By way of comparison, here's a 'ground fog' test image I did last year, using Volume. This shader is much simpler than the above DepthCue shader, just using a P-node to directly control the Volume Density parameter, with some noise thrown in.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)