I'm not sure if it was talked in the class or I just missed this part. Why the denominator of f is Pi instead of 2Pi, the solid angle of a hemispheres?
@Febgreen The denominator is chosen such that conversation of energy is preserved for all albedo values between 0 and 1. Try sticking the reflectance function f_r = 1/pi (white albedo) into the energy conservation equation on this slide; the integral of (1/pi) * cos(\theta) over the hemisphere is equal to 1 (and not 2).
f_r = 1/pi
(1/pi) * cos(\theta)