Alpha doesn't work additively for pretty much anydrawing packages: http://en.wikipedia.org/wiki/Alpha_compositing http://en.wikipedia.org/wiki/Alpha_compositin

Alpha doesn't work additively for pretty much anydrawing packages: http://en.wikipedia.org/wiki/Alpha_compositing 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.