MistThe Mist shader can create both a true layered fog, along with a realistic depth fading effect - superior to that of Softimage's built-in depth fading. In addition, it allows for increased control by providing the ability to specify directional variation with a texture map.
Perhaps Mist's most interesting application is on containers; applying Mist as a volume shader of a transparent object gives the object a murky interior, without adversely effecting rendering times.
To use as an atmosphere:
Choose Atmosphere->Depth Fading.
Select Mist as the depth fading shader; make sure that the Softimage Depth Fading check box is checked.
Edit Mist's parameters.
To use on the volume of an object:
In the Material dialogue box, select Mist as the volume shader.
Edit Mist's parameters.
Transparency controls the fog's maximum transparency. The fog will never become more opaque than this value. Low values give a more opaque fog, higher values give a more transparent fog.
Effect Alpha-Channel controls whether or not Mist applies itself to the alpha channel. With Effect Alpha-Channel turned off, Mist will not alter the alpha-channel value. (Be warned: Mental Ray has strange rules for alpha-channels, and may alter an image's alpha-channel even though you've deselected Effect Alpha-Channel. See the Mental Ray User's Manual for more information.)
The artist has two ways of setting the fog's color. With Solid the artist chooses one color that is applied in all directions, while From Image changes the color depending on viewing direction, much like a reflection map, useful in situations such as a sunset, where the fog color is different when looking toward or away from the sun.
To get the appearance of fog laying on a surface, Layering changes the fog's thickness as a function of height. Baseline controls the layer's vertical position, measured in SoftImage units from the world center. Height controls how tall the transitional layer is, and thus how abruptly the fog varies from opaque to transparent.
The Falloff settings control the rate at which the fog becomes opaque over distance.
Linear is the traditional method used by SoftImage. At the Start distance the fog is transparent, and the End distance the fog is opaque. Distances between interpolate linearly.
Realistic uses an exponential function, similar to a true, natural fog. Where the Linear falloff has an abrupt end (as an object passes through the End distance, it almost seems to spring into view) the Realistic falloff is more gradual. With Realistic, the artist has no direct control over starting and ending distances, only the fog's overall thickness, controlled with Density.
Custom allows the artist to specify exactly what he wants as the falloff curve. At the Start distance the fog is transparent, and at the End distance the fog is opaque. At the Middle distance the fog is exactly halfway between transparent and opaque. Distances between are interpolated on a spline curve. (See the discussion on Custom Curves.)
NOTE: If banding occurs when using Mist, simply turn on "Dithering" in the Render dialogue box to correct it.