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S & T each feed a triwave generator, which are passed through
smoothstep to truncate the tops and bottoms to produce
a simple 'tire tread'. S is used as an index into a spline
color lookup table, used to create the colors in the tread valleys.
The 'S' value walks the sphere parallel to the equator,
and 'T' walks the sphere parallel to the polar axis.
Here's the desktop for the above image:
Detail:
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