means that the computer sees (assuming white's turn)1) a white piece dissapear at some place
We can place one sensor under each square, sensors like CNY70, they are cheap.
How will you assure that a sensor under one square does not see a piece on another square? How will you assure that the player places a piece where the sensor can see it?
The more I read and think about it, the more I understand that it is a very very big and hard job to realize it.