Gouraud Shading:

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

Figure: Variable Shading of an Individual Polygon

In order to attain this, the colour must be calculated for each pixel instead of one colour for the complete polygon. By ensuring that the method we use to calculate the colour results in the neighbouring pixels across the border among two polygons end up with approximately the same colours, we shall be able to blend the shades of the two polygons and avoid the sudden discontinuity at the border.