Starcraft AI blog

In 2017, I posted breaking scouting assumptions about a crazy trick terran can play: Lift off the initial command center and land it elsewhere, so that the opponent does not scout you at any starting base.

Today will go down in history, because somebody played this trick versus Steamhammer on SCHNAIL.

The game is LiberTY_ vs Steamhammer on Fighting Spirit, at 21-05-19 08:40:04 in SCHNAIL server time. Let the timestamp be forever honored in memory. Some background: Liberty (under a couple of related names) has played many practice games against Steamhammer, having some trouble at first but steadily improving and now winning nearly all games. This is someone who has learned Steamhammer’s weaknesses and has exploited them with increasing efficiency. Maybe the game was to accept a handicap and make Steamhammer a serious challenge again. Or maybe it was just a fun try to see if the bot would do something silly in the silly situation.

Liberty started in the bottom left. Here the command center is approaching to land in the bottom middle base. See the worker counts—lifting off your base interrupts mining for a long time in the critical early game, so it is a severe setback. Terran has no chance unless the trick breaks Steamhammer somehow. Steamhammer’s overlord is only halfway across the top of the map on its scouting trip, so it would have been possible to lift off later, losing less ground. A little less.

Steamhammer scouted the bottom right base last, so it believed that was Liberty’s start base. The drone scouted it thoroughly, over and over, even after finding it empty.

Wasting time with the scout drone was Steamhammer’s only real misreaction to the trick. The scout manager does not update its idea of what location needs scouting merely because the location is empty, but the information manager understands that no enemy base has been found. The initial 6 zerglings set about scouting the map for the terran buildings that must exist, checking base locations first (including bases that were already scouted). At about 5:30, a watch squad zergling arrived to keep a watch on the empty enemy “main”, and the scout drone could return home. By coincidence, at the same time, the now more numerous lings found the terran encampment and did some damage.

This was not a scouting squad, it was a combat squad, and it was only scouting because it hadn’t found a place to attack. The lings had speed and there were only 4 marines. The game could have been over right there, but Steamhammer controlled its units poorly (in part because of crummy code trying to join clusters that became separated), and terran suffered only moderately.

But zerg had made no fatal mistakes and was too far ahead. With little room on the tiny mesa for ground production, Liberty chose wraiths. The first 2 wraiths arrived at a zerg base and were scourged before they could do damage. Soon the wraiths did better with cloak, but paper airplanes were not going to stop any zerg army for long.

Steamhammer’s reaction was about as good as I expected, and easily good enough. Does your bot survive this crazy trick? Probably! The only critical skill is the ability to notice that an enemy base is not where you “knew” it was, and you need that skill in case an enemy command center lifts off or burns down, which can happen in a normal game.

The number of games on SCHNAIL exploded recently. I assume it’s related to the new SCHNAIL stream showing random replays of past games. Is there another source of publicity that I didn’t notice?

Nice job, Sonko and Bytekeeper! I especially like the viewpoint switching and the treatment of fog of war in the replay viewer. Of course, it is a universal constant that there is room for improvement. I wonder: Are new viewers able to tell the bot player from the human player? I can tell easily, but I already know all the bot names.

I learned a new trick from watching the ASL 11 games. The game was Sharp versus Best, the second game of group B in the round of 16. Watch with English commentary by Nyoken and Scan on Afreeca or on Youtube. On the Afreeca vod, the game starts a little after 55 minutes in. The Youtube vod was trimmed and it starts closer to 46 minutes.

Sharp scouted Best’s base with an SCV, which then left the base. Best blocked the entrance to his main with a zealot to prevent further scouting. But when the SCV returned for a second look, it mineral-walked through Best’s blocking zealot and got a look around anyway. I was surprised, and Nyoken (who knows more about Starcraft than I do) was also surprised: How did the SCV get vision of Best’s minerals to do the mineral walk? Sharp had no apparent way to see the minerals.

Scan explained it. The SCV was click-moved to a location far from the base (a location chosen so that the second-scout timing would come out right). As it was leaving, while it still had vision of the minerals, Sharp shift-clicked back to the minerals. That was the last time Sharp had the necessary vision. The SCV followed its orders to move far away, then followed the queued order to mineral walk back into the base. I had never seen it before.

I sometimes see bots using queued commands to move units across the map through waypoints. (In BWAPI you do that by setting the optional shiftQueueCommand parameter of a command to true; it defaults to false.) It is usually a mistake. When you queue a move command, the moving unit completes the command by coming to a stop at the destination before the next command in the queue executes. It adds a delay—a very visible delay. If you’re following waypoints, it is normally better to issue a new move command in real time before the unit reaches the waypoint, so that it never slows down or stops, and reaches its final destination sooner. But in the case of mineral walk, you may not still have vision of the minerals later when it’s time to issue that command. If you want to issue a sequence of moves ending with a mineral walk, you may want to queue them all while you still have vision. It will execute later whether you have vision then or not.

I tried it myself, and it works. What a complicated game!