{
	BAD fog simulation effect using a converted z-file, eg:

	pvalue -r -df -b -h `getinfo -d < scene.pic` scene.z | \
		pcomb -e VIS:20 -f fog.cal scene.pic - > foggy.pic

	The constant VIS must be set to the visibility distance,
	ie. the distance at which half the contrast is lost.

	Substitute a local brightness model for fog_v to improve
	the effect.
}

dist = gi(2);			{ distance given by second file }

fog_v : .5;			{ fog particles are uniform grey }

prob = 1 - 2^-(dist/VIS);	{ probability of intercepting fog particle }

fog(v) = prob*fog_v + (1-prob)*v;

ro = fog(ri(1));
go = fog(gi(1));
bo = fog(bi(1));