In my current scene I would like to render a single sided plane with blurry refractions when using the Standard Surface in 'thin walled' mode. When changing the Spec Roughness value, it works fine for the Specular, but not for the Transmission. That only happens when 'thin walled' is enabled.
Is that a bug, or am I missing something obvious ?
Cheers,
I can confirm this behavior.
It could be a bug. We will look into it. Thanks for reporting.
Thanks Lee.
This looks like a normal behavior, since the roughness is distance based (a point exactly at the surface has no "blur") and since you have no distance computed in thin walled you dont get it.
@Bruno, remember my issue is for HtoA. It seems that your are using Maya to illustrate this render. Also, Adding thickness to the plane does not fix the issue, when "Thin Walled" is 'on', there is no Roughness happening.
My versions:
HtoA 3.2.1
Arnold 5.2.2.0
Houdini 17.0.459
I know, and it does not matter, the renderer is the same, you can export a standin and I would render it exactly the same in maya. My example is not using thin walled, I showed you this so you can see that transmission roughness is distance based and when you turn on thin walled you dont get this computed.
Like this:
In maya the roughness value for transmission is even locked when you turn on thin walled.
You cannot have rough transmission with thin walled enabled.
documentation is a bit misleading because treats with the same regard both refr/transp and sss/trans and it obfuscates a bit the practical point. that is .. when to use 'thin walled' stuff ?
with refraction..
of course if you have already a single sided 'thin walled' object.. you don't need thinwalled to be enabled at all.. it's already thinwalled luke :) and it's not a matter of roughness at all (which doesn't make any sense to define as 'distance based') .. with 'thin walled' there ain't any refraction, it's just 'transparency'. so when to use thinwalled transparent materials ? when one doesn't want to model explicitly the inner sheets of a double glass window for example or the same for anything that is not solid glass because the sheets of glass are so thin that refraction is negligible... the main material appearance is that of 'being transparent' and not that of 'being refractive'. the fact then that transparency generally doesn't support roughness depends on the implementation, refraction is implemented as a microfacet BTDF, transparency not.
with SSS..
this is a very different scenario than the above. almost the opposite. if for refr/transp you use 'solid' geometry that you want rendered as 'thin' geometry.. here you really use thin geometry and help arnold rendering it telling him in advance.. it's thin (single sided) and should not be consider for example as a volume for randomwalk sss.. so instead of sss you get translucency.
You are right, but you misread a little bit, what I meant is that the distance the refraction ray travels in thin walled is 0, hence no refraction. Also, a single sided plane is thin walled in the sense, but not the same as Arnold thin walled, as it does not behave the same, like the water surface on a pool for example. Like the example below, using depth also:
On a single sided plane the refraction ray never exits. Or am I completely wrong?
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