Q. Describe briefly phong shading algorithm.  

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Write short note on phong shading.   

Phong shading   Phong shading or normal vector interpolation shading is an accurate method developed by phong Bui Tuong for rendering a polygon surface. It interpolates normal vectors and then apply the illumination model to each surface point. It displays more realistic highlights on a surface and greatly reduces the Match- band effect. A polygon surface is rendered using Phong shading by carrying out the following steps: 1. Determine the average unit normal vector at each polygon vertex. 2. Linearly interpolate the vertex normal over the surface of the polygon. 3. Apply an illumination model along each scan line to calculate project pixel intensities for the surface point. Phong shading method requires considerably more calculations. Surface rending with phong shading can be speed by up using approximations in the illumination model calculations of normal vectors. Fast phong shading approximates the intensity calculations using a Taylor- series expansion and triangular surface patches. Fast phong shading takes approximation twice as long as to render a surface then with Gouraud shading. Normal phong shading using forward difference takes about six to seven times longer then Gouraud shading.