Starcraft AI blog

An enemy lurker or dark templar is in your base, and you have absolutely no form of detection nearby. Can you defeat it anyway? In many cases, yes.

An enemy unit can be in sight range but undetected if it is burrowed—a spider mine or a burrowed zerg unit—or if it is cloaked—a wraith, ghost, dark templar, observer, or a unit under an arbiter’s cloaking field. I’ve written before about countering spider mines, so I’ll skip that.

burrow versus cloak

When a cloaked unit is in sight, a bot knows where it is. Without detection the cloaked unit cannot be targeted, but BWAPI gives its type and position.

A burrowed unit does not give itself away. If the bot did not see the enemy burrow, and did not detect it since it burrowed, then BWAPI does not reveal that it is there. Exception: A lurker can give away its position when it attacks. The lurker spines are a bullet, in BWAPI parlance. However, if you try to build on top of a burrowed unit, construction will fail. Many bots use that fact to infer that a spider mine is blocking an expansion position, but the principle is general. In theory, you could locate burrowed zerglings by starting buildings in different positions. It’s rare, but I have seen a game where hold lurkers waiting in ambush in an expansion were discovered by trying to build a turret before any mining SCVs arrived.

spider mines

Spider mines will attack cloaked enemies. They are popular for fending off dark templar.

splash damage

An undetected unit can’t be targeted, but it can be hit by splash damage. Psionic storm is the simplest way; burrowed and cloaked units are affected like any other. Nuke may be overkill, but is effective.

An example: For protoss, the most common way to deal with a single undetected lurker is to park a friendly zealot on top of the lurker and attack the zealot with an archon. The zealot will remain stoic as it takes full damage, and the lurker will take splash damage. You may need 2 zealots to finish off the lurker. Skynet by Andrew Smith has this skill, and I wrote it up in 2016.

Any unit or spell that does splash damage will work—well, it has to do ground splash for ground enemies or air splash for air enemies (archons do both ground and air splash). Besides archons, popular choices are reavers, lurkers, and sieged tanks. Corsairs and valkyries are good for air splash. Firebats and infested terrans can theoretically work, though I’ve never seen it done. You can also do the eraser: Irradiate a unit (preferably an air unit) and keep it next to a dark templar or ghost—but then, if you have a science vessel, you have a detector, so the eraser is for when you have no other forces around. Spider mines do not attack friendlies and are the only splash damage unit that cannot be used, but they do attack enemies so you don’t need to. Mutalisk bounce does not count as splash; glaive wurms do not bounce to undetected enemies. Also devourer acid spores do not splash onto undetected air units—they will reveal a cloaked unit that they are attached to, but they can only attach to a unit that is detected.

Also, any target that will splash onto the undetected enemy is good—anything directly next to the undetected enemy. Sometimes you can target a building or an enemy unit.

revealing cloaked units

The queen’s ensnare and the defiler’s plague are area effect spells that work on cloaked units, though not on burrowed units. Besides affecting cloaked units, the green or red goo reveals cloaked units so that they can be targeted.

EMP

Wraiths and ghosts need energy to cloak. Hit them with EMP, and they are forced to become visible. Of course, the science vessel can also directly detect them. EMP might be worth it if the vessel is at risk (perhaps from the wraiths, or perhaps from ghost lockdown), or if one EMP can hit a group of enemies which will then have to build up energy to cloak again.

drilling cloaked units

Sometimes you only need to gain a little time before you can deal with the cloaked enemy. You can delay the enemy if you can force one of your units to overlap with it; both units will move randomly, unable to act, until they separate. Zerg can do this by unburrowing from under the unit, or by turning a larva into an egg when the unit is very close to the larva. All races can do it with a worker drill: Send workers to a mineral patch so that their path will take them through the position of the enemy—mineral walk through the enemy—then stop or attack when they are overlapping.

Any other techniques? I won’t be surprised if I forgot a few.