In the physically based Blinn-Phong equation presented by my own company, tri-Ace, the specular is multiplied by the Fresnel term and the diffuse is thus multiplied by (1 - Fresnel) in order to maintain energy conservation.
It is the first equation in this paper: http://research.tri-ace.com/Data/s2012_beyond_CourseNotes.pdf
Oren-Nayar by itself is energy-conservative when considering only the diffuse term, so logically if you want to add some specular you have to somehow modify the diffuse.
I am adding the specular component of Blinn-Phong from that paper and am not sure how to correctly adjust the diffuse term.
My initial thought was to do the same as before, by multiplying it by (1 - Fresnel). But Oren-Nayar is already view-dependent and that is like doubling its view-dependence.
Is it as simple as I thought or is it more complicated to normalize the combination of specular and diffuse here?
What do I need to know in order to balance speculars and diffuses of other models on my own without having to run online every time?
Looking at Cook-Torrance here, the diffuse term is just Lambert. That is not physically plausible, is it?
How do you make it so?
L. Spiro