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This shader simulates light scattered by a thin, uniform atmosphere. It produces shafts of light and volumetric shadows cast from geometric objects. It works with point, spot, and area lights, but not with distant or skylights. This is a scene-wide volume shader (or an atmosphere shader in Arnold's terms).  

Note
  • atmosphere_volume used to be called volumetric_scattering and should not be confused with volume rendering of fluid type objects.
  • atmosphere_volume only works with 'local' lights that have a precise location and size and inverse-square decay. It does not support lights at an infinite distance, such as the Skydome light or directional light.
  • Light volume_samples do not apply to the atmosphere_volume shader. To improve the quality of the atmosphere_volume shader, you must increase the atmosphere_volume samples.
  • Currently, atmosphere_volume does not compose well against volumes. This is because atmospheres return a single flat result that is opacity mapped on top of whatever is in the background of the pixel.

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    atmosphere_volume cannot 'penetrate' through the cloud volume

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atmosphere_volume should be composited using an 'additive' mode such as 'screen' because volumetric scattering is the light that cannot be represented in the alpha channel.

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enable_matte enabled for standard_surface shader assigned to wall geometry (left). atmosphere_volume render composited using 'screen' (right). Rollover images.


The example below demonstrates the effect of atmosphere_volume through a medium. It consists of a polygon plane with a circular ramp texture connected to the opacity of a standard_surface shader. The spotlight is pointing at the plane and atmosphere_volume is enabled.  

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 Polygon plane with circular ramp texture -> opacity of a standard_surface shader

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