CMSC 435 Autumn 1998  Introduction to Interactive Graphics
Smooth Shading vs. Faceted Shading
Faceted Shading:

1 color per polygon

produces "unsmooth" objects

direct rendering of the geometry

fast
Smooth Shading:

assumes object is really a curved surface, approximated by
a collection of polygons.

more realistic rendering

slower

Two Types: Gouraud and Phong
Gouraud Smooth Shading (1971):

Calculate Average vertex normal

Perform illumination calculation to get vertex colors

Interpolate vertex colors accross the face.

Suffers from Mach banding :
Whenever the slope of the light intensity curve changes
abruptly,
the surface appears brighter (inflection is concave)
or darker (convex)

May miss highlights within a face.
Phong Smooth Shading (1973):

Calculate Average vertex normal

Interpolate vertex normals accross the face.

Perform illumination calculation at each pixel

More accurate highlights

Fewer Mach Band effects

Why do they occur?
Comparison of Shading Types



Facted Shading 
Phong Smooth Shading 
Phong Smooth Shading
with Volumetric Steam 
David S. Ebert
November 1998