Author: Don Dailey
Date: 22:07:29 05/13/98
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On May 13, 1998 at 21:59:41, Howard Exner wrote: >Some of the games I've managed to win against computers, and these are >few in comparison to the many defeats I've suffered, contain the theme >of the "trapped piece". Common themes are Knights trapped in the corner >and basically pieces that have their mobility stripped. > >To illustrate here is a line that I've played from the Crafty vs >Rebel game from KKup2. > >[FEN "8/2N1R3/7p/R4r2/3pkr1P/1P6/2P5/2K2b2 b - - 0 1"] > >1... Kf3 2. Rxf5 Rxf5 3. Rd7 Ke4 4. Kd2 Rh5 5. Ne6 Ke5 6. Nxd4 Rxh4 7. >Nf3+ Ke6 8. Nxh4 Kxd7 9. Nf5 Bh3 10. Nxh6 Be6 11. c4 Kc6 12. Kc3 Kc5 13. >b4+ Kc6 14. Kd4 Kb6 15. Kd3 Kc6 16. Kc3 Kb6 17. Kd4 Kc6 * > >Granted it was played rather quickly so I doubt very much that this >will transpire in the real game. I took black's side here and had >Rebel 8 play for white. After the move 10. Nxh6 Rebel thinks its >position is quite good, but the move Be6 traps the Knight. This kind >of position is nothing new to the readers here but it made me wonder >how programmers deal with this type of thing. How can positions were >pieces get trapped be avoided? I see no way for white to make any >progress from the above line despite the material plus. Black simply >has its king prevent the advance of the white pawns. > >Would any program play the move 9 ... Bh3 so as to trap the Knight >if Nxh5? 8/3k4/4b2N/8/8/1P6/2PK4/8 w - - Whites advantage is pretty small in this position, I see a couple tries but they lead to draws. But the point is having some concept of a trapped piece. It would not be hard putting this particular pattern in a chess program but like many other chess principles it becomes difficult to come up with proper scoring. Pehaps in similar positions the knight can be extracted. In this position extracting the knight costs a pawn (and half a point.) The problem isn't so much detecting a pin piece (which of course is not easy either but would be useful to do) but figuring out how it relates to all the other moves and the rest of the game. Maybe it's ok, maybe it's not. As soon as a human see's this position, he immediately begins to reason about it in a way that computers do not currently do (as far as I know.) This becomes critical and powerful information for the human. Suddenly things occur to him, plans to extract the knight, he knows exactly where the king must be to extract it, or he reasons that the bishop might be forced to defend thereby freeing the knight. And he considers the repercussions of all of this and knows how to integrate the whole process into a serious of quality moves. But the computer does not even really know the knight is trapped! As far as the computer is concerned, h6 is the best square for the knight since the other squares lose it. And that's all it cares about. It of course knows other squares are better but it is completely isolated from this knowledge because the search says it's not relevant here! Cool and efficient in a way. Do not worry about something you cannot have! Several years ago, Rexchess (on route to winning a human tournament) captured a rook on a8 getting its knight trapped. It was the classical computer "blunder" because material had been sacrafice in order to "win" the rook. To save the knight, Rex suddenly woke up and began a "reign of terror" on it's poor unsuspecting opponent. It began a beautiful and furious attack which quickly won the game. I recognized immediately that it was the horizon affect kicking in with a vengance and vowed right then and there I would fix this problem right away. But after looking at the game later, it became quite clear that the whole plan was quite sound! The rook was completely takeable although most of us watching laughed (except for me, my heart broke) when the computer played this "silly move." This game made me see the horizon affect problem from another perspective. In a sense there is no such thing as the horizon affect, or if you prefer it's an artificial concept we made up to explain the way we humans look at it. If this is unclear, let me state it another way. Every single move searched by a chess program (except ones ending game theoreticially) are horizon affect moves, but we only use that terminology when WE see something the computer doesn't. We never call it horizon affect when the computer gets gradually outplayed. I was fascinated by the fact that the computers extreme naivety is what caused it to win. It's like the fighter who doesn't know he's supposed to lose and so goes on to win. I've discovered over time that the computer seems to make this decision correctly most of the time. This is due to greater search depths. I remember Bob Hyatt telling me that early Blitz program had this rule and it was useful, later he felt like it hurt the program. I can believe this. So many programs do have trapped piece code in them, but I really doubt they are really integrated very well into the overall strategic decision making, they are probably more prophylactic in nature. - Don
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