Author: Uri Blass
Date: 04:47:36 02/26/04
Go up one level in this thread
On February 26, 2004 at 07:38:39, Sune Fischer wrote: >On February 26, 2004 at 07:11:24, Uri Blass wrote: > >>On February 26, 2004 at 06:59:37, martin fierz wrote: >> >>>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 >> >>I do not know about crafty but it is certainly possible that a program does >>better without tablebases. >> >>Without tablebases it can capture a pawn and get KR vs KPP that is drawn when >>with tablebases it may prefer to get an endgame when KR is losing against KPPP. >> >>Uri > >This is only possible with draw scores, right? > >I mean you can't be on the wrong side of a mate score, so TB mates should be >safe despite the discontinuity. > >-S. correct for mate in less than 51 moves(otherwise the mate may turn out to be a draw). Uri
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