How is control of board determined?

I recently played a game (as black) where I thought I had a corner under control:

I’m speaking of the group of stones I have (as black) that include G1, at the end of the game. The computer scored them for him.

Do these belong to my opponent because he could in theory take them with optimal (or perhaps even suboptimal?) play? I am very curious about the algorithm used to determine territory at the end, as I find myself confused by it fairly often.

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That’s not a question of control, but simply of Life&Death.
Look at the black group of question and you’ll see that it has one big eye. (It’s called bulky five if you want to look it up.)

Question: Can White play such moves that the black group will be ultimately captured?
Answer: Yes, simply by playing J2; because Black won’t be able to create two eyes after this move and is surrounded by a group which has even more than two eyes.

Can’t Black do anything to make his group alive? The answer is again J2. It’s the so called “vital point”.
One of you should have played it.

Stones are said to be "alive" if they cannot be captured by the opponent, or if capturing them would enable a new stone to be played that the opponent could not capture. Stones which are not alive are said to be "dead." 

—japanese rules 

So, yes. White could capture with J2, Black needs to play J2 to make its stones alive.


That is very helpful thanks. So if we both passed, and either of us could have taken it with J2, then they are considered dead.

I am still wondering about the algorithm used to decide this. Is there a simple set of patterns that it is able to recognize (e.g., bulky 5)? Or does it actually explore the possible game trees and calculate the potential outcomes?

The simple answer is this:

Eyes make life.

In AGA (and BGA and FGA) rules, they distance themselves from the esoteric bullshit that is normally called “Japanese rules.”

Under AGA, if you think you can take it, you have to prove it by actually doing it.

This is, in part, because, culturally, we cannot and will not give out jobs for people to be judges for every single game. The players need to fight it out until one side comes out on top.

In regards to your game, the lower right needs to finish. Whoever gets j2 first will take control of the area.

Bulky or not, black’s move into the lower right is about reduction of white, not territory for black.

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This sounds reasonable, and is also very helpful. Still I wonder what is the actual algorithm the program uses when the players don’t fight it out 'til the end?

If it was automatically marked dead, which I doubt, then it was because it could not find an eye.

Computer do not recognize patterns nearly as well as people. That’s why we have a counting phase here (and all other go servers.) The system gives a best possible guess but it’s up to the players to define what is living in what is dead.

That’s why both players must agree about what they see before concluding the match.

When you don’t complete securing or killing stones, you get situations like yours.

Black should have won the game. I’m not sure either of you would benefit from having it reversed.

You came looking for clarification. That is good. White, however, might steal a game from somebody else unintentionally.

Thanks I need to build up experience with life/death determination…I will attempt to just play it out next time…

You probably already know this, but just for the record, and since this has not been explicitly mentioned, you can click on groups to change their status from alive to dead. The computer makes mistake once in a while (even if this was not one). So unless it is because you are interested in algorithms in general, you don’t need to worry about how the computer determines life&death (I don’t know either). If the computer is wrong you just click the group.

Now, if you and your opponent disagree you can resume the game (there is a button for this) and play it out. This is how you are supposed to handle it under AGA, Chinese, and most other rule sets. As @SunPin mentions, Japanese (and Korean) rules are strange in this regard. You can play it out, but the act of playing it out might change the outcome of the game. The correct way to handle the situation under these rules is by “agreement and confirmation” according to Japanese rules. I guess this means you just argue until you reach agreement.

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