Here is my test scene.

Ground plane.

Some overlapping primitives.

My EnvSphere, self lit, with a very gentle light blue-to-white gradient.

One infinite white light at 20% intensity.

All materials are simple white diffuse-only with diffuse value at .7 or .8, so there's no question of bounced light intensity blowup.

Being that the GI will largely be driven by the EnvSphere, and it exists in all directions, there's no problem with a photon "finding" a light source.

Using the D3D Render Firefly dialog I set very aggressive (slow, high-quality) values on everything:

Irradiance Sample Size=5 (half of normal, creating four times as many sample points as usual)

Bounces=6 - plenty enough to find the sky. Note that when two planes meet at 90 degrees, sky is found in 2 bounces at all times, and within 3 bounces at a 3-plane corner. I used 6 just to be sure that prop-to-prop bounces would work as well. Since the sky is reached in 3 bounces at any corner, then a pair of corners reaches sky in 6, no matter what. 99.9999% of the samples in the render reached the sky within 3 bounces.

Samples=1000 - Each point IC sample sent 1000 rays to examine the rest of the world. Very aggressive.

Irradiance Cache - 70. Fairly aggressive - does not use the cache unless preliminary sampling indicates less than 30% error. Note that the artifacts show a luminance error far, far in excess of 30%.

The results are attached. Very bad.

But it was only a 24 second render!!!

 

 

 

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