Gouraud shading is a method for making a surface built from flat polygons look smooth. Rather than giving each polygon a single uniform brightness, which makes a curved object look faceted, it computes a light intensity at each polygon vertex and then linearly interpolates those intensities across the interior of the polygon. The result is a continuous gradient of shading that hides the polygon boundaries and approximates the appearance of a smooth curved surface.
The technique was introduced by Henri Gouraud in “Continuous Shading of Curved Surfaces,” published in IEEE Transactions on Computers, volume C-20, number 6, pages 623 to 629, in June 1971 (DOI 10.1109/T-C.1971.223313), based on his University of Utah doctoral work. Gouraud’s insight was to attach a surface normal to each vertex, averaged from the normals of the polygons that meet there, and to evaluate the lighting once per vertex. The interpolation that fills in the rest is cheap, which mattered greatly given the limited computing power of the time.
The economy of the method is its lasting virtue. Because the expensive lighting calculation happens only at vertices and the per-pixel work is a simple linear interpolation, Gouraud shading is fast enough for interactive and real-time rendering. It became a standard feature of graphics hardware and was the default smooth-shading model in early 3D APIs and game consoles. The same interpolation machinery that smooths intensity is also used to interpolate colors and texture coordinates across polygons.
Gouraud shading does have known limitations. Because specular highlights are only sampled at vertices, a bright highlight that falls in the middle of a large polygon can be missed or smeared, and the interpolation can produce visible artifacts along edges. Bui Tuong Phong’s later work addressed these by interpolating the surface normals themselves rather than the final intensities. Even so, Gouraud’s vertex-interpolation idea remains a cornerstone of the rendering pipeline.