Author: Dieter Buerssner
Date: 10:59:02 09/16/02
Go up one level in this thread
On September 15, 2002 at 20:25:46, martin fierz wrote:
>> 8/8/4K1p1/4PpB1/3p1P1P/5n2/4k3/8 b - -
>
>hi dieter,
>
>funny position! the reason your engine fails here is simple: computer eval
>functions are incredibly dumb when it comes to winning positions. you count
>material, which normally is good, but when it comes to winning a winning
>position, this is just a bad idea.
>when a human master is winning, his only remaining aim is to get into a simple
>technical win. computers continue to count material. their eval functions prefer
>a pawn endgame a pawn down to a rook ending two pawns down, even if the side
>with two pawns less has some counterplay. human masters don't count the
>material, instead, they think about winning probability. how likely is it that i
>will win a pawn ending with a clear extra pawn? very close to 100%. how likely
>is it that i win a rook ending with two extra pawns against an active rook?
>likely, but not as close to 100%.
I agree, with almost everything. However, I think, that typically the method of
the chess engine works as well. Indeed, I am partitially using ideas you
mention, but your input may motivate to do this better.
>here's where yace goes wrong in this position: it looks at the winning
>continuation d3 Kf7 d2 e6 d1Q e7 Qd7 Kf8 Nd4 e8Q+ Ne6+ Qxe6+ Qxe6.
I think, in this position it still is a bit more complicated, because of Bf6
instead of Kf8, to make Nd4 impossible. I think the mainline goes like this:
d3 Kf7 d2 e6 d1Q e7 Qd7 Bf6 Ke3 Kf8 Kxf4 e8Q Dxe8 Kxe8 and win the h4 pawn (for
ex. Kg4 Kf7 Nxh4). Now black has knight +2P vs. B, and will win. I am not
totally sure, that it is the only winning path. But what chess engines may do:
Don't move Kxf4 fast and do some useless Q moves. Then, an engine that counts
the first repetion as draw, will not move back to d7 with the Q again (there
seems to be no way, to get the e7 pawn, without saccing the Q). So this
basically wins a P less. Black must hope, that he can win with one pawn more
(KQNPPKQBP or KNPPKBP). Lines I tried, yielded in draw. BTW. In very fast game,
an engine may even spoil this to a loss - it may at an inconvenient point see,
that a repetion will arise with "normal" means, and sac the N ("We'll, I still
have a Q vs. B"). Done "clever" enough, and black may win.
>i'm sure it
>sees this line very quickly - it's only 9ply + a few checks/captures which you
>probably extend. now it counts: Q-B: +6. note that white has zero compensation
>here - not even a passed pawn anywhere, no possibility to attack any black
>piece, this is a dead loss.
>then it looks at it's game continuation, and gets into that drawn ending, and it
>counts: Q-P: +8. perhaps, you give the pawn a bonus of 1, because it's on the
>7th rank. +7. and you play Nxg5??... of course, you are "unlucky" not to win
>this: a queen against an e-pawn alone would win easily, because of Qf6+ at some
>point, but you don't have this thanks to white's "lucky pawn" on g5. so how do
>you fix this?
>when you are *totally* winning, say +5 or more, check if the losing side has
>some form of compensation. like a passed pawn. or worse, a passed pawn on the
>7th rank. then give that a *big* score - much bigger than usual. if you give a
>passed pawn on the 7th a +3, yace will solve this position instantly... i don't
>think there are any harmful side-effects, but maybe i'm overlooking something.
So, with the line above, probably +3 for the pawn on 7th will not be enough. But
your idea is of course valid and very interesting. So, leaving other factors
out: Yace evaluates the KNPPKB fast slightly lower than +3. The material balance
in the SxB lines is +8. I think, it would be risky, to give a positional score
of over 5 for a passed pawn (when you have no other reasons). It may well make
the engine draw in other positions, that would have been won.
Another thing came to my mind. I was watching that game, and was chatting with
the operator. At some point (slightly earlier in the game) I said more or less
"now f4 is dead and an easy win". Why didn't Yace do this? I extend
significantly for trading into pawn endgames. So, this may have "helped" to see
fast, that the d-pawn will promote fast. Of course, without trading into pawn
endgames, this would have seen too, but with relatively less depth behind it.
Now, this was in a phase of the game, where the score was still rising - so the
deeper line going through the pawn endgame, would have gained more score due to
the deeper depth.
I hope my analyisis is correct. If yes, I wonder, how fast club level chess
players see all this. Is there another path to win (Besides delaying Kxf4 with
some Q moves)? Can black take f4 with Q somewhere different as d7?
Thanks for your interesting thoughts.
Dieter
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