Starcraft AI blog

One of the items on my list is good lurker targeting: Lurkers should aim for the target that will bring about the most total damage, with the lurker’s line of splash damage. In the current release, Steamhammer’s lurkers use the same targeting as most units, choosing the closest available target when other things are equal. It’s not so smart, but improving lurker targeting has been a low priority; the full analysis is more effort than it’s worth for the time being. Lately I hit on a simple idea to improve targeting without doing any deeper analysis. It’s a rule of thumb that I notice when I play myself, but for some reason I never thought of implementing it before.

Suppose a lurker is to choose one of 2 targets, and it knows how far away each is but nothing else. It has to choose blindly. Which target is the better choice? Does one of the targets offer a better chance of blindly landing splash damage on the other, with the lurker spines reaching out in a line? It might be counterintuitive, but geometry gives an answer: The more distant target is better. If you aim for the near target, the far target subtends a smaller angle from the point of view of the lurker. If you aim for the far target, the near target subtends a larger angle, so you are more likely to hit it by chance. If you don’t believe me, draw some circles on paper, or start up the game and try it!

The same reasoning works if there are 3 targets; pick the farthest. If targets are dense, it doesn’t matter where you shoot, and if targets are sparse and you do no further analysis, then aiming for the farthest is best. It’s dead simple, so I implemented it in Steamhammer, choosing the highest priority target at the greatest distance rather than the smallest distance. It is a small but real improvement, and it was hardly any effort. There is a disadvantage, which is that the more distant unit may be better able to sidestep the lurker spines, since it has more time to react. But not many bots dodge lurker spines.

Of course it’s much better to do the full analysis: For each target, calculate which enemies the lurker will hit and what the hit is worth. Ideally, predict how the enemy units will move, taking into account their likely decisions, and correlate their paths with the extending line the lurker spines will follow. And optimize the fire of all lurkers at a location as a group, so that they cooperate to kill more efficiently (2 lurker shots kill a marine, so these 2 lurkers should aim here and here to overlap their splash damage). It is an elaborate calculation, and I don’t think it’s worth the effort for now; better to improve Steamhammer’s other decisions first.