references)
+- [Appendix: Full Khronos Copyright Statement](#appendix-full-khronos-copyright-statement)
+
+## Overview
+
+`KHR_materials_volume` defines surfaces as interfaces between volumes and provides tools for specifying attenuation via absorption in homogeneous volumes, but it lacks a definition for scattering. Scattering refers to the wavelength-dependent redirection of light upon particle interaction, while absorption is the wavelength-dependent reduction of light energy along a path. This extension builds on the definitions in `KHR_materials_volume`.
+
+<figure style="text-align:center">
+<img src="./figures/volume.svg"/>
+<figcaption><em>Interaction of light rays inside the volume and at the boundaries. E.g. a homogeneous volume with an index of refraction of 1.5.</figcaption>
+</em></figure>
+
+## Extending Materials
+The scattering properties are defined by adding the `KHR_materials_volume_scatter` extension to any glTF material that also uses `KHR_materials_volume`.
+
+```json
+"materials": [
+    {
+        "extensions": {
+            "KHR_materials_volume_scatter": {
+                "scatterAlbedo": [ 0.572, 0.227, 0.075 ],
+                "scatterAnisotropy": 0.3
+            },
+            "KHR_materials_volume": {
+                "attenuationColor": [0.9, 0.9, 0.9],
+                "attenuationDistance": 0.01
+            }
+        }
+    }
+]
+```
+
+## Properties
+
+The extension defines the following parameters to describe the scattering behavior.
+
+|                       | Type        | Description                                         | Required                 |
+|-----------------------|-------------|-----------------------------------------------------|--------------------------|
+| **scatterAlbedo**      | `number[3]` | The multi-scatter albedo.                           | No, default: `[0, 0, 0]` |
+| **scatterAnisotropy** | `number`    | The anisotropy of scatter events. Range is (-1, 1). | No, default: `0`         |
+
+## Scattering
+
+### Attenuation
+`KHR_materials_volume` defines the attenuation coefficient as a sum of absorption and scattering coefficients:
+$\sigma_t = \sigma_a + \sigma_s.$ It further notes that attenuation from scattering events is ignored by assuming $\sigma_s$ to be zero. Combining `KHR_materials_volume_scatter` with `KHR_materials_volume` lifts this restriction on the scattering part.
+
+The two derived parameters attenuation color $c$ and attenuation distance $d$ that relate to the attenuation coefficient as
+
+$$
+\sigma_t = \frac{-log(c)}{d}
+$$
+
+now not only consider absorption but also scattering events. The same applies to the definition of the volume transmittance
+
+$$
+T(x) = e^{-\sigma_t x}.
+$$
+
+Instead of only taking into account absorption, $T$ now corresponds to a change in radiance along a path of length $x$ as light travels through a medium with **absorbing and scattering** particles.
+
+### Single-Scatter Albedo
+Where the parameterization of `KHR_materials_volume` provides control on the density of a medium that absorbs and scatters, this extension provides control on just the scattering part of the medium interaction.
+
+While `KHR_materials_volume` allows control over the density of a medium that both absorbs and scatters, `KHR_materials_volume_scatter` controls just the scattering part of the medium interaction.
+
+The ratio of scattering to total attenuation is given as single-scatter albedo $\rho_{ss}$
+
+$$
+\rho_{ss} =  \frac{\sigma_s}{\sigma_t}.
+$$
+
+Rendering algorithms that implement volumetric scattering are usually interested in the scattering coefficient.
+Given $\rho_{ss}$ and $\sigma_t$ the scattering coefficient is
+$$
+\sigma_s = \sigma_t  \rho_{ss} = \sigma_t - \sigma_a,
+$$
+
+and for completeness, the absorption coefficient:
+$$
+\sigma_a = \sigma_t (1 - \rho_{ss}).
+$$
+
+
+### Multi-Scatter Albedo
+The single-scatter albedo could be used to parameterize scattering directly, but it has its drawbacks.
+In reality, light scatters multiple times in the medium until it leaves the volume. Depending on the number of bounces, the overall perceived color of the medium differs drastically from what is given by the single-scatter albedo.
