A Simple Model of Wood
Wood grain can easily be modeled as a collection of concentric
cylinders. By specifying two parameter the RingWidth and the
GrainWidth determining the color for a given point is a function of
that points distance from the axis of the cylinder.
If the cylinders are defined to be centered on the z-axis the color
function is just a matter of determining which ring the point falls in
based on the radial distance from the z-axis.
D = R/RingWidth;
D = (D - (int)D) * RingWidth;
if( D < GrainWidth) return GrainColor;
else return PulpColor
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