Gouraud Shading

The faceted appearance of a Lambert shaded model is due to each polygon having only a single colour. To avoid this effect, it is necessary to vary the colour across a polygon :

In order to achieve this, the colour must be calculated for each pixel instead of one colour for the entire polygon. By ensuring that the method we use to calculate the colour results in the neighbouring pixels across the border between two polygons end up with approximately the same colours, we will be able to blend the shades of the two polygons and avoid the sudden discontinuity at the border. Lambert shading is based upon calculating a single normal vector for the surface (which is then compared to the lighting vector and the viewpoint vector to determine the colour). Gouraud shading is based upon calculating a vertex normal rather than a surface normal. A vertex normal is an artificial construct (a true normal cannot exist for a point such as a  Visual Realism vertex). A vertex normal can be thought of as the average of the normals of all the polygons that share that vertex.