Author: martin fierz
Date: 03:59:37 02/26/04
Go up one level in this thread
On February 25, 2004 at 12:30:38, Robert Hyatt wrote: >On February 25, 2004 at 12:09:16, Daniel Clausen wrote: > >>On February 25, 2004 at 10:52:27, Robert Hyatt wrote: >> >>>On February 25, 2004 at 05:56:16, martin fierz wrote: >> >>[snip] >> >>>>i don't know whether i should believe the eval discontinuity thing. i know >>>>somebody recently quoted a paper on this, but it's just a fact: exchanging any >>>>pieces necessarily changes the evaluation. sometimes not by very much. big >>>>changes are usually the exchange of the queen, the exchange of the last rook, >>>>the exchange of the last piece. these eval discontinuities are *real*. i don't >>>>believe in smoothing them out. perhaps if you write an eval with >>>>discontinuities it's harder to get it right that everything fits in with each >>>>other, and that's why it's supposed to be bad?! >>> >>>No. When you have a discontinuity, you give the search something to play with, >>>and it can choose when to pass over the discontinuity, sometimes with >>>devastating results.. >> >>The arguments of you two could be combined to this: >> >> Eval discontinuities are _real_ but it hurts the search too much and >> therefore it's better to be a tad less realistic in eval here in order >> to get maximum performance out of the search+eval. >> >> >>Does that make any sense? >> >>Sargon > > >That is not quite the issue. Consider the following X-Y plot of your >eval function (Y axis) against some positional component (X-axis): > > | > | > | > | * > E |* * * * * > V | * * > A | * * > L | > | > | > | > | > | > | > | * * * * * * * * * * * * * * * * * * * * * * * > |_________________________________________________________________ > some feature you are evaluating > >Notice the sudden drop to zero. If you start off in a position where the score >is non-zero for this term, and you can search deep enough to drive over the >"cliff" for this term and hit zero, strange things happen. The search can use >this as a horizon-effect solution to some problem. And it will be able to use >that sudden drop (when something goes too far) as opposed to the big bonus just >before it goes too far, to manipulate the score, the path, the best move, and >possibly the outcome of the game. > >This is what Berliner's paper was about. I suspect that anybody that has worked >on a chess engine for any length of time has run across this problem and had to >solve it by smoothing that sudden drop so that there is no "edge condition" that >the search can use to screw things up. another reason for not believing this stuff: your above graph shows *exactly* what happens when you go from a non EGTB position to an EGTB position (or, for that matter, what happens when you go into any position your program can recognize as a draw whether it has tablebases or not): your eval thinks it's doing great, but the exchange of something leads to a drawn position in your tablebases. are you going to claim that crafty plays better without TBs? :-) cheers martin
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