Some doubts about extra roughness parameter in aiStandard shader

[Francesco_Furneri]

Hi there,
I have a question to ask.
I was studying the transmission in AiStandard shader and I found a parameter called extra roughness. I conceptually understood what it mean but I was wondering if the result of having an Extra roughness equals -1 reflects the reality. I explain myself better.

If I have a glass with some high specular roughness, I expect that the transmission roughness is high as well and that's ok. But Is It possible to have a low transmission roughness and a High specular roughness in reality (like setting -1 for extra roughness)?
My thought was that if the specular roughness is high, the rays passing through a glass and existing should also be scattered because of the roughness of the surface and we shouldn't see them clear and sharp.

Is the reasoning correct? If so, the extra roughness is just used for artistic purposes?

Thank you very much for your help

[lee_griggs]

You can have low specular_roughness with high transmission_extra_roughness.

From the docs:

Adds some additional blurriness of refraction computed with an isotropic microfacet BTDF. The range goes from -1 to 1, where 0 means no roughness. It is computed as

transmission_roughness = specular_roughness + transmission_extra_roughness.

[Francesco_Furneri]

Oh ok. thank you very much.

So to complete my question, that situation you shown me is not possible in reality but just used for artistic purposes. right?

thanks.

[lee_griggs]

It can be used to add more creative control over the specular/transmission roughness.

[nwoolridge]

It's very possible in reality, given a material that is non-homogenous and has varying IORs internally, but has a polished surface... Think of e.g. fibreglass: non-homogenous (glass weave and epoxy/resin) with a sanded and polished surface.