Starcraft AI blog

I wasn’t planning to implement the larva trick now, because it’s not important. But after testing it for yesterday’s post and finding it easy, the extra work to complete the feature took only minutes, so I went ahead.

Steamhammer determines “the direction to the minerals” from a base’s hatchery by averaging together the offsets of the mineral patches from the center of the hatchery, giving the location of the average mineral patch. (There was already code to do this for another reason, I only factored it out, and it’s barely a few lines anyway.) Once every 20 frames, starting on frame 1 (not 0), it sends Stop indiscriminately to every larva at a base hatchery whose minerals are to the left. It desists after an early game time limit, or when the spawning pool finishes; almost all benefit should be in the early game, and zerglings may be slowed down by starting at the edge of the mineral line.

There are weaknesses to the implementation. When morphing a drone, Steamhammer does not select the larva closest to the minerals, so it often chooses a larva that is still in the process of moving left. Judging by eye, I think this weakness eliminates a third to half of the potential benefit. It’s possible that the indiscriminate Stop commands could sometimes prevent a drone from morphing on time; I didn’t check, but if so it should be rare. Probably a little extra benefit could be captured by doing it all game long, but more smarts would be needed.

The benefit is tiny, as I mentioned yesterday. I tested by playing Steamhammer versus itself with the same opening on Heartbreak Ridge, so that the left base uses the larva trick while the right base does not. In Steamhammer’s implementation, the edge gained by moving drones to the left, so that they begin to mine minerals earlier, is smaller than the natural variation between runs caused by the game’s random differences in timing and position. The technique is worth something, but so little that it was hardly worth implementing, even though it was easy.

An ideal implementation would be better. If you know what units you want to produce in the near future, and what their jobs will be, you can select the larvas of left-facing hatcheries that will become local mining drones and use the larva trick only on them, and do it in time to get them moved into position before you morph them. But that is no longer a simple job, and the benefit will still be small.

The other major use of the larva trick is to control egg blockades. If you have a wall consisting of a hatchery on the right and terrain or another building on the left, and zealots come knocking, you can move a larva to the left of the hatchery and morph it into an egg that blocks the gap. See the game Shark vs Reach (2008), explained on this TeamLiquid thread (it’s an example game on Liquipedia's larva trick article). I’m also fond of a famous 2x2 game from 2006—it may be from before the larva trick was discovered, but the video segment is short and worth seeing.

A larva follows a predictable path after it is tricked. If it is next to the upper half of the hatchery, it takes a route around the top; if in the lower half, around the bottom. The larva moves smoothly, so you can predict where it will be when. With good timing, you can morph an egg where you want, for example to push a unit away from your hatchery to gain time.