Make brush pattern "patchy"

[bschwind]

Notice how there are fairly long streaks of no ink:

http://www.creativeroots.org/wp-content/uploads/2010/02/japanese_calligraphy.jpg

I'll probably need to use a seed in the random number generator to have more predictable bristle positions.

Here's another example:

http://lh5.ggpht.com/_ONiSfl3Iud0/S6GymR0-vuI/AAAAAAAAEtg/Jxh_ADSZ7gs/s800/IMG_9939.JPG

[bschwind]

This is a great video of a brush spreading ink across a canvas:

http://www.youtube.com/watch?feature=player_embedded&v=G0ASmb3QFKE