Starcraft AI blog

Mutalisk bounce attacks are complicated.

In lore, the mutalisk attacks with a glaive wurm. A glaive is a sword. BWAPI spells it “glave”, which is a less common alternate spelling. Anyway, the attack does 9 normal damage directly. If an enemy unit is close enough, it bounces and hits the enemy, doing 1/3 as much damage, 3. If a second enemy is close enough, it bounces a second time and does 1/3 as much damage again, 1.

upgrades

If you upgrade mutalisk attack +1, the bounces are upgraded too. The first attack is 9+1=10; the second attack is 10/3 = 3.33 (rounding to 2 digits); the second attack is 10/9 = 1.11. The total gain is 1.44; for +2 upgrade, 2.88; for +3 upgrade, 4.33.

Brood War does track fractional damage behind the scenes, though you can’t see it in the user interface. There is also a hard floor; every attack, even one that is smaller than the target’s armor, does at least 0.5 hp of damage. At least that’s my understanding of how it works.

If you’re under attack by mutalisks, +1 armor applies to all 3 bounces, so it prevents up to 3 points of damage per shot.

For ZvZ air battles, zerg players disagree on whether it is better to get +1 armor or +1 attack for mutalisks. +1 armor is more popular; it costs more (150/150 instead of 100/100) but prevents more damage (3) than +1 attack causes (1.44), and it also protects scourge and overlords. Nevertheless, some strong players prefer to get +1 attack. The best choice can depend on the game situation—for one thing, upgrades don’t use up a larva; for another, scourge have 25 hit points and are vulnerable to bounces.

you don’t have to attack an enemy

It doesn’t matter what the mutalisk attacks, the glaive wurm will bounce to an enemy if one is close enough. You can attack a harmless enemy building, or a neutral building or neutral critter, or your own unit or building. Mutalisks have a short range of only 3 tiles. By bouncing from a safe object, they can do some damage to enemies while keeping at longer range. In the best case, the bounces can hit marines while the mutalisks are out of range of marine fire.

Human players do this when a good opportunity presents itself. I’ve never seen a bot do it.

which way does it bounce?

This TeamLiquid translation from Korean gives 2 rules for how glaive wurms bounce. You can have some foreknowledge of which enemy each bounce will hit.

Rule 1: It bounces to the left if possible; otherwise, it bounces down if possible. If it can’t bounce left or down, it will go another direction. This rule applies no matter what direction the mutalisk attacks from. It means that separated mutalisks which attack the same target at the same time will get the same bounce pattern, which is good for the attacker because it focuses fire. It also means that the defender can arrange units to try to minimize the bounce harm, for example by putting healthier units or units with higher armor on the left of the formation.

Rule 2: If you attack from directly to the left, it bounces the minimum possible distance, hitting the closest enemy. If you attack from directly to the right, it bounces as far as it can. It can make a difference when fighting marines, if the mutalisks are numerous enough that the bounce can kill a marine. In that case, other things being equal, it’s better to attack from the left and kill a close marine that is in range to shoot back, rather than from the right and kill a distant marine that may be idle. You’ll escape taking that much less fire.

These rules aren’t enough to tell exactly which unit the bounces will hit. It’s coded into OpenBW, so there should be people who know....