[AlexP]

I have to make series of shower cabins, some of them have colored glass. I don't need to see refraction but I really need to see caustics behind glass (colored shadows) because lights are usually on camera side of glass. How should I make it?

[eric nixon]

Do it as real solid glass, use roughness 0.

[AlexP]

I almost always use roughness 0 on glass and did same here but still too dark behind glass, even on fully transparent (transmitance 255).

[Mihai]

It's not really possible to get the same look as seeing colored shadows through glass, but you can fake it a bit:

1. Assign a normal glass material to your shower glass - hide it from camera.
2. Duplicate the shower glass object and make it just slightly larger than the original object - the idea is just to avoid overlapping of geometry.
3. Assign a typical AGS material to this object. To make some reflections on it, have rough (99) BSDF and add a second BSDF, or a second Layer with a BSDF and set the roughness low, or 0. This will be your shiny layer. Set this second Layer to Additive to avoid too much darkening on the final look of the AGS.

By playing with the Layer opacities of this AGS material you can get some acceptable results. For example, comparing real glass with this approach:

[AlexP]

Thanx Mihai, it works.
Still this color layer (99 roughness) makes all greyish-green... doesn't it? - what I mean it doesn't darken with color like multiply in PS.
I'm thinking about making "green" in PS but then I will have to make second render for reflections. For now have no idea how to get reflections for that glass element only to composite...
Also singlesided would be very helpful for coloring glass, but even with very low coef (0.01) there is some blur visible.

[Mihai]

Yes, it's not a perfect solution, the green shadow will always lose some intensity and color...
You can do a reflections only pass (you have this option in Studio in the render options panel (diffuse or reflections only, or both (default)), and select the window pane using matID or objectID later in Photoshop. Not sure which mode you would use to composite the reflection, I think it's Screen or Lighten.

[AlexP]

I always composite in 32bit so linear dodge will be best option.
But always need two renders right? One with glass hidden to cam and refl and second "black" opaque glass to composite reflections.

[Mihai]

Yes, but still if you even do it that way, you're not going to get a green glass visible, just a non colored "glass" made visible by reflections. Or you plan on just adding a transparent green surface in PS?

[AlexP]

Yep, green coloration in ps.
I'm thinking about possibility to get internal reflections (from back side of glass) but no idea right now.