Occlusion Map (occlusion map / AO map) is a technical term that is extremely important in the rendering and optimization of game graphics (technical art). An explanation of the technology for defining ultra-light and precise shadows on a material-by-material basis, in addition to post-processing's SSAO, using textures that are pre-baked to reflect the ``inability of light to reach (environmental light shielding degree)'' in deep gaps such as wrinkles, screw holes, and joints in 3D models.
An example from the real world: When painting a figure's face, the professional technique of applying ``shadows (smudges)'' with black paint to the thin grooves of the sculpture and the edges of the eyes in advance so that the three-dimensional effect will not be lost even if the light hits it
Occlusion map is a monochrome defensive texture that creates a shadow mask in advance (pre-baked) that says, no matter how strong the light is shined from all directions, this gap is ``the depths of darkness where no light can reach''. post processing's SSAO is a rough shadow based on the screen resolution, but occlusion map can express ultra-high-definition shadows that closely adhere to the material itself (ear grooves, wrinkles on clothes, etc.), completely preventing the "overexposure of the three-dimensional effect caused by light" that is characteristic of CG.
Illustration: Occlusion Map (Occlusion Map / An infographic that clearly illustrates the basic processing flow and mechanism of AO Map) in Japanese.
Detailed mechanism and operating principle
Create a "occlusion map (monochrome texture)" with the gaps painted black (0.0) using 3D paint software, etc., and register it in the "Occlusion" slot of the material.