Gouraud shading

Henri Gouraud's 1971 University of Utah dissertation Early 3D objects were built from flat polygons, and lighting each facet as one block of color made a sphere look like a cut gem. Gouraud's method computes the lighting at each corner of a polygon and blends the colors smoothly across the face in between. It is cheap, which is why it mattered: a simple rendering method that dramatically improved how curved objects looked without needing more polygons. Its weakness is highlights, which smear or vanish when they fall between corners, and that gap is exactly what Phong attacked next.