Computes the accessibility factor in real-time, which must be done efficiently.

This was first used in the game Crysis, using multi-pass rendering.

Rather than evaluate the integral directly, a random (Monte-Carlo) sampling technique is used.

Approach:

• Pass 1:  Render the scene to texture.
• Pass 2:  Randomly sample the depth buffer around the current point p (this can based on the normal if available).  Count the points that are closer; these block the light coming to p.  Encode occlusion as a value [0, 1].
• Pass 3:  Blur the output from Pass 2, and darken fragments based on the AO term.

Tutorial: john-chapman-graphics 
Video: SSAO

There are also details in the SuperBible, including shader code.

High