+
+[Kulla and Conty (2017)](#KullaConty2017) use an alternative, more intuitive term, the multi-scatter albedo $\rho_{ms}$. Assuming commonly used values for scatter distances, it is a good approximation to the perceived color of an object after many bounces. $\rho_{ss}$ can be calculated from $\rho_{ms}$ as follows
+
+$$
+\rho_{ss} = 1 - (4.09712 + 4.20863 \rho_{ms} - \sqrt{9.59217 + 41.6808 \rho_{ms} + 17.7126 \rho_{ms}^2})^2
+$$
+
+<figure style="text-align:center">
+<img src="./figures/diffuse-sss.png"/>
+<figcaption><em>A simple, diffuse-only material (left) and a material that uses dense volume scattering (right). The base color of the diffuse material is set to the same color as the scatter color of the scattering material. Due to the multi-scatter albedo mapping the final color of the object is very similar.</em></figcaption>
+</figure>
+
+### Phase Function
+
+For a single scattering event, the phase function $p(\mathbf{v},\mathbf{l})$ determines the probability density of outgoing directions $\mathbf{l}$ given an incident direction $\mathbf{v}$ at a scattering event. We use the phase function developed by [Henyey-Greenstein](#HenyeyGreenstein). It is controlled by a single parameter  g in the range [−1,1], the `scatterAnisotropy` parameter.
+
+$$
+p(\mathbf{v},\mathbf{l}; g) = \frac{1}{4 \pi} \frac{1-g^2}{(1+g^2+2g(\mathbf{v} \cdot \mathbf{l}))^{3/2}}
+$$
+
+For any pair of incident and outgoing directions $\mathbf{v}$ and $\mathbf{l}$.
+
+## Scattering & (Diffuse-)Transmission
+
+For dense subsurface scattering materials like skin or wax (where the scattering coefficient $\sigma_s$​ is high), it is recommended to use `KHR_materials_diffuse_transmission` over `KHR_materials_transmission`. In such cases, the visual difference between diffuse and regular transmission is minimal, as volume scattering primarily determines the appearance, with boundary interactions having a small effect.
+
+Using diffuse transmission signals the renderer that the material is dense, eliminating the need to analyze geometry and scattering distance. For instance, a small object with low scattering might appear transparent, but a larger one would appear denser at the same scattering coefficient. Diffuse transmission also enables a translucent look regardless of object size.
+
+This cue allows renderers to use diffusion approximation rather than random walk subsurface scattering, producing near-ground-truth results for dense materials with greater efficiency, essential for both real-time and offline rendering. [Christensen and Burley (2015)](#ChristensenBurley2015) demonstrate the mapping of physical parameters to diffusion-based reflectance profiles, while [Jimenez et al. (2015)](#Jimenez2015) introduce a real-time method using a separable kernel for reflectance profile rendering.
+
+<figure style="text-align:center">
+<img src="./figures/transmission-translucency.png"/>
+<figcaption><em>Comparison of combining denes volume scattering with either transmission or diffuse transmission. Left: Rough transmission and scattering. Middle: Diffuse transmission and scattering. Right: Diffuse transmission without scattering using a thin-walled material. Colors have been adjusted manually in order to account for differences in distances and to minimize the impact of energy loss from the rough microfacet BTDF.</em></figcaption>
+</figure>
+
+## Schema
+
+- [glTF.KHR_materials_subsurface.schema.json](schema/glTF.KHR_materials_subsurface.schema.json)
+
+## References
+
+* [Christensen, P. and B. Burley (2015): Approximate Reflectance Profiles for Efficient Subsurface Scattering](https://graphics.pixar.com/library/ApproxBSSRDF/paper.pdf)<a name="ChristensenBurley2015"></a>
+* [Louis G. Henyey and Jesse L. Greenstein: Diffuse radiation in the galaxy. Astrophysical Journal 93, 70–83, 1941.](https://ui.adsabs.harvard.edu/abs/1941ApJ....93...70H/abstract)<a name="HenyeyGreenstein"></a>
+* [Jimenez J., K. Zsolnai, A. Jarabo, C. Freude, T. Auzinger, X.-C. Wu, J. Pahlen, M. Wimmer and D. Gutierrez (2015): Separable Subsurface Scattering](http://www.iryoku.com/separable-sss/)<a name="Jimenez2015"></a>
+* [Kulla C., Conty A. (2017): Revisiting Physically Based Shading at Imageworks](https://blog.selfshadow.com/publications/s2017-shading-course/imageworks/s2017_pbs_imageworks_slides_v2.pdf)<a name="KullaConty2017"></a>
+
+
+## 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,
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