u/Ascolom

This is for anyone who has seen the "arc slop" change to mutalisks and wonders what it does.

Tldr:

Mutas refuse to attack way less and feel a lot smoother against targets running past them or when attacking while gliding over units.

Short version:

1. Projectile attacks cancel if the projectiles travel direction faces to far away from where the attacker was originally facing when he initiated the attack.
2. Mutalisks had a problem, where when attacking while flying over units or fast units running under them, this rule made them cancel a lot of attacks.
3. Arc slop determines how much leeway in the angle an attack has before it is cancelled.
4. The increase in arc slop from 45->180 effectively increased the allowed angle for an attack from 135° -> 270° area around them.
5. That means the target does not have to be in front of them, but could also be sideways of the direction they were facing when ordering the attack.
6. Attacks will only cancel if the attack target is in a 90° cone behind the mutas original attack direction when the attack is fired.
7. Mutas cancel way fewer attacks and feel a lot smoother against targets running past them or when attacking while gliding over units.

Long version:

It is a mechanic for projectile attacks in SC2. That is with a visible projectile like stalkers, mutalisks, unlike hit-scan attacks, like marines, thors, etc.

When a unit attacks, the attack has several phases:

1. Ordering of the command.

2. Attack anitmation wind-up.

3. Projectile created.

4. Attack animation wind-down. (the remaining animation after the actual projectile was shot).

Attack arc:

Units can only attack enemies in front of them or have to turn to attack them. But it does not need to be perfectly aligned in front of them. Every unit has a hidden "arc" stat, that allows them to shoot in a not perfectly straight angle. For mutalisks this is 90, which means it can start attacking as soon as it has turned so the target is in a 90° degree cone in front of it.

Attack canceling:

In most games, you cancel the attack if you order another command before the projectile is created. SC2 works differently. In sc2, to allow you to already turn and move another direction smoothly while the windup is still playing.
However, there is another mechanic at play. Units can "dodge" attacks, or rather the attack gets canceled, if the target is not inside the attack arc from earlier anymore when the missile actually fires.

So what is arc slop?

Arc slop is the extra leeway the game grants before an attack is cancelled due to being out of bounds of the arc angle.
So they do not have to be in the inital targeting angle, but in a wider area. The exact calculation is nowhere 100% confirmed, but most likely this is additive.

The exact way the angle is calculated is important for the mechanics. We are measuring the angle with vertex at the attackers position at the time of the check (missile creation /after wind-up), the targets position at the same time and the direction the muta was facing at the start of the attack (before wind-up). So between the line of the units new positions and attackers old facing direction. If you are imagine the cone, imagine it attached to the attacker, moving but not changing direction with it. That means:
- An attack can be canceled, by either attacker or target moving in different directions
- it can't be canceled by either unit turning.

The muta problem:

Muta are short range and have the flyer deceleration mechanics. The result is, they are often microed to attack while gliding over enemies while attempting to keep momentum. However, the problem was:

1. Attack starts: target is inside arc cone
2. During Attack windup muta glides over the target.
3. At arc check the enemy unit is in many cases to much sideways or behind the muta - attack cancels, muta flops uselessly around.

The angle only covered only 135° or 67.5° on each side. So unless the unit was still almost in front of the muta, it would fail.

The new change:

Much larger leeway for the valid angle of unit to attack direction. arc slop 45->180. Meaning, the cone for the missile cancel check goes from 135° -> 270° (twice as big). This means the side areas are covered and unless the target is almost excalty behind the attacks original angle when the attack is supposed to fire, it will hit.