I am trying to render a beam of light (going through a fog volume (for visibility) and collide with a surface. The surface has a mirror like material, but the light beam is not reflected back into the fog volume.
I am trying to simulate a visible laser beam going from mirror to mirror.
For the “laser” I am using a spot light, with a cone angle of 0.1, and cone delta of 0.5. The spot has a bit of an angle, to the surface to not reflect the light right back at the source.
If you increased all of the minimum ray counts to a million, you'd probably get the reflection. However, because a ray needs to go from the camera to the volume, then happen to scatter off the volume to the extremely tiny little point where the light hits the mirror at just the right angle to reflect back to the point in the volume, it's not very feasible.
Long story short, you need to add a GI Light node (of some sort) to the scene. I'm definitely not an expert with photon maps, but it will do a photon map generation pass before the main render, sending light rays out from the lights, and the main render will effectively use that as a light, so it'll hopefully avoid the issue you're seeing. I've only used them a bit, though, so I don't know how best to use them.
I'd suggest that the best approach to this would be to cheat.
The setup you're describing works fine in the real world, because there are millions of photons piling out of the laser, and billions of billions of air molecules to scatter and reflect the light.
Asking Houdini to simulate all those interactions is obviously impossible, but it'll do the best approximation it can, even though to get good looking results could take it a year of deep thought and calculation.
What's the visual end result you're after?
- if the beam has to be visible in the “air”, you only need to use volumes at all if the air has to be uneven/cloudy/patchy. If you're happy with a consistent beam, you don't need the volume at all; just create the beam as geometry with a suitable shader and/or a glow effect.
- whether your beam of light is geometry or a “real” light, I'd handle the reflection myself with an expression and a second beam.
You could even do the old laser bouncing zig-zag between two parallel mirrors thing pretty quickly with geometry - but it'd be utterly impractical to try doing that with a simulation.
For more help, let us know: after you've got your laser beam bouncing off that mirror, what happens then? What's the end result of the scene supposed to look like?
