First and foremost, I would ask: Does your scene need to be path traced?
Path Tracing is most effective where there is a lot of geometry (e.g. imported object populations), or generally more potential for bounced light. It handles complex interactions of light bouncing better than the normal renderer. But for example in a mostly-terrain scene with a few scattered objects and a lot of direct sunlight, you're probably not going to see much benefit. If you move the sun lower in the sky to create lots of shadows, a sunset for example, then you might see more benefit. But the greatest gains will tend to be in thicker vegetation, or on more complex objects.
Path Tracing also doesn't really affect the atmosphere, its main effects are on the ground, the terrain and anything sitting on it. So don't enable it if you are hoping for some improvement in the sky. It can, however, create more realistic lighting on the ground from the sky, to some degree.
So, if you decide your scene would benefit from Path Tracing, then the render settings do often need to be different than regular renders. We are working on a guide for this, but in the mean time some quick tips (not necessarily in order of importance):
#1: Don't use high MPD
Path Tracing enables "Defer All" shading. Defer All is a rendering method which makes much more detailed terrain (similar to "Defer Atmo" which makes smoother clouds). This means MPD values can be lower for equivalent detail. You would be fine with 0.5.
You can test how Defer All compares to regular rendering on its own, without Path Tracing. Just enable Defer All separately (without Path Tracing), and maybe render a crop of terrain and test the render time. You can use MPD 0.8 for the non-Defer All render and MPD 0.5 for the Defer All. See which is better and what the render times are.
#2: If you have Soft Shadows Enabled, Reduce Samples
Again, because of Defer All being enabled with Path Tracing, you get higher quality surface shading. This includes Soft Shadows. So you can use fewer samples. I recommend a value of 2 when Defer All or Path Tracing are enabled. Again you can test this with a crop.
#3: Use Robust Adaptive Sampling (especially in v4.4)
The new Robust Adaptive Sampling helps a lot to reduce render time with Path Tracing, while maintaining quality. You can use a value of AA8 or AA6 with Robust Adaptive and 1/64th samples as a starting point. Increasing the amount of adaptivity, i.e. the "First sampling level", will reduce render time but can add to noise. If you use AA8 you can consider 1/256, but at AA6 I would stick to 1/64.
Then adjust the Pixel Noise Threshold *only* to get the level of noise under control. Start with the default value and go down if you want less noise, or up if noise looks fine but render time is too long. You may find you can get away with a lower Pixel Noise Threshold. Do renders in crops to reduce time needed for checking the best values, and you should include an area of shadow in your crop for most accurate results.
Note that Robust Adaptive is improved in v4.4 vs. 4.3.
#4: Don't Bother with High GI Values
The GI Cache Detail and Sample Quality values do have an effect with path tracing, but they only control GI contributions from the atmosphere into the path tracer, which is used for the terrain illumination. I.e. the Path Tracer essentially replaces the cache-based GI system when rendering the terrain. So these GI settings can generally be fairly low (e.g. 2/2 or 3/3) because the atmosphere contribution is fairly diffuse in most cases, and the terrain lighting will be made accurate by the path tracing itself.
Put simply: you don't need high Cache Detail or Sample Quality values to get good path tracing results on your terrain.
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That should get you a good ways toward faster path traced renders. We'll put out more info in the near future.
- Oshyan
