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Computations in the register combiners are performed in 8 bit (+ 1 sign bit)
fixed point, this results in precision and dynamic range problems as shown in
Figure 10. In order to isolate precision problems from
interpolation and normalization issues the used sphere model is highly
tesselated.
Figure 10:
Comparison of Precision Problems
|
In the shader with register combiner normalization, the
exponentiation is computed by repeated self multiplication, for higher
specular exponents this results in visible banding. The shader with table
lookup performs much better. In this shader the result of a high precision dot
product between
and
is used to access a texture where the
specular intensity is stored. These values are stored as 8 bit values, but
through the use of linear interpolation between the table values the quality
is very good. The standard Gouraud shader is used only for reference purpose
as no per-pixel shading takes place. It should be noted, that in the inner
zone of the highlight the Gouraud shader performs still better than the
texture shader, because the exponentiation function gets very steep near one
and there are too few values in the table to interpolate perfectly
[6].
Next: Bump Mapping
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Gerald Schröcker
2002-03-21