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The geometry of light reflection at a surface is illustrated in Figure 2. The polar and azimuthal angles of incidence and reflection are given by (θ_{i},φ_{i}) and (θ_{r},φ_{r}), respectively, and the solid angle of the incident light is given by dω_{i}. The polar angles are measured from the surface normal. For isotropic surfaces, the BRDF is independent of φ_{i} because of rotational symmetry.
Figure 2: Geometry of incident and reflected light
We can express the incident energy flux dE_{i} in terms of the incident radiance I_{i}, the incident angle θ_{i} and the incident solid angle dθ_{i}
dE_{i} = I_{i} • cosθ_{i} • dω_{i} | (2) |
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