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bsdorra authored Jan 18, 2021
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28 changes: 12 additions & 16 deletions extensions/2.0/Khronos/KHR_materials_translucency/README.md
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Expand Up @@ -21,18 +21,15 @@ Written against the glTF 2.0 spec.

This extension adds a Lambertian diffuse transmission BSDF to the metallic-roughness material. Thin, dielectric objects like leaves or paper transmit some of the incoming light to the opposite side of the surface. For thick media (volumes) with short scattering distances and therefore dense scattering behavior, a diffuse transmission lobe is a phenomenological plausible and cheap approximation.

<p float="left">
<img src="figures/leaves.jpg" height="350" /> <img src="figures/thin_translucent.jpg" height="350" />
<p><em>(Thin) translucent leaves</em></p>
</p>

<figure float="left" style="text-align:center">
<img src="figures/leaves.jpg" height="335" />
<img src="figures/thin_translucent.jpg" height="335" />
<figcaption><em>(Thin) translucent leaves</em></figcaption>
</figure>
<br/>
<figure float="left" style="text-align:center">
<img src="figures/hand1.jpg" height="300" />
<img src="figures/hand2.jpg" height="300" />
<figcaption><em>Skin modeled with translucency</em></figcaption>
</figure>
<p float="left">
<img src="figures/hand1.jpg" height="350" /> <img src="figures/hand2.jpg" height="350" />
<p><em>Skin modeled with translucency</em></p>
</p>

## Extending Materials

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### KHR_materials_transmission
Both `KHR_materials_translucency` and `KHR_materials_transmission` replace the diffuse BRDF with a BTDF that transmits light onto the opposite side of the surface. In case of `KHR_materials_transmission`, this is a microfacet BTDF that shares its roughness with the microfacet BRDF. In case of `KHR_materials_translucency`, on the other hand, this is a diffuse BTDF. Since the diffuse BTDF does not have controls for roughness, the roughness parameter acts only on the reflective part of the surface. By decoupling the reflection and transmission parts. By decoupling the reflection and transmission parts it is possible to configure materials which have a smooth reflection and a diffuse transmission, as shown in image below (right).

<figure style="text-align:center">
<img src="./figures/transmission-translucency_thin.jpg" width="500" />
<img src="./figures/transmission-translucency_thin_angle.jpg" width="500" />
<figcaption><em>Emissive sphere behind material sample<br/>Left: Opaque diffuse. Middle: Rough transmission. Right: Translucency.</em></figcaption>
</figure>
<p float="left">
<img src="./figures/transmission-translucency_thin.jpg" width="350" /> <img src="./figures/transmission-translucency_thin_angle.jpg" width="350" />
<p><em>Emissive sphere behind material sample<br/>Left: Opaque diffuse. Middle: Rough transmission. Right: Translucency.</em></p>
</p>

If `KHR_materials_transmission` is used in combination with `KHR_materials_translucency`, the transmission effect overrides the translucency effect. The calculation given in `KHR_materials_translucency` still holds, but uses the modified diffuse term defined above: *f*<sub>*diffuse*</sub> = (1 - *F*) * (1 - *T*) * *diffuse*.

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