The new pen alpha feature allows drawing with semi-transparency. However, it's not easy to get a uniform transparency because of the 'overlap problem' - if you draw over the same region twice, you get 'double-strength' opacity. The triangle fill here uses a rastering method to draw only horizontal lines that don't overlap each other, thus ensuring a uniform fill. Interestingly, Flash renders horizontal lines *much* quicker than vertical (but it makes no difference to the actual script execution time). Consequently, the performance isn't a total flop when compared to the really fast hybrid method (from which this project was remixed).
