LuxCoreRender Forums

You could try to place a glass box (with dispersion enabled) between the picture and the camera. The box should have an inner homogenous volume with scattering enabled.

By playing with volume scattering factor, absorption and glass dispersion, you may able to achieve the result you are looking ...

addition: I could render R G and B effects separately, resulting in 3 images which I then stack and add in photoshop but that is not workable - getting a preview of the effect needs to many actions in postprocessing.

FiatLux wrote: Tue May 08, 2018 8:32 am Are you by any chance tryin to emulate the cycles' trick to fake dispersion?

Nope, for that part I switched to Lux

It is the first time someone ask for this feature, probably because there is an "add" texture. However, it can not fix your problem if you need a color dependent roughness.
At the moment, I can not think to a work around, you could try to post an image of the material you are trying to render to ...

Switching from Cycles to Lux, and playing with nodes in my materials. There I noticed there is no "add shader" as blender and cycles have, just a mix shader.
This makes it impossible to split my transparent material in 3 colors (with each a different roughness) which then get added, I can only mix ...