Using Blender 2.79b + LuxCore, I'm trying to set up a scene, which would support, at the same time, both A) volumetric caustics and B) Hemi lamp + HDRI backdrop.
When I assign a homogeneous volume with scattering (called Dust) to the World as it's volume, I get expected results on the side of light propagation as seen on picture 1. But the HDRI backdrop is obstructed by, quite obviously, the whole world full of dust. It is invisible.
Intuitive solution is to limit the Dust volume to only as small area as possible, just around optical elements. To do so, I removed the Dust volume from the World and assigned the very same Dust volume (through material) to the Cube, which contains the whole scene. Only to get unexpected results regarding light propagation as seen on picture 2. Segments 1, 2 and 3 from the first picture are missing. Remember, the only 2 things changed: World got rid of Dust volume and the Cube with Dust volume got turned on. (HDRI backdrop is now visible).
When I tried to randomly turn off some optical elements, I started to see a pattern. It looks as though only the segment of light after the last optical element is visible. Others are either completely absent or only very faint.
Example with second lens turned off:
Example with prism turned off:
I would expect that a cube with a volume assigned to it, would locally create very similar optical conditions, as World would, with the very same volume assigned to it. But this is not the case.
You can see the text "Not supported by Bidir engine." in material editor. Is that the answer? That, while in case of the World, it is just volume, which is assigned to it, but in the case of the Cube, it is the unsupported material (with the very same volume as an input), which is assigned to it?
(I was also thinking about compositing, but it seems that you can't have two worlds (dust and air) each in different render layer.)
How to set up a scene in Blender 2.79b + LuxCore, which would support, at the same time, both A) volumetric caustics and B) Hemi lamp + HDRI backdrop?