With latest mental ray progressive sampling advancements together with rombo.metal we can do sort of progressive pathtracing.
This is an experimental implementation available in rombo.metal that turns the material in for global illumination. You have to use all rombo.metal materials in the scene. And for the while you can only compute diffuse GI without any reflections/refractions but in a future we may add at least reflections.
First of all disable FG or GI Next 🙂 Then set both rombo.metal UV roughness exactly to 1 and multiscatter to at least 10, this will engage a special mode in the material. Add a simple environment shader or arealight and the basic setup is done. You can control GI bounces with reflection trace depth in the renderer settings panel.
Here some results.
With environment shader alone.
And with area light and DOF (20mins).
Here a video: Progressive Pathtracing