volume
blendfog ( float maxdistance =  1.0;
	  color background  =  0.5)
{
    float d;
    float x, y, z, dist;
    
    /*x = xcomp(P);
    y = ycomp(P);
    z = zcomp(P);
    dist = sqrt(x*x + y*y + z*z);*/
    dist = length(I);

/* EXPONENTIAL FOG  */
    d = 1 - exp( -dist/maxdistance );

/* LINEAR FOG  */
/* d = clamp( 0.0, 1.0, dist/maxdistance);*/

    Ci = mix( Ci, background * Oi, d );
/*    Oi = mix( Oi, color (1,1,1), d );*/
}