SSAO (screen-space ambient occlusion)
A screen-space approximation of ambient occlusion that darkens creases and contact points from the depth buffer alone.
True ambient occlusion — how much surrounding geometry blocks diffuse light at each point — is expensive to precompute. Crytek's trick, shipped in Crysis in 2007, was to estimate it in real time from the depth buffer alone: for each pixel, sample nearby depths, and those sitting in front imply a nearby occluder. The effect adds the soft contact shadows that make objects feel grounded rather than floating. Being screen-space, it misses occluders off-screen or behind others and tends to halo, which later variants (HBAO, GTAO) refined. It remains near-universal as a post-process because it buys a lot of perceived depth cheaply.