Surface imperfection [Color Composite Node] not working

[zooropa]

Hi.
I am trying to create a wood material with poliigon textures.
After inverting the textures map in PS (the Color Invert Node does not work) I plug my roughness texture (contrast between wood's grain) into the Background [Color Composite Node] and the surface imperfection texture (light scratches) into the Source.

After doing that  I change the Blend Mode to Screen. This allows me to have the roughness of the grain contrast + the scratches on top of it. The preview (C key) shows me exactly what I would like to achieve. Unfortunately the render outcome does not show the effect. The scratches are not there.

The alternative that worked consisted in using Overlay as Blend Mode. The problem with overlay is that the scratches take the whole map and the grain texture map disappears. I can reduce the Source Alpha (scratches) in order to make the grain texture roughness emerge again. This works in the render, but it is an "eyeballing" solution: A ratio between the transparency of the scratches texture and the grain texture...meaning that the actual values of the grain are affected by the subtle transparency of the source alpha. Of course If I intend to have the original grain texture I will need to put the Source Alpha value to 0 ...meaning that my scratches are gone.

Hope some insights could help me.

[Ryan Day]

The scratches should probably be a bump map (inverted), and not simply a roughness map; especially since your wood is already quite rough, there's not much difference in surface roughness for the scratches to show up. If you base material was shiny you'd probably see them, then, with the right lighting. I suspect the issue is simply the lack of contrast in the surface roughness combined with lighting that doesn't have enough contrast.

Unless you're routing this composite through a colour to number node, white in a roughness map means a roughness value of 1 I believe. I've never used a roughness value over ~0.2 for anything, I don't think (except for an untreated wood, I guess). With such a high roughness value you're losing all of the contrast in your lighting and coming out with, essentially, the same result as a matte material.

So, what I'd do here; use the scratches textures as a bump map instead of a roughness map, and use a negative bump value so that they cut into the surface (since your texture has white scratches, it's 'backwards'). You might want to decrease the roughness a bit, too.