Comparing Shading Modes

In the flat shading mode, the following pyramid would be displayed with a sharp edge between adjoining faces. In the Gouraud shading mode, however, shading values would be interpolated across the edge, and the final appearance would be of a curved surface.

Gouraud shading lights flat surfaces more realistically than flat shading. A face in the flat shading mode is a uniform color, but Gouraud shading allows light to fall across a face more correctly. This effect is particularly obvious if there is a nearby point source.

Gouraud shading smoothes out the sharp edges between polygons that are visible with flat shading. However, it can result in Mach bands, which are bands of color or light that are not smoothly blended across adjacent polygons. Your application can reduce the appearance of Mach bands by increasing the number of polygons in an object, increasing screen resolution, or increasing the color depth of the application.

Gouraud shading can miss some details. One example is the case shown by the following illustration, in which a spotlight is completely contained within a polygon face.

In this case, Gouraud shading, which interpolates between vertices, would miss the spotlight altogether; the face would be rendered as though the spotlight did not exist.