Normal vector interpolation or Phong Shading Method

In this method, normal vector is interpolated. Following calculations are done for displaying polygon surface:

1) Calculate average unit normal vector at each polygon vertex.
2) Linearly interpolate the vertex normals.
3) Determine projected pixel intensities by applying illumination method at each scan line.

Normal Vector

Where, N1, N2 and N3 = Surface normals of polygon

V = Vertex of the polygon

Nv = Normal vector at vertex.

For any vertex V,

Normal vector for any vertex

Interpolation of surface normals along a polygon edge

Normal Vector Phong Shading

Where, N = Normal vector

Then incremental methods are used to evaluate normals between scan lines. After that intensity at a point is determined by applying illumination method.


1) Mach Band effect (Dark intensity streaks) is reduced.
2) More accurate results are obtained by this method.
3) Displays more realistic highlights on a surface.


1) More calculations are required.
2) Cost of shading is more in this method.

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