Atmospheric effects like fog describe what happens to a ray when it travels between surfaces. In a perfect vacuum, a ray travels without being reduced, and no light is scattered along the path of the ray from the light sources. This is typically the case in most computer graphics images. A clear, hard-edged look results. However, in real life, dust or smoke particles float in the air; they obscure distant objects and scatter light to produce a mist color.
The appearance of the atmospheric effects depends on several factors.
One factor is the distribution of particles in the atmosphere. A column of smoke has an evident concentration of particles in particular locations. Distinct puffs are visible. This type of atmospheric effect can be emulated by using the sCloud procedural texture. See the section on sCloud in this chapter for further information on using this procedure.
Alias has three different types of fog:
| Constant density fog | equal densities of particles at all locations: distant objects fade equally in all directions. View in any direction blends to a constant color if there are no obstructing surfaces. |
| Max/Min fog | This is the same as Constant density fog except that the max and min distances are used to ‘clip’ the fog. That is, the fog particles only exist between the max and min distances from the camera. |
| Layered fog | fog density varies with altitude. |
In the fog procedure, the color for infinite distances is defined by selecting a Fog color. The relative scattering of light is controlled using the Fog Depth slider. Optical depth, roughly speaking, is the distance away that a surface must before it has half its light is scattered by the intervening air. For an outdoor scene during the day, the fog color should be white. For smog, try using a high layered fog and a brownish color.
To create a scene with the impression of great distances, give distant objects a blue tint by using a light fog colored blue.
