I have a question about rendering workflows when using Karma for explosions and Arnold for other elements in a scene.
In many studios, Arnold is used as the main renderer by the lighting department. However, for large-scale FX like explosions, there are some challenges. Arnold doesn’t handle volume lighting as efficiently as Karma or Mantra—especially with dense smoke and effects that rely heavily on scattering. These kinds of explosions, typical in Houdini, tend to produce excessive noise in Arnold, making them very difficult to render cleanly.
Because of this, FX artists often render just the explosions in Karma, while the rest of the scene is rendered in Arnold.

To integrate the explosion lighting into an Arnold-rendered scene, a common workaround is to convert the smoke into geometry and use it as a geometry light in Arnold. Then, we render the explosion in Karma and composite both renders using deep compositing.

However, we’ve encountered a couple of issues with this method:
Geometry lights don’t show up in reflections, so on highly reflective surfaces, the explosion isn’t visible.
Geometry lights don’t cast light onto overlapping or intersecting geometry, so when we deep composite, objects inside the explosion aren’t lit properly.

Has anyone found a good solution or workflow for this kind of hybrid rendering setup? Any tips for getting more accurate lighting interaction between Karma-rendered volumes and Arnold-rendered geometry?

Thanks!