Starcraft AI blog

A zerg creep colony has a base health of 400 hp. A zerg sunken colony has 300 hp. The way it works is, just when a creep colony finishes morphing into a sunken, it suddenly loses 100 hp. If it falls below 0 it doesn’t die, though; there is a floor of 1, and zerg healing has instant effect so the sunken colony completes with 2 hp (I think it takes 1 frame for the second hit point to be granted). When you start morphing a sunken, the UnitType immediately changes from creep colony to sunken, but the hit points are not removed until the morph completes.

Good players exploit this. I don’t know of any bot that takes advantage. Steamhammer certainly doesn’t. It could be hard to notice in a bot’s play, though. Does anybody know of one?

If you’re attacking morphing sunkens, then you should avoid doing unnecessary damage. If a morphing sunken will heal to no more than 100 hit points before it completes, and your units will still be in range then, and there is something else in the area worth attacking, then you should probably switch to attacking the something else. You will normally be able to kill the brand new sunken with one hit before it can get a shot off, saving the time it would have taken to inflict nearly 100 hp of damage. Depending on the situation, you may also be able to stop early if for some reason you’re pre-emptively hitting creep colonies before they morph. (I think most bots don’t waste their time on that, though.)

If you’re considering morphing a sunken, you may want to hold off if the creep colony has less than 100 hp left, or any low number that would leave the sunken helplessly weak. That may be best if an enemy attack is near; don’t waste 50 minerals on a defense building that will die instantly, get a pair of zerglings instead (if you have a larva and supply). On the other hand, if you don’t expect enemy action for a long time, you may want to morph the sunken immediately. By losing less than 100 hp in the morph, you are effectively speeding up zerg healing compared to the case of waiting to morph the sunken later.

It’s a strange behavior, and the considerations it induces are strangely complicated. You can only make the best choices if you have skill in seeing the unclear future. Taking all considerations into account is too hard, but I think the top bots have become strong enough that they could gain from taking into account the basic considerations.

Spore colonies have 400 hp and don’t show the same strange behavior.