I see what you mean.

Just to make sure we're on the same page: Intended behaviour is that when max_volume_bounce is set to 0, we trace one scatter event inside the volume from which we gather emission inside the volume, but not any direct or indirect light? So volume emission will illuminate volume scattering, but not other surfaces (since we don't have next event estimation for volume emission)? And once we get next event estimation for volumes emission, it should then also illuminate surface shaders too?

The intended behavior is that when max_volume_bounce is 0, we gather direct lighting and emission from surfaces and volumes. The latter is also considered to be "direct lighting", so that a mesh light behaves the same as any other light. Using next event estimation or not should not make any difference in the converged render result, but it is different in this case due to an inconsistency with the handling of transparent surfaces.

If you run the regression tests you'll see the difference, but here's an example where you can see how surfaces and volume handle indirect light differently after the fix:

	Before Fix	After Fix
Max surface and volume bounces 0
Max surface and volume bounces 1

Currently I'm thinking the fix could be something like this:

• Move the max bounces test into path_state_next(), setting a state->flag to indicate the path is to be terminated.
• If that flag is set shader evaluation at the next bounce will remove all closures except for emission and transparency (or not add them in the first place: D2938).
• The path can then continue for transparency but not other BSDF/BSSRDF types.

Maybe there's a simpler way to do it, I'm not sure.

That sounds like a good approach. I suspect that in general, transparent intersections can lead to incorrect path termination. Note how for example how an environment light with diffuse bounces = 0 will not provide direct illumination through transparency. Volume emission currently also provides direct light to surfaces inside the volume's bounds, but not to surfaces outside of the volume's bounds.