How would you shade that? Dirt on glass

[Robert]

Ok I do that a lot in Modo. If I have a light with a glass in front I want an effect that I see in reality. Because the light is so bright all the fine dust on the glass in front get lit from behind and produce bloom effect. Well its actually just translucent. You can even do that for tiny scratches. The cool thing is, that it only exist where light shines from behind.
So to do that in Modo I simple turn on SSS, set the distance to 0.0 cm and it works. With SSS map I can add additional texture effects.

In Vray so, there is no translucency in the the base shader. I tried a few settings but none gave me the result I wanted. Wax is to blurry. Water doesn't give me anything (glass is just 5mm) and also on hybrid I got no result. Maybe I have try harder, but I couldn't find way yet.

The solution I then used was creating an own polygon with a 2-sided shader, that is weighted to the backside (else it would be a grey overlay). However it also kills all reflections behind it. So It need to be behind the glass. It works, but its difficult to set up (think of CAD models of bend surfaces) and it is not accurate. Plus it renders a bit.

Anyone with more Vray knowledge?

[John_Do]

Do you have an image or an example to better seeing the effect you're looking for ?

From what I understood, I'll do that with the Refraction Gloss and Refraction Color channels. Usually when I do glass with dirt on it, I use the reflection gloss/color map also in refraction gloss/color channels, in a color correction node or with a specific blend amount with base refraction color/gloss.

[Robert]

Here is a simple flat sample of a display. You can see that effect around the LED Lights.
Refraction gloss would blur the light behind? Thats not what I want.

[Robert]

I found a second way of doing the same effect with a 2 sided Material. The trick here to use the 2 sided mat on the front side of the glass only and then use an identical Glass Shader but with SSS Wax shading enabled. With the translucency color in the 2 sided Shader you can then control how strong the effect is. Easy.
But to be fair the effect should also work in the Glass Shader a standard Vray material, but there I have no control about the intensity. All the settings under SSS seem to have no change on the rendering.