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When Direct3D renders a polygon using Gouraud shading, it computes a color for each vertex by using the vertex normal and lighting parameters. Then, it interpolates the color across the face of the polygons (See Face and Vertex Normal Vectors). The interpolation is done linearly. For example, if the red component of the color of vertex 1 is 0.8 and the red component of vertex 2 is 0.4, utilizing the Gouraud shading mode and the RGB color model, the Direct3D lighting module would assign a red component of 0.6 to the pixel at the midpoint of the line between these vertices.
The following figure demonstrates Gouraud shading. This teapot is composed of many flat, triangular polygons. However, Gouraud shading makes the surface of the object appear curved and smooth.
Gouraud shading can also be used to display objects with sharp edges. For details, see Face and Vertex Normal Vectors.