Before using smooth sampling for DFAO, adaptive sampling was used which meant that flat surfaces did less work compared to denser scenes with many surfaces, like foliage. It also meant that
there could be a lot of splotches in clean environments.
In the example below, AO computations are now fast enough to get rid of the adaptive sampling, so occlusion is much smoother.
The smooth sampling requires a longer history filter, which can contribute to "ghosting" (or a trail behind a moving object), especially when shadow casters are moved. In Unreal Engine 4.16 and later,
Ghosting has been improved by having the Distance Field temporal filter store a confidence value. It is used to track leaking of occlusion during the upsample and then flush those leaked values through
the history faster. Overall, this should help reduce ghosting that can happen when the camera or a dynamic object is moving fast.