Phong Shading

Phong shading too is based on interpolation, but instead of interpolating the colour value, it is the normal vector, which is interpolated for each point and a colour value calculated for each pixel based on the interpolated value of the normal vector. The interpolation is (like Gouraud shading) based upon calculating the vertex normals (red arrows in Figure 3.28), using these as the basis for interpolation along the polygon edges (blue arrows) and then using these as the basis for interpolating along a scan line to produce the internal normals (green vectors).

Phong shading allows us to counteract the fact that we are using a flat surface to approximate to a curved one. The arrows (and thus the interpolated vectors) give an indication of the curvature of the smooth surface, which the flat polygon is approximating to.