I know that the BRDF can be used to calculate one ray casted from a specific incidence direction to a view direction.
I also know that I can use the Monte Carlo method to integrate the BRDFs casted
from all the incidence directions to one view direction in offline rendering.
But as far as I know the Monte Carlo method is rather expensive in real time case,
so I want to understand how the real time game engines such as unreal engine 4,unity 3d efficiently integrate the BRDFs?
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