Excellent, that makes sense. Thank you. There is a small error in your implementation of my sample function. It should be:

my $z = sqrt( (1/(0.5 + 0.02*$x**2)*cos($x))**2
            + (1/(0.5 + 0.02*$y**2)*cos($y))**2 );
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But the illustration of the technique was the important thing!