> [!abstract] Summary
> Arnold provides two main atmosphere/environment shaders: **Fog** (distance-based contrast reduction for outdoor environments) and **Atmosphere Volume** (volumetric light scattering that produces god rays and volumetric shadows)
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# Ressources
[Arnold for Houdini — Environment Documentation](https://help.autodesk.com/view/ARNOL/ENU/?guid=arnold_for_houdini_ah_Environment_html)
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# Setup in Houdini
1. Create a **Material Builder** node in your material network.
2. Inside it, create the desired atmosphere shader and connect it to the **OUT_environment** output.
![[file-20241022190738500.png|525]]
3. Connect the Material Builder to the **Environment** input on the Arnold ROP.
![[file-20241022190828266.png|550]]
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# Fog
The **Fog** shader simulates atmospheric light scattering — distant objects appear lower in contrast and desaturated, which is typical of outdoor haze or mist.
> [!tip] Fog is purely a contrast/density effect based on distance. It doesn't produce visible light shafts or volumetric shadows — use Atmosphere Volume for that.
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# Atmosphere Volume
The **Atmosphere Volume** shader simulates a thin, uniform participating medium across the entire scene. It produces:
- **Shafts of light** (god rays) visible from point, spot, and area lights
- **Volumetric shadows** cast by geometry into the atmosphere
> [!warning] Atmosphere Volume does **not** work with **distant lights** or **sky lights** — only with point, spot, and area lights.
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# Setup in Maya
![[image-20250330.png]]
## Key Parameters
| Parameter | Description |
| --------- | ----------- |
| **Attenuation** | Controls how quickly the fog attenuates with distance — how far rays travel before scattering |
| **Attenuation Color** | Attenuation multiplied by a color tint — use to add warm or cool haze |
| **Anisotropy** | `0` = scatter light in all directions evenly; `1` = scatter predominantly toward the camera (forward scattering, produces brighter shafts) |
> [!tip] A slight positive anisotropy value (e.g. `0.3–0.5`) gives more realistic god rays by biasing scattering toward the camera, similar to how real atmospheric scattering behaves (Mie scattering).