Volumetric Caustics + HDRI Backdrop Goal

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.

Problem

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:



Example with first lens turned off (exception from the pattern):


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.)

Question

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?

Scene: volumetric_caustics_30.blend
You have to specify the exterior volume of the objects inside the fog cube.
Plug a pointer to the fog volume into the "exterior volume" slot of the two glass materials of the lenses and prism.

Note, the beam between lens on the right and prism still stays very weak. To be honest, I don't know what's up with that. It could be another problem in the volume setup or mesh normals, or it might be a bug in LuxCore.
You can try asking about it in our forum where our main LuxCore developer Dade might be able to tell you more about it.
In the forum I have also written a bit of an explanation about interior/exterior volume setup, maybe it helps you: https://forums.luxcorerender.org/viewtopic.php?f=4&amp;t=1178&amp;p=13864#p13879



Before:
(HDRI was not packed, so I used a green color instead)



After:



(30 second renders)

The prism

The last piece is the prism. Originally, I did my prism from the cube by eliminating one face and merging disconnected edges. Now I tried cleaner way by adding cylinder and setting number of faces to 3. This did the job. The two ways obviously don't lead to the same mesh.

Result:


Link to fixed scene is here and link to HDRI image here.