Starcraft AI blog

I got a suggestion by e-mail to theorize about ground, air, and detection. It’s a vague request, but I take it to be about the systems of counters designed into the game, many of which are arranged rock-scissors-paper style: valkyries counter wraiths, battlecruisers counter valkyries, wraiths counter battlecruisers. So, in hope that this will be of use to some people....

Instead of facing the question head on, I’ll start with air as an example. The design of the air combat units in Brood War is more straightforward than the design of the ground units. The terran and protoss designs are closely similar in outline:

I’m leaving out spell units like science vessels and spells like wraith cloaking. Not to mention all the details that I’m not mentioning.

Each has a ground harass unit with a strong anti-air attack, a splash anti-air unit to counter mass air, and a capital ship that is strong all-around and resistant to direct splash attacks. The difference is that for terrans, the ground harass wraith is cheap and the splash valkyrie is expensive, while protoss has it the other way around, a cheap corsair and an expensive scout. Corsairs counter scouts, scouts counter carriers, carriers counter corsairs—at least notionally. A carrier has base armor 4, so a corsair with attack 5 can barely scratch its hull, but corsairs counter interceptors, which can be as good. Head to head, 1 carrier can easily defeat 4 corsairs which cost more than the carrier and its interceptors. With enough corsairs and a situation that allows them to stand off and attack interceptors, the costs go the other way. The moral is, the outline is only an outline and details count.

“Corsairs counter scouts” is another simplification. Air units stack to attack together (and there’s a limit to how widely they can scatter and still act as a fleet), and air splash damage also stacks. So the anti-air splash units become more cost effective in relation to the enemy as the enemy fleet gets larger. The larger the fleet the harder it can be countered, so the net effect on strategy in terran and protoss matchups is to make ground combat more important, at least until late game when capital ships come out.

The zerg design is different—

—because the strengths and weaknesses are split up differently and have some twists. As suicide units with low hit points, scourge are risky and difficult to use in large numbers, but they’re efficient when they hit. Devourers don’t do splash damage directly but by splashing acid spores, so that devourers become support units—that’s good game design, it means players have to coordinate units. Mutalisks are the only air unit of any race that is both cheap and versatile, and they’re faster than battlecruisers or carriers, but they are countered by scourge and by devourers plus anything that can attack air, and they don’t directly counter any air unit of any race except guardians.

Consider the timings of the anti-air splash units. Corsairs fly early, so terran and zerg should be reluctant to commit to large air fleets vP until late game. Valkyries are later and more expensive, so zerg can harass early with mutalisks vT; scouts are too expensive to be harassment units. Devourers fly late and zerg needs overlords, so opponents vZ can go air if they choose. That and the high mobility of air units is why ZvZ is usually a mutalisk-scourge battle until late game (which is not often reached).

The cross-race unit counters are understandable in outline from these tables plus a little background knowledge: Battlecruisers can one-shot scourge, so scourge counters carriers harder than cruisers. Devourers splash in a way that counters capital ships with their heavy armor, unlike valkyries and corsairs. That’s most of the greater-than/less-than knowledge of counters needed for air-to-air fights. It doesn’t say how much greater than; to code it, you need numbers. Knowledge of counters is about knowing which units to build and which units to send; combat itself ain’t so simple.

The idea of today’s post is to set a point of view. It’s like setting a nail. Now when I get down to it you should know what I’m hammering on.

Tomorrow: Air versus ground, in general terms.