Substance Designer Metal/Roughness to Spec/Gloss Converter

arnareli

Hey everyone, I'm currently working on a material in Substance Designer that I need to get a Specular and Glossiness outputs from.
I started the material with the PBR Metallic/Roughness preset and made the full material and now I need the spec gloss maps and I'm using the Basecolor/Metallic/Roughness Converter node, I connected all the right inputs and used the default target PBR/Specular/Gloss. 
But I'm being told that my specular map is incorrect and should have details like the roughness or Gloss, it comes out as a uniform value for the whole map (V:59, Dark grey). Other maps seem to come out right tho, is this a known problem or is this the right map coming out of the node?

rollin

Well.. this is not "incorrect" but it's also not perfect. It's the way allegorithmic went with the specular conversion. The problem is that you can not physically correctly transfer the one into the other as the one is not physically correct. 
The best solution I have found yet is to output an extra channel "my custom spec map" and tweak it manually. 

gnoop

It all depends on what your target shader is going to do.  Typically  metal/roughness PBR  uses  GGX  kind of highlight shaping   and calculates  shape and  intensity of highlight spot  + fresnel styled environment reflection  on its own.

Nothing prevents  you from doing same manually having same GGX highlight shaping  and proper fresnel values.    Glossiness is usually just inverted roughness that form highlight spot size     and spec texture is highlight spot color and intensity .  Could be perfectly physically correct too. 

     Intensity is a key usually.       So yeah, spec map couldn't be flat since stone and moss could't have same highlight spot  intensity .   Moss/dirt parts should be darker and in cracks it should be 100% black.    

I would rather use flattish values for glossiness unless it's something super polished there and being that bright the glossiness would make you a tiny highlight spot like polished surfaces  do usually. 

   I would say your Glossiness would be ok as spec texture ( maybe a bit more contrast and daker) and  that flat gray as the gloss one.   

But finally it depends a lot on your target shader /engine  implementation, highlight spot shaping math of it especially.

Best practice is just test it manually and find proper values scope.     For old fashioned  Phong highlight spot  it could be quite reasonable to use some sort of deviation from physically correct values. Otherwise you wouldn't see any highlight at all in half of materials .  It may backlash with some weirdness in certain light angles  so you have to find some trade off manually anyway. 

poopipe

Is this pbr spec/gloss? (in which case you can do a perfectly good conversion minus specific dielectric specular values)

Or

Is this old school blinn/phong style shading? (in which case, eyeball it)