ypes of smooth shading include:
Gouraud shading
Phong shading

**SMOOTH SHADING**

In contrast to flat shading with smooth shading
the color changes from pixel to pixel. It assumes that the surfaces are curved
and uses interpolation techniques to calculate the values of pixels between the
vertices of the polygons.

Types of smooth shading include:

Gouraud shading

Phong shading

**Gouraud shading**

1.
Determine the normal at each polygon vertex

2. Apply an
illumination model to each vertex to calculate the vertex intensity

3. Interpolate
the vertex intensities using bilinear interpolation over the surface polygon

*Data structures *

Sometimes vertex normals can be computed directly
(e.g. height field with uniform mesh)

• More
generally, need data structure for mesh

• Key:
which polygons meet at each vertex

*Advantages *

Polygons, more complex than triangles, can also
have different colors specified for each vertex. In these instances, the
underlying logic for shading can become more intricate.

*Problems *

Even the smoothness
introduced by Gouraud shading may not prevent the appearance of the shading
differences between adjacent polygons.

Gouraud shading is more
CPU intensive and can become a problem when rendering real time environments
with many polygons.

T-Junctions with
adjoining polygons can sometimes result in visual anomalies. In general,
T-Junctions should be avoided.

**Phong shading**

Phong shading is similar to Gouraud shading,
except that the Normal’s are interpolated. Thus, the specular highlights are
computed much more precisely than in the Gouraud shading model:

a. Compute a
normal N for each vertex of the polygon.

b. From
bilinear interpolation compute a normal, Ni for each pixel. (This must be
renormalized each time)

c. From Ni
compute an intensity Ii for each pixel of the polygon.

d. Paint
pixel to shade corresponding to light.

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