Aggro

Aggro is an invisible statistic that determines the distance at which enemys will begin to attack the player. Aggro is short for "aggression," and is a common mechanic in video games. Terraria features various items that can alter this statistic when equipped.

All enemies target the player based on criteria set in their AI. Most will constantly try for a target, while others may only target if the player is in water, etc.

Currently, the Stardust Guardian is the only summon able to divert an enemy's target away from the player. It overrides the player's presence, but only if it is closer than 200 pixels from said enemy. Otherwise, an enemy may target the player or the Guardian &mdash; whichever is closer. It is best to summon the Stardust Guardian closer to hordes of enemies, rather than a small few, to avoid being targeted when a few enemies disperse or die.

Armor and other items that "increases/decreases enemy targeting" make enemies "think" a player is closer or farther away than they actually are. So if you have a Stardust Guardian close to an enemy, but have accessories that increase your aggro, then you could be considered "closer" than the Stardust Guardian is, and you may temporarily be targeted instead of the Guardian.

If the Eternia Crystal is present, the Old One's Army will target it by default, only targeting the player or Guardian if they get "in the way".

Decrease
These items reduce the player's "aggro" statistic, meaning that NPCs will have to get closer to target the player.


 * Aggro reductions of -750 (or more) nearly guarantee the player will not be targeted.
 * Vortex armor provides the largest aggro reduction available in the game.

Increase
These items increase the player's aggro statistic, meaning that NPCs can be farther from the player and still target them.

Formula
Most NPCs will only attack the target they are closest to. They will determine the player's distance first, then the Stardust Guardian's (if applicable) and finally the distance to the (if applicable). The distance for the player formula is abs(target player.x + 10 - npc.x + (npc.width / 2)) + abs(target player.y + 21 - npc.y + (npc.height / 2)) - target player.aggro The result from that formula is then compared to the guardians formula result: abs(guardian.x + 25 - npc.x + (npc.width / 2)) + abs(guardian.y + 40 - npc.y + (npc.height / 2)) - 200 Whichever results in the higher value is targeted.

Old One's Army formula
Enemies in the Old One's Army follow a separate formula due to having to attack the player, as well as the crystal. An NPC will first determine the distance between the Eternia Crystal and it with a simple distance formula. Then, barring the chance the enemy is currently "ghosted", the NPC determines the "distance" to the player with the following formula. distance(npc.position, target player.position) - target player.aggro Even though this formula does account for aggro, a player will be ignored if they are not within 200 pixels of the specified enemy. Like with regular NPCs, if the Guardian is closer to an NPC than the player is, and is within 400 pixels of it, then the Guardian is targeted instead of the player.

History
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