Page 194 - DCAP504_Computer Graphics

Unit 12: Color and Shading Model

Phong Shading
Phong shading is also called as normal vector interpolation shading. Phong shading can be used to
interpolate vector N that is normal to the surface, instead of interpolating the color values.
In figure 12.12, to determine the normal vector N at point E in a triangle, we
first interpolate N1, and N2, to get N', then N’’ can be calculated by
interpolating N2 and N3. Finally, normal to the surface can be calculated
through the interpolation of N' and N".

Figure 12.12: Finding Normals for Phong Shading

Source: Xiang. Z., Plstock. R. (2006). Computer Graphics. 2nd Edition. Tata McGraw Hill. Pg no. 299
Phong shading method consumes relatively more time since the illumination model is evaluated at
every point of interest using the interpolated normal vectors. However, Phong shading is helpful when
we need to deal with specular highlights.
Phong shading results in specular highlights which are much less dependent on the underlying
polygons. The drawback of Phong shading is that it requires much calculation as it determines the
interpolation and intensity of each pixel.
Consider the following figure 12.13 which depicts the Phong shading of an image.

Figure 12.13: Phong Shading of an Image

Source: http://images.yourdictionary.com/images/computer/_SHADING.GIF

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