I'm not very familiar with computer graphics.
Currently I'm reading a paper "Image-Space Horizon-Based Ambient Occlusion - Louis Bavoil, Miguel Sainz, Rouslan Dimitrov"
Here's the problem
In the first paragraph they state general form of ambient occlusion illumination and mention that V is a function, that equals 0 if no occlusion and 1 otherwise.
And in the second paragraph they expand it to cosine.
I can't find any explanation about how it happens. Does anyone know how did they do it?