What is the "official" VRay workflow for rendering refractive objects on a transparent background?

[Art48]

I have a model of a personal bathroom scale which has a glass top. In VRay I set up my scene very basic, with a vrayplane and white material als ground, then I put the scale there, light it and now I want to render it in a way that allows me to do two things:

1) Change the color of the background of the image and thus also the color "below" the glass panel but keeping reflections etc (I know, I render it on white and it will only look right on white, yet I'd like to have the freedom of changing the background color or ad gradients and the like)
2) Render a seperate shadow and comp it under the scale instead of the shadow that just happens when lighting it regularly.

How can I go about that in the most realistic and consistent way possible. Is it possible at all with VRay? What steps are involved regarding render settings and (exr-)import in Photoshop? Can someone walk me through the technical steps here?

[MANUEL_MOUSIOL]

The most straight forward way might be to have two renders, with the scale separately and its own alpha channel. that way everything behind it can be freely graded. But of course then you have more render time and additional work...

[Svetlozar Draganov]

The official workflow is to actually render it
This will produce the most realistic and consistent result.

There is no official way to fake stuff, everything depends from the scene and the needs.
If you could send us a real-scene or example-one that demonstrates your setup we might be able to provide some tips, otherwise you should render the scene on separate passes and then comp the render layers in a way to get the effect you are looking for.

[Art48]

I see Thanks for the clarification. Maybe I will send it.