Import sections copied from the paper:
Decoding:
A move in chess may be described in two parts: first selecting the piece to move, and then selecting among possible moves for that piece. We represent the policy ⇡(a|s) by a 8 ⇥ 8 ⇥ 73 stack of planes encoding a probability distribution over 4,672 possible moves. Each of the 8⇥8 positions identifies the square from which to “pick up” a piece. The first 56 planes encode possible ‘queen moves’ for any piece: a number of squares [1..7] in which the piece will be moved, along one of eight relative compass directions {N, NE, E, SE, S, SW, W, NW}. The next 8 planes encode possible knight moves for that piece. The final 9 planes encode possible underpromotions for pawn moves or captures in two possible diagonals, to knight, bishop or rook respectively. Other pawn moves or captures from the seventh rank are promoted to a queen.
Encoding:
