Rendering equation ($L_o=L_e+{\int }_{{\Omega }^{+}}{f}_r{L}_i\cos {\theta }_{i}d{\omega }_i$)

A balance of emitted and reflected radiance at a surface. A bidirectional reflectance distribution function weights incident radiance from each direction, and a cosine accounts for projected area. Indirect illumination makes the incident field depend on other surfaces.