We all know about the Add and Mix nodes - but is there a way to subtract one shader from another.
To illustrate the concept - if you mix two glossy shaders with different roughness values together - you get a bright highlight where the light hits the surface - surrounded by a more diffuse highlight. This technique is used in materials such as “bloom metal”.
What I would like to do is subtract the glossy shader with the lower roughness value so that i’m left with a ring/halo shaped highlight around the light sources.
I tried using a trick I learned from the emission shader - whereby if you set the emission strength to negative, you actually emit darkness rather than light. I tried applying the same concept to a glossy shader by trying to make the glossy colour negative (figuring that of I added the two shaders together - the negative colour of one shader would cancel the positive colour of the other) - however Blender wont accept negative RGB values.