Starcraft AI blog

Today’s topic of ground versus air is more complicated than yesterday’s topic of air versus air, so I don’t go into as much detail. Not that anybody was overwhelmed with details yesterday. It’s all Quite Abstract, ahem, allow me to adjust my spectacles.

Two systems of counters are designed into the game for ground units as a group versus air units as a group.

you need anti-air

The first and most basic system of counters is:

1. Not all ground units can shoot upward.
2. Those ground units that can shoot upward are more efficient than air units.

#1 means that as soon as the opponent has wraiths, your tanks need to bring goliaths along with them everywhere, or to stick to places where you can build turrets, or something. In every matchup, you have to worry about air defense. In PvZ, corsairs and not ground units are the favorite air defense, but that doesn’t change the principle.

#2 means that bringing goliaths is all you need. A few goliaths can beat a lot of wraiths, so you don’t have to give up on tanks altogether. Even if the opponent has mass carriers, goliaths are efficient and you can keep tanks. #2 also means that ground units tend to be more important than air units. The more efficient unit can overwhelm the enemy with strength; the less efficient unit will need to win with skill if it can win at all.

You may be surprised how thoroughly and carefully the units are designed so that ground has an advantage over air. 8 unupgraded marines, the same mineral cost as a battlecruiser (and much less gas cost), are about equal with a battlecruiser. 8 marines with stim can easily shoot down a battlecruiser. 2 hydralisks, the same mineral cost as a guardian, can shoot down a guardian. And so on.

air units have countervailing advantages

The second system of counters has to do with properties of air and ground units that hand air units an advantage sometimes, even though air loses head-to-head.

1. Air units can go anywhere.
2. Air units stack.

#1 means that air units can outmaneuver ground units to gain a local preponderance of force. They can make hit-and-run attacks, trying to stay a step ahead of the defenders, or to spread them thin so they can be defeated piecemeal, or to convince the enemy to spend on static defense. Ground beats air in a head-to-head fight, so air will avoid that and go for guerilla action until it gets far enough ahead to win straight up.

#2 means that air units can use mass more effectively than ground units. A group of mutalisks can stack up so that they all attack at the same time. If they are picking off marines, then rear marines may be unable to return fire—mutas will try to hit when marines are in poor formation. The same for other air units. This advantage of mass air is why each race has an air splash unit, as I talked about yesterday—the air splash unit means that the defender’s advantage remains even against mass air. Air splash at least forces mass air to scatter and lose its stacking advantage.

Also, every race has an air siege unit that can win the game if unopposed. If the map is mined out and nothing is left but a couple battlecruisers for one side, the cruisers can slowly yamato away static defense and blast down everything else. Protoss has carriers and zerg has guardians. Even when opposed, the long range attacks make cliff positions valuable.

Tomorrow: Detection. Though I haven’t thought of much to say about it yet.