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Atmospheric effects such as fog, dust, haze, or visible gas may be
simulated by a media statement specified in the scene but not
attached to any object. All areas not inside a non-hollow object in the
entire scene. A very simple approach to add fog to a scene is explained in
section "Fog" however this kind of fog does not interact with any
light sources like media does. It will not show light beams or
other effects and is therefore not very realistic.
The atmosphere media effect overcomes some of the fog's limitations by calculating the interaction between light and the particles in the atmosphere using volume sampling. Thus shafts of light beams will become visible and objects will cast shadows onto smoke or fog.
Note: POV-Ray can't sample media along an infinitely long ray. The ray must be finite in order to be possible to sample. This means that sampling media is only possible for rays that hit an object. So no atmospheric media will show up against background or sky_sphere.
Another way of being able to sample media is using spotlights because also
in this case the ray is not infinite (it is sampled only inside the spotlight
cone).
With spotlights you'll be able to create the best results because their
cone of light will become visible. Pointlights can be used to create effects
like street lights in fog. Lights can be made to not interact with the
atmosphere by adding media_interaction off to the light source.
They can be used to increase the overall light level of the scene to make it
look more realistic.
Complete details on media are given in the section
"Media". Earlier versions of POV-Ray used an
atmosphere statement for atmospheric effects but that system was
incompatible with the old object halo system. So
atmosphere has been eliminated and replaced with a simpler and more
powerful media feature. The user now only has to learn one
media system for either atmospheric or object use.
If you only want media effects in a particular area, you should use object media rather than only relying upon the media pattern. In general it will be faster and more accurate because it only calculates inside the constraining object.
Note: the atmosphere feature will not work if the camera is inside a non-hollow object (see section "Empty and Solid Objects" for a detailed explanation).
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