volume
depthcue( float mindistance = 0, float maxdistance = 1e6,
	  color background = 0, string units = "common" )
{
//    float d = transformu (units, length(I)); // transformu is unsupported
    float d = length(I);
    d = clamp( (d - mindistance) / (maxdistance - mindistance),
		0., 1. );
    C = mix( C, background * opacity, d );
} 