Linearly interpolate - Modeling and Rendering
I_{4} = I_{1} + t (I_{2} - I_{1}); here t = (|y_{1} - y_{2}|)/(|y_{1} - y_{2}|)
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.