In the above equation, x and y are points on the scene and an object respectively
s is a function which returns 0 if point x is shadowed, 1 if not
R returns the reflectance of point x in the scene
cos(theta) is the dot product between the object normal and the light direction
cos(phi) is the dot product between the light normal and the light direction
(y-x)^2 is the distance squared between points x and y
Radiance(Reflection) is the color of the image where the reflected ray hits
