E is the emitted radiance from a point: E is non-zero only if x’ is emissive

V is the visibility term: 1 when the surfaces are unobstructed along the direction _, 0 otherwise

G is the geometry term, which depends on the geometric relationship between the two surfaces x and x’

project onto a finite set of basis functions (piecewise constant)

Photon tracing computes L [D | S]* E

Radiosity only computes L [D]* E