Ambient shading

Another heuristic is ambient shading, which causes an object to glow without being illuminated by a light source. This lights surfaces that fall into the shadows of all lights; otherwise, they would be completely black. In the real world this does not happen because light interreflects between objects to illuminate an entire environment. Such propagation has not been taken into account in the shading model so far, thereby requiring a hack to fix it. Adding ambient shading yields

$$L = d I \; (0,n \cdot \ell) + s I \; (0,n\cdot b)^x + L_a,$$ (7.5)

in which $L_a$ is the ambient light component.