Using per-fragment shading for improved realism

When the shading equation is evaluated within the vertex shader (as we have done in previous recipes), we end up with a color associated with each vertex. That color is then interpolated across the face, and the fragment shader assigns that interpolated color to the output fragment. As mentioned previously, this technique is called Gouraud shading. Gouraud shading (like all shading techniques) is an approximation, and can lead to some less than desirable results when, for example, the reflection characteristics at the vertices have little resemblance to those in the center of the polygon. For example, a bright specular highlight may reside in the center of a polygon but not at its vertices. Simply evaluating the shading equation at the vertices would prevent the specular highlight from appearing in the rendered result. Other undesirable...