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what is "1/2 point"? [#559]

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jonathan: what is "1/2 point"? (2006-07-27 00:41) [#1958]

jfc: you may want to point out that the 1/2 point you get for taking the button is an area scoring 1/2 point i.e. the button taker gets 1/2 added to her score while her opponent gets 1/2 point deducted from his score.

I think some folks who come from a territory scoring background forget the area scoring rule that the total score must add up to 361 (or what ever the board size is) and consequently +x points for one side is implicitly ''-x points" for the other side.

jfc: It looks to me as if button scoring is just new name for Ikeda's [ext] Last Competitive Play rule.

Bill: Re: what is "1/2 point"? (2006-07-27 04:27) [#1959]

Actually, the scores normally add to 361.5 with button go.

The rationale is different, but Ikeda's rule is, in a sense, a form of button go. By adding 1/2 point to White while subtracting 1/2 point from Black, White gains 1 point in terms of the net result.

As games, the button looks like this:

        /      \
      0.5     -0.5

Ikeda's "button" looks like this:

       /     \
      0      -1

The button is neutral, with a mean score of 0. Ikeda's "button" favors White by 1/2 point. It's a question of adjusting the komi to make them equivalent.

Two points:

1) Taking the button removes ko or superko bans, which Ikeda's pass, I think, does not.

2) Ikeda focused on the "last competitive move", which he did not have an adequate definition for. He said, "If we can only give a complete definition of the last competitive move, we can claim to have a set of rules that is optimal in both theory and practice."

Button go avoids that difficulty.

jonathan: Re: what is "1/2 point"? (2006-07-27 05:30) [#1960]

The button is neutral, with a mean score of 0. Ikeda's "button" favors White by 1/2 point. It's a question of adjusting the komi to make them equivalent.

jfc: this claim is just hand waving. Ikeda's button does not favor white. What it does is guarantee that black and white receive the same number of points for dame regardless of whether or not black gets to play one more stone than white (i.e. fill the last dame).

Your button {0.5 | -0.5} and Ikeda's button {0 | -1} give the same result. Whether or not taking the button removes the ko ban is orthogonal to the value of the button.

Thanks for the clarification that your button results in the sum of the player's score equaling 361.5. That was not clear.

I prefer Ikeda's button because I like the score to be exactly equal to the number of spaces on the board. Of course arguing about this is like arguing whether current flows from negative to positive or vice versa.

Bill: Re: what is "1/2 point"? (2006-07-27 06:55) [#1962]

Ikeda's button does favor White. Suppose that we use the +/- 0.5 point button, and also subtract 0.5 point from Black. Then the result is like that of Ikeda's button.

The main problem with Ikeda's rule is, as he says, that of defining the last competitive play.

jonathan: Great Button Debate of 2006 (2006-07-27 20:31) [#1966]

I don't see the problem of defining the last competitive play.


  • you choose to fill a dame (worth 2pts)
  • you choose to be the first player to pass (worth 1 pt)

Heck, black could chose to pass as his 3rd move (i.e. take the ikeda button) or even his first move but I expect a skilled player will only take the button when doing so is the largest move -- when the only plays of value left are dame and there are an even number of those making them miai.

Ignoring the last move issue above and points in seki for a moment ...

The formula for finding equivalent komis between traditional territory and area scoring is simply:

:: area_komi * 2 == territory komi

So, if the Korean's are using a 6.5 komi and we want to count the same game using area rules we use a 3.25 komi (black -3.25, white + 3.25). Ikeda's button does not change this komi equivalence formula.

Does Bill's Button require a different komi equivalence formula? If so, what is it?

returning to the Great Button Debate of 2006, given:

  1. Territory scoring (the most sensible set you know of. Japanese 89?)
  2. Area scoring + Ikeda's button {0 | -1}
  3. Area scoring + Bill's button {+0.5 | -0.5}

Can you construct a position in which Bill's button gives the same result as territory scoring but Ikeda's button gives a different result?

I claim no such position exists i.e. that Ikeda's button does just as good a job of matching territory scoring results as does Bill's button. If you show me one I will happily ingest some Corvidae Corvus.

Bill: Re: Great Button Debate of 2006 (2006-07-28 16:50) [#1970]

Ikeda gives a position where it may not be clear, even to strong amateur dan players, whether a protective play is required or not. If it is, then it can be the last competitive play. If it is not required, then it is not a competitive play.

Or suppose that player A takes a ko at the end of the game, and player B marks time by filling territory. (He could also pass.) Next, the player A wins the ko and player B makes another play. What was the last competitive play?

Or, more simply, player A makes a play, player B passes, player A makes a play, player B makes a play, both pass. Who made the last competitive play? For instance, player A might have filled a dame, and then, after player B's pass, played a sente. Both filling the dame and answering the sente gain something. But we really want filling the dame to be the "last competitive play". (See Pass Fight.)

I do not recall Ikeda's discussion of the question, but button go avoids it entirely. :-)

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