The Gouraud Shading shading method applies the illumination model on a subset of surface points and interpolates the intensity of the remaining points on the surface. In the case of a polygonal mesh the illumination model is usually applied at each vertex and the colors in the triangles interior are linearly interpolateded from these vertex values.

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The linear interpolation can be accomplished using the plane equation method discussed in the lecture on rasterizing polygons.

Notice that facet artifacts are still visible.