[cscheid]

Alpha doesn't work additively for pretty much anydrawing packages:

http://en.wikipedia.org/wiki/Alpha_compositing

When overplotting, the usual compositing operator gives a final alpha of

1 - (1-alpha)^N

So your alpha = 1/5 overdrawn 5 times would give a final opacity of ~0.673. By its very nature, there is no alpha < 1 which when composited together a finite number of times gives alpha = 1.

[mxfh]

I was aware of that this approach towards alpha was oversimplistic to begin with, should have pointed that out. Thanks for posting the correct formula.