Linearly interpolate - Modeling and Rendering

I₄ = I₁ + t (I₂ - I₁); here t = (|y₁ - y₂|)/(|y₁ - y₂|)

I_D = I_A + t (I_B - I_A); here t = (|AD|)/(|AB|)

As same in I_E = I_C + t (I_B - I_C); here t = (|CE|)/(|CB|)

And I_P = I_D + t (I_E - I_D); here t = (|EP|)/(|DE|)

Here I_p → Intensity of points over the surface of polygon that is in Gourand shading the intensity of point - 4 is linearly interpolated by intensity at vertices 1 and 2, as the same in point 5 too is interpolated by intensity at vertices 3 and 2. Intensity of points P is linearly interpolated by intensity at point 4 and 5.