Atmosphere Volume

Rollover image

 

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 terms).  

  • 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.
  • Currently, atmosphere_volume does not compose well against volumes. This is because atmosphere's return a single flat result that is opacity mapped on top of whatever is in the background of the pixel.

    Atmosphere Volume cannot 'penetrate' through the cloud volume

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.

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.  

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

A simple scene file can be dowloaded here.

 

The shader can be found in the Create menu in the Material Manager window:

 

Once you have created the material, drag it onto the Atmosphere attribute in the Environment section of the Main Arnold Renderer settings.

Main

These parameters control the scattering of light by the participating medium (fog, smoke, etc).

Color

The density control is multiplied by this RGB value (so a blue value here means the blue light is scattered). 

It is also possible to map textures to this parameter to get interesting volumetric effects. The example below uses noise texture to simulate a cloudy fog effect.

noise -> color

Density

Increases/decreases the atmospheric volume density. The following images show the effect of altering density.


Attenuation Color

The attenuation control is multiplied by this RGB value (so a red value here means red light is attenuated).


Attenuation

The attenuation parameter sets the rate at which the rays of light traveling through the scattering medium are extinguished and how much light coming from the background is blocked. A high value means that light will only travel a short distance through the volume, while a low value means that light will travel a long distance through the volume.

Anisotropy

Henyey-Greenstein Anisotropy coefficient between -1 (full back-scatter) and 1 (full forward-scatter). The default is 0 for an isotropic medium, which scatters the light evenly in all directions, giving a uniform effect. Positive values bias the scattering effect forwards, in the direction of the light, while negative values bias the scattering backward, toward the light. Changing the eccentricity, therefore, means that you will get a different effect depending on whether the camera is looking toward the light or away from the light. 

Note that values very close to 1.0 (above 0.95) or -1.0 (below -0.95) will produce scattering that is so directional that it will not be very visible from most angles (and what scattering you do see may be noisy), so such values are not recommended.

Samples

The samples are distributed according to the volume density. More samples will refine the quality of the solution.


Contribution

The following attributes allow you to control the contribution that the atmosphere makes to different types of rays in Arnold - camera rays, diffuse GI rays, and reflection rays.

Camera

Controls the degree to which the atmosphere_volume affects camera rays.


Diffuse

Controls the degree to which atmosphere_volume affects GI_diffuse_rays. 

Thin volumetric spotlight with a small cone angle

Care should be used when enabling these settings. Non-zero values mean slower renders because the GI rays now need to also do volume calculations, and most of the time this effect is subtle and not worth the extra effort.

Specular

Controls the degree to which atmosphere volume affects specular rays.


 

 

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