Really nice pictures over there. I left a few comments…
Just to clean up my previous blunder, here is the setup and result for a correct light rig.
The lights producing the mirrored beams have to be on mirrored positions (of course…). I abandoned the 3D Shadows Invert setting and went for excluding the mirrors from the lighting instead. The mirror in front of a reflected beam light is on the “All objects except:” list of this light. So the light as well as the light cone go through the mirror and show properly between both mirrors.
With the mirror reflection set to 90%, each reflected beam is set to 10% less than the previous one (light cone intensity and light brighness).
The “Fog/Use gel” setting in the Light Cone dialog also reacts to the Angular Falloff setting of the lights, which adds to the credibility of the fake beams.
Masking out the surplus beams turned out to be much easier than I had expected. I rendered the complete scene, then only the stage, and at last only the two mirrors with the Scene Background color set to white, and with Multi-Pass element Background switched on. The latter provided the contours of the mask to cut out the complete scene. Pasted that onto the empty stage, and got the cleaned-up result.
- Light positions and beams
- Complete render and stage render
- Background pass and mask
- Final composite