Author: Robert Hyatt
Date: 05:54:20 08/14/00
Go up one level in this thread
On August 14, 2000 at 00:40:06, Ed Schröder wrote: >On August 13, 2000 at 20:51:38, Robert Hyatt wrote: > >>On August 13, 2000 at 18:47:42, Alvaro Rodriguez wrote: >> >>>On August 13, 2000 at 18:20:33, Robert Hyatt wrote: >>> >>>>On August 13, 2000 at 16:21:34, Uri Blass wrote: >>>> >>>>>On August 13, 2000 at 15:12:25, Christophe Theron wrote: >>>>> >>>>>>On August 13, 2000 at 11:42:57, Mike S. wrote: >>>>>> >>>>>>>On August 13, 2000 at 10:59:22, pete wrote: >>>>>>> >>>>>>>>(...) >>>>>>>>[D]3r1rk1/2p1Rppp/p4n2/1p1b4/3P4/3B3P/PPPN2P1/4R1K1 b - - 0 1 >>>>>>> >>>>>>>It seems to me that Tiger, when playing 21...Bxa2?, cannot have expected 22.b3. >>>>>>>Maybe he expected something like 22.Rxc7 Rxd4 23.Ra1 Bd5 24.Rxa6 or similar. I >>>>>>>would be interested if Tiger "knows" this standard motif of locking up a bishop >>>>>>>after it captured a border pawn on the 2nd (7th) row. I think, in such cases the >>>>>>>lines beginnig with b3 etc. should be examined more closely than usual (?). >>>>>>> >>>>>>>Regards, >>>>>>>M.Scheidl >>>>>> >>>>>> >>>>>> >>>>>>Chess Tiger 12.0e has a partial knowledge of this "standard motif". It knows >>>>>>that the bishop is in trouble if it cannot leave a2, but the evaluation penalty >>>>>>I give in this case does not prevent it to take the pawn. >>>>>> >>>>>>That means that if another move could lead to a positional advantage, Tiger >>>>>>would play the other move. If there is no such move, Tiger will take the pawn >>>>>>with the bishop. >>>>>> >>>>>>I know it sounds a little bit strange, but I have been thinking about this >>>>>>problem for quite a while, and I have not found a good solution. For every >>>>>>example of a trapped bishop that gets lost I have seen the opposite example >>>>>>where the trapped bishop eventually escapes or completely shreds the side it has >>>>>>been trapped in, which leads to a big pawn majority and a winning endgame. >>>>> >>>>>The question is what happens in cases when you cannot find by a search of few >>>>>minutes that the bishop can escape and cannot find by a search of few minutes >>>>>that the bishop is trapped. >>>>> >>>>>I believe that in most of these cases moves like Bxa2 are wrong but I may be >>>>>wrong because I did not see a lot of examples when search cannot solve the >>>>>problem. >>>>> >>>>>Uri >>>> >>>> >>>>I prefer to be conservative here. Rather than trying (a) if the bishop isn't >>>>lost, then take the pawn, I prefer (b) if the bishop can't get off of a2 by >>>>the time the evaluation is called, then it is trapped. >>>> >>>>Works well for me, very inexpensive to test for. >>> >>>Crafty takes the pawn if the analysis shows that it can get out? So crafty takes >>>no risk.. Interesting to see what the other program does in this positions.. >>> >>>Regards, >>>Alvaro >> >> >>That is correct. It has to see taking the pawn, _and_ the bishop getting off >>of a2, within the search. Otherwise it assumes that the bishop is trapped and >>gives it a huge penalty. > >How huge is huge? > >The penalty Rebel uses is 1.25 > >Never seen the problem again when I implemented the rule 15 years ago >after losing too many games because of not understanding the danger. > >Ed > I believe mine is 1.5. for Crafty, 1.25 wasn't quite enough. And I agree, I did it in Cray Blitz, I did it in Crafty. It is nice to not have to watch and worry about that happening (of course, there is plenty else to go wrong.) :) Bob > > >>I haven't seen it fail very often, and when it did fail, the position was >>complex enough that it wasn't possible to understand it with a simple static >>eval trick anyway. >> >>The amazing thing is that I _still_ see it happening on ICC... I got tired >>of seeing crafty do that pretty quickly. I decided that sitting in a game >>and worrying about whether it will play a move that even a 1600 player would >>avoid was simply something I didn't want to do. As a result, I don't. :) >> >>A 2500 (GM-level) program simply can _not_ play such a move. If it does, and >>a GM sees it, it will lose the next N games because he will set that trap over >>and over... and the program will bite over and over.
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