cylindrical pattern creates a one unit radius cylinder
along the Y axis. It is computed by: value = 1.0-min(1, sqrt(X^2 +
Z^2)) It starts at 1.0 at the origin and decreases to a minimum value of
0.0 as it approaches a distance of 1 unit from the Y axis. It remains at 0.0
for all areas beyond that distance. This pattern was originally created for
media but it may be used anywhere
any pattern may be used.