SketchUp has added a nice Fog feature in SU 6. IRender now reads these settings and creates similar fog effects.

Fog sample in SketchUp and IRender.
(One of these crosses is blue and has a long arm, because I wanted to be able to see where the fog was supposed to stop.)

Note: SU has a "start fog" setting which we are not able to emulate in IRender.

Note: Su does not apply fog to shadows, but IRender is able to do that.

See also