If you want to recognize single letters and shapes alike I would suggest using GestureOverlayView
.
Read about it here:
http://android-developers.blogspot.com/2009/10/gestures-on-android-16.html
here:
http://www.vogella.com/articles/AndroidGestures/article.html
and here:
http://jayxie.com/mirrors/android-sdk/resources/articles/gestures.html
It is said to be good for casting HarryPotter-like spells too.