I created a snow material so it just sits on top of any object, but now the problem is how to apply to all materials. So, either function / layered / other, what’s the simplest way to get this working ? Using functions I would need to create one for each material, on the other hand may have to copy the material nodes into all other materials.
I followed this for function set up, 1 material has no specular and no normal whilst the other does. I used the simple blend as it mentions and when blended together has no spec and no norm, however individually plugged into mat attributes do show ok.
Which ever way I set this up, ends up being individual materials. Is there a way to affect many materials at the same time ?
Usually we use master materials if we want to keep the number of unique materials down, but generally you still do end up putting things like snow as material functions into every object. You could conceivably do it in the post process pass but I wouldn’t recommend it since you won’t be able to tweak the actual material values the same way.
If I use a master material, how can I affect multiple materials at the same time ?
You don’t. With a master material, it means most of your assets are just material instances of the same master material, so if you add it to the master it is automatically part of all the ones that are just instances of the master.
So, if I make the master material and set up the instances, I see inputing textures is simple, but am I then stuck only being able to input a texture I have to pre make, or can I construct the param in a graph ?
You can actually do a parameter for everything on your master material (in the material or/and material fonction), I’m a little bit sad you can’t do that without edit the master material.
Just put switches to enable or disable it 
I just have to check it to enable it, and I can change the textures if I want.
It’s a shame that there is no selection node or function switch.