HBAO
A screen-space ambient occlusion method that measures how much sky each point can see by reading the horizon in the depth buffer.
Where plain SSAO just samples points around a pixel and counts how many sit closer to the camera, HBAO (Horizon-Based Ambient Occlusion) is more physical: for each pixel it marches out in a few directions through the depth buffer and finds the highest horizon angle that blocks the sky. Integrating over those angles gives the occlusion, which yields softer, more accurate contact shadows in creases and corners.
Presented by Nvidia in 2008 (Bavoil et al.). The HBAO+ variant was a common PC graphics option from roughly 2012 to 2018 before GTAO took over as the reference.