Phong Shading

where P_a, P_b, P_c are pixels covered by this polygon

(approximation to curved surface)
Ideally: shade from normals of curved surface

Approximate with normals interpolated between vertex normals:
N_a = | P_a - P₀ | / | P₁ - P₀ | N₁ + | P₁ - P_a | / | P₁ - P₀ | N₀