Starcraft AI blog

I like Johan Hagelbäck’s writings (see this post), but one point bugged me. He kept repeating the idea that units should engage at their maximum shooting distance. Well, he talks about various kinds of unit micro that you can achieve with potential fields, and most of it is simplified from the ideal, but this one simplification bugged me.

Keeping near your maximum shooting distance is often a good idea. You’re close enough to harm the enemy, but being at the maximum range increases your options (you could run away or kite out of range during cooldown) and decreases the enemy’s (maybe they have to approach to hit you at all). If you have only one behavior, then staying near maximum shooting distance is a good one.

But look at the giant exceptions:

• If you have a group of ranged units, you want as many as possible to fire. If there are too many to fit into a single arc (you have a large number, or you’re fighting in a narrow space), then some of them must come closer. If you’re going to stand and fight, then bring the most possible firepower to bear. When your reinforcements join the battle, they have to be able to reach the front line—it’s a kind of coordination and hard to implement, but it’s critical to good micro. The front line can make gaps to allow reinforcements through (a skill also useful for reaver scarabs), or can move forward to let reinforcements into range.

• Sieged tanks can shoot farther than they can see, so you have to consider vision range too.

• Sieged tanks have a minimum range, so if you’re attacking them you may want to snuggle up. Also, if the enemy has sieged tanks on the field, any enemy unit you snuggle up to is at risk of taking splash damage from its own side. Few bots seem to know.

• If you’re blasting down static defense (cannons, say) with siege tanks, don’t siege just inside tank range like most bots, siege just outside cannon range. You’ll probably be able to destroy more cannons before you have to unsiege and move, so you’ll clear the cannons faster and be more likely to win overall (carriers may be coming, yes?). This advice will usually be right when the enemy front line is static defense, but might be wrong if the enemy has other units in wait. Humans commonly siege tanks at different ranges, partly to save micro but also because it’s more robust against surprises from different directions.

• If you have ranged units and there’s an obstacle between the armies, then your only reason to keep your distance is so that fewer enemies can shoot at you. Dragoons on a cliff can take potshots all day at zerglings down below, and the same for guardians uphill from marines.

• If you’re going to win the battle, then staying at maximum shooting distance gives the enemy better chances to escape. If you can’t surround, then keep close so you can pick off more escapees.

In full generality, the distance at which one unit should seek to engage another (given that you want to) depends on the range, damage, cooldown, speed, and acceleration of both units, plus the terrain, with consideration of special abilities (psionic storm, mine laying, medic healing...), and if you want to be utterly thorough, the full game situation too. For most unit pairs the basic calculation is easy enough. For example, if you have more speed and more range (speed vulture versus slow zealot, ranged dragoon versus unupgraded marine), who cares how close you get, as long as you don’t get hurt?

For army on army, the formation and distance you want depend on the sizes and unit mixes of both armies, plus the terrain, plus the game situation. The maximum shooting distance is a good base to start at, but you’ll need to expand beyond to win.