Nodes are the essential building blocks in the Advanced interface to the Poser Material Room. They are the graphical representation of mathematical function calls, that is: calculation procedures which turn parameters (inputs) to a result (output).
All nodes in the Specular group respond to specular light, which refers to specular light from direct (point- spot- infinite) sources only. The regular ‘specular’ node itself, either separate or embedded in the PoserSurface definition directly is fast to calculate, but falls short for a neat illusion of about any real stuff except hard plastic.
This is why Poser offers alternatives, like Blinn, or Anisotropic for microscopically grooved surfaces. Effectively Blinn relates to Specular as Clay relates to Diffuse: it’s the improved version for generic purposes. In other words: when I do need Clay (or other improvements, like Scatter) to represent surface roughness, then I do need Blinn for the according specularity. Just nullify or black out the regular Specular slot in PoserSurface, and plug the Lighting > Specular > Blinn node into Alternate_Specular.
Above: the left pawn has regular specularity (highlight size 0.05) while the right pawn has Blinn. Parameters at 0.15 / 0.7 / 0.7 make a sort of matching specular strength, but note the strong center of the highlight in Blinn.
Above: the right pawn has parameters at 0.15 / 0.7 / 0.7 as before, now the left pawn at 0.5 / 0.1 / 1.0 representing a porous, soft, diffusing surface. There still is some resemblance of highlight, though. So the question is: what do the parameters mean, and how do they relate to regular specularity?
Well, Specular_Color and Normals_Forward are the same, while Reflectivity sort of matches Specular_Value. It serves the same purpose at least, though I need different values for a similar effect. When Eccentricity is low (< 0.1) the highlight is sharp like regular specular, and when it’s 0.2 or up the represented stuff looks mildly porous like the roughness of clay. Values will be quite different though, and differ from the regular Highlight_Size as well, so it’s a matter of experimenting what works best in my scene. The SpecularRollOff also affects the impression of porosity.