These parameters control the scattering of light by the participating medium (fog, smoke, etc).
Increases/decreases the atmospheric volume density. The following images show the effect of altering density.
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
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.
The attenuation control is multiplied by this RGB value (so a red value here means red light is attenuated).
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The samples are distributed according to the volume density. More samples will refine the quality of the solution.