Ambient Occlusion (AO)
AO: Adds "shadowing" to objects without adding proper shadows. These darkened areas where geometry would block access to light adds perceived realistic depth to objects and the scene, and combats "flat" lighting.
AO Types
Screen-Space Ambient Occlusion (SSAO)
SSAO attempts to measure the amount of occlusion of a pixel based on the depth of surrounding pixels. This implementation of AO is generally regarded as the least realistic as it depends on the view of the camera in-game. SSAO is best if there is no other AO alternative, or if the alternative is no AO.
Horizon Based Ambient Occlusion (Plus) [HBAO(+)]
HBAO and HBAO+ are superior than SSAO as it considers geometry in the scene, and not just pixels and relative depths. This process, however, is more computationally intensive to run and may be more demanding with increasing amounts of geometry. Enabling HBAO(+) is typically a trade worth making even with its performance impact. This is especially true in single-player or slow-paced games.
