Multiple light sources

Typically, the virtual world contains multiple light sources. In this case, the light from each is combined additively at the pixel. The result for $N$ light sources is

$$L = L_a + \sum_{i=1}^N d I_i \; (0,n \cdot \ell_i) + s I_i \; (0,n\cdot b_i)^x ,$$ (7.6)

in which $I_i$, $\ell_i$, and $b_i$ correspond to each source.