Figure 2:

a. Map-view conceptual diagram showing that the region where direct and scattered radiation is to be evaluated (E_d+s) is nested within the direct-only region (E_d) and the overall shape model (W_m). b. Cross-sectional view of the geometry involved in photometric computations. Line-of-sights (L_s → P₁, L_s → P₂, P₁ → P₂, P₁ → O, P₂ → O) may need to be evaluated, and the photometric function evaluations at P₁ and P₂ will also involve the surface normal unit vectors at these points, ${\widehat{\mathbf{n}}}_{\mathbf{1}}\hspace{0.17em}\text{and}\hspace{0.17em}{\widehat{\mathbf{n}}}_{\mathbf{2}}$. The point P₁ is within E_d, meaning only direct illumination is considered. The point P₂ is within E_d+s, meaning both direct and scattered illumintion need to be considered. A₁ and A₂ are the albedos of P₁ and P₂, while dS₁ and dS₂ are their respective surface areas.