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I've noticed some of the architectural materials I've downloaded use a second bsdf layer with the texture map desaturated, then inverted with contrast increased. I've seen this in a couple of mxm files (which are excellent by the way)...why is this done?

I've looked for some tutorials on these kinds of materials but can not find them. How can I learn why this is done and what is does for the texture (I"ll do some trials to see what difference this second bsdf layer makes.

Just wondering how someone figured out the benefits of doing this?

are these desaturated textures in the roughness slot?

In the roughness slot...indeed it is. However, this second bsdf layer is also in the refl 0 slot...same greyscale map used for the roughness.

I noticed a similar thing in another mxm (brick) material, where a second bsdf greyscale map was used for both the color and roughness....seems like adding a greyscale texture at 20% would have the effect of desaturating the image some....it really came out great in the render.

One used normal mode, the other additive.

Can you link to the mxm's? I'd like to download and disect!

The mxm is in the mxmgallery.maxwellrender.com. It is called Weiberner Tuff 1 (looks like a beige limestone).

I did "disect" is as well, and see that the contrast-increased inverted greyscale image in the second (normal additive) bsdf layer has the effect of darkening the color. The same map is used for the roughness.

I rendered 3 versions side by side with the MW plugin for C4D, #1 with the second bsdf layer completely deleted, #2 with the default grey for the second bsdf color, and#3 with the contrasty-inverted greyscale image. #3 reads to me as the richest texture. I find it interesting that the inverted greyscale second bsdf did not decrease the darks.

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That is made to control the shine regardless the base color used. Something like plastics.

Yes, it's so you can have some reflective specs on the material, which you can control via the second bsdf in several ways - the weightmap of the bsdf, the rougness which can also be mapped, ND etc.

Btw, I'm moving this thread to MXM section, this section should be only for posting tutorials.

I'll look for it in the mxm section. thx. Gotta keep alert here