Vertex Normal:

Vertex Normal can be calculated from :

where, N is the number of polygons sharing the vertex.

Figure: Find the Vertex Normal at Each Vertex

Having found the vertex normal for each vertex of the polygon we want to shade we may calculate the colour at each vertex by using the same formula that we did for Lambert Shading. Calculating the colour for all of the remaining pixels in the polygon is simply a matter of interpolating from the vertices, that means if your half-way along one of the edges, the colour value ought to be half-way between the colour values at the ends of the edge. A value for the colour may be given more formally by considering a scan-line through the polygon.