Gouraud Calculations

Ia = I1 - (I1 - I2) (y1 - ys)/(y1 - y2) Ib = I1 - (I1 - I3) (y1 - ys)/(y1 - y3)

(1) calculate intensity at vertices (I₁, I₂, I₃) from illumination model.

(2) interpolate vertex intensities along edges (I_a, I_b)