detecting hand position

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Hi, I know how to detect hands with Haar or skin color, but I want to detect exact hand positions for ASL-American Sign Language for deaf people. http://www.lifeprint.com/asl101/fingerspelling/images/signlanguageabc02.jpg

For instance, Haar would detect both letter A and B as hand from my image, because...well that's what it does, it detects hands. But how can I make it detect the exact hand position and make a difference between A and B?

Any answer helps, thanks for your time.