*Doylle's shader from game-artist.net ;) IIRC, I've used a Composite material with the "show in viewport" thing turned on, but this was from a big while back.
IIRC, in that example 'Rho' is a constant that represents something like specular intensity, for which you probably could (and probably should) indeed use a texture :)
Comparing 7 photogrammetry systems. Which is the best one? http://arc-team-open-research.blogspot.co.uk/2016/12/comparing-7-photogrammetry-systems.html