In the flat shade mode, the Direct3D rendering pipeline renders a polygon using the color of the polygon's material at its first vertex as the color for the entire polygon. 3-D objects that are rendered with flat shading have visibly sharp edges between polygons if they aren't coplanar.
The following figure shows a teapot rendered with flat shading. The outline of each polygon is clearly visible. Flat shading is computationally the least expensive form of shading.