How do you ignore the palm of the hand when doing multitouch drawing?

Several multitouch drawing applications have implemented features that prevent interference when a user rests their palm开发者_开发问答 on the screen while drawing.

How do they filter out touch events from the palm of the hand, but allow the correct input from the drawing finger or stylus?

Ignore any multi-touch inputs that are close to each other, only taking the data from the single point (the finger).

Take the top y component from the group of touches.