you'd need to know where 'Q' was , absolutely , for you to know where "W" , "E" , "R" , "T" and "Y" were relative to that.

But pretty cool idea though ...

Why do you need absolute positions? Just see if the finger tapping the table is extended up/forward relative to other finger(s).

how would the computer know which key your finger was on to figure out what keys are relative to that finger?

You could assume that the non-thumb fingers of a hand span at most two rows and that one of these is the "home" row.

When you detect a tap visually, you could then infer which row that finger tapped by looking at where the other non-thumb fingers are.

i dont understand , are we still talking about a qwerty keyboard? or a special kind with just two rows, one of which is a "home" row that is placed wherever certain fingers are?

This would work with three rows, with the home row being the middle one.