Volumetric Lighting As a basic principle, volumetric lighting uses the rays of light that travel from the source to the surface to mark out a "volume" (picture a cone or pyramid) that can be altered in such a way to give the impression that it contains things like smoke, fog, floaty bits, etc. It can be something as solid as a whole person, as demonstrated by the image to the right - the arms, legs and body of the person blocks out some of the light, leaving the rest to reflect of particles in the air. It generates huge amounts of realism for such scenarios. In the past, volumetric lighting has been simulated by adding polygons to a scene and colouring the shape so that it's translucent, giving the impression of light beams. One could argue that the modern methods are still only simulations as we're not really "seeing" beams of light occluded by bodies and reflecting off particles; but it's the fact that the latest techniques are fully real-time and dynamic that makes them so much better. For example, compare the two images below:
The screenshot from Alan Wake (left) is an example of modern methods; the character and environment occlude rays from the distant light source, which strongly highlights the fog in the air due to the contrast of light and darkness (something that the game heavily features). The other example shows that this effect was produced in the days before pixel shaders and HDR existed - the static nature of such attempts limits how realistic it looks. The differences lies in that the modern system doesn't just add coloured shapes into the frame to give the impression of light beams. Instead, each relevant pixel in the frame is coloured by a process of calculating the visibility of objects along the light rays, as well as what materials the ray will go through. The end result is a mobile, reactive system that immerses the viewer into a more genuine world of foggy, dirty air.
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