I have a test scene where a very thin and bright spotlight (almost like a laser) shines on a diagonal mirror in a volume that scatters light, such that the light beam is directly visible. The mirror uses a 100% glossy material. The intention is to make the light bounce on the mirror. However, it seems the light beam disappears after bouncing in the mirror.

This is the scene in the viewport: enter image description here

And this is the rendered result: enter image description here

As you can see, the light beam is visible from the light source to the mirror, but then the light beam stops: it doesn't continue from the mirror to the cube. Is this a limitation in Cycles or do I need to enable some options somewhere? As far as I'm aware, I'm using Cycles' default settings. I've already tried increasing the maximum number of volumetric scattering events to no avail.

(I'm aware you could fake the effect by placing another spotlight on the surface on the mirror, pointing in the direction of the cube, but I'd like to do it the "proper" way if possible)

  • $\begingroup$ Under Light Paths enable Reflective Caustics and maybe increase the maximum number of Bounces if needed, also if you want switch to Full Global Illumination preset if you also need other things cranked up. $\endgroup$ – Jaroslav Jerryno Novotny Jul 28 '17 at 9:04
  • $\begingroup$ @Jerryno Unfortunately that didn't work - Reflective Caustics is on by default, and switching to Full Global illumination also gives the same result. $\endgroup$ – Bauxite Jul 28 '17 at 9:50
  • $\begingroup$ Well then you may need to use a bi-directional renderer for this job. I do get correct volumetrics with a mesh light but the second reflected ray is very hard to clean from severe noise. It is only fireflies around 10k samples and non-existent below 1000 samples.. $\endgroup$ – Jaroslav Jerryno Novotny Jul 28 '17 at 12:36
  • $\begingroup$ An effect like that is not possible using cycles. A glossy object will not reflect light like it does in th real world. blender.stackexchange.com/q/8645/1853: $\endgroup$ – cegaton Jul 29 '17 at 8:54
  • $\begingroup$ You need to use a different render engine to to proper caustics. Luxrender would work. $\endgroup$ – cegaton Jul 29 '17 at 9:00