Starcraft AI blog

I made an improvement to overlord production in Steamhammer, and it reminded me: Making overlords is a difficult skill. Supply is tricky for terran and protoss too, but zerg overlords are trickier because overlords have more uses and are more vulnerable.

When to make overlords. If you know ahead of time what units you will produce when, you can plan overlords at exactly the right times. Of course, you usually have only a general idea. If you scout that the enemy is getting mutalisks much earlier than you, then you have to make spores and that is that; an overlord in the egg will turn out to be unnecessary.

The tactical situation matters: Are corsairs out? Are you planning a doom drop? If you’re at risk of losing overlords, you may want extras to avoid being supply blocked. Ideally, bots should weigh the risk of supply blockage (which depends on events in the game) against the cost of extra overlords. I don’t know whether any bots do that; Steamhammer certainly doesn’t. When CherryPi loses overlords and gets supply blocked, it likes to hugely overproduce overlords and try to stay ahead by sheer volume. I don’t think that’s good play, but it seems at least as successful as Steamhammer’s method of producing the minimum number of overlords.

Conversely, if you find yourself ahead in supply, you may be able to take bigger risks with your overlords, at least for a while. Sacrifice one to scout and you won’t miss it.

The strategic situation also weighs in on overlord production. If you get supply blocked, instead of producing overlords, you could start a fight to reduce your army size. You want to compare the cost of the fight versus the cost of the overlords. Even if you’re not supply blocked, the cost of buying supply to feed your growing army can provide some impetus to attack. If you suddenly lose a lot of army in battle, you may even want to cancel overlords in the egg.

The result of all the complications is that bots have to rely on heuristics or approximations. Most bots do well enough with overlord production despite living by simple rules of thumb. But the stronger the bot, the more the fine details matter.

Where to make overlords. Since overlords have uses, it also matters where you produce them, especially before overlord speed is researched. In the early game, strong players plan ahead where to place and how to maneuver overlords to see as much as possible of key areas. Where each overlord is produced figures into the plan. Later, bases and armies may need overlord coverage to spot dark templar or to gain vision of high ground. New overlords should be made near where they will be needed—unless other units are higher priority. If corsairs are marauding, most overlords may have to be produced near air defenses (lacking this skill hurts Steamhammer).

Overlords are a tiny aspect of the game. It’s dizzying how complicated it all is.