NPC spawning

In Terraria, enemies, critters, and other NPCs will spawn off-screen and enter the screen area based on their AI. The rate at which enemies are spawned, the maximum number of enemies, and the type of enemies spawned are all dependent on the time of day, biome, the tile type of the ground, and other factors.
== Spawn rates ==
=== Basics ===
The game determines new enemy spawns every tick. There are two variables it takes into consideration when doing so: spawn rate and max spawns, with the spawn rate setting the chance of an enemy spawning at each tick, and the max spawns limiting the number of enemies that are active at a time.
The default spawn rate is 1/600. This means that for each tick, there is a chance of 1/600 that an enemy is spawned. Since there are 60 ticks in 1 second, there is a 9.52% chance that at least one enemy will spawn in a given second.
The default max spawns is 5. This means that a new enemy can only be spawned if the sum of the NPC slots of all current active enemies is lower than 5. Most enemies occupy 1 NPC slot by default; see the following table for a list of all enemies with a higher or lower number of NPC slots. Active enemies exclude many enemies considered part of the same enemy, for example, body and tail parts of , because a Giant Worm, which has 6–8 segments (i.e. individual enemies), would otherwise occupy a disproportionate number of NPC slots. Town NPCs are not considered active NPCs for the purposes of NPC spawning. See the following table for a list of enemies that are not considered active.
 NPCs with non-default NPC slots 
{| class="terraria sortable"	NPC	NPC slots
 NPCs that are never considered active 
=== Natural spawn rates ===
 Key
1/x: spawn ratey: max spawns
Spawn rate and max spawns have set values, which are different depending on the environment. They are formally defined for the environments mentioned in the following tables. Events that are not listed usually employ separate mechanisms to control spawn rates; bosses usually occupy enough NPC slots to block most other spawns.
==== Pre-Hardmode ====
Biome/Event/Boss fight 	Surface 	Surface 	Surface 	Underground	Cavern	Underworld
Biome/Event/Boss fight 	Day 	Night 	Blood Moons only affect the spawn rates above the surface.	Underground	Cavern	Underworld
Weather additionally affects spawn rates and max spawns in the Snow biome. 	1/ 	 	 	 	 	
 	 	 	 	 	 	
 	 	 	 	 	 	
 	 	 	 	 	 	
Only applied if the Meteorite biome does not overlap with the Dungeon, the Underground Desert, the Desert during a Sandstorm, the Jungle, or the evil biomes. If it does overlap with any of those biomes, then the overlapping biome's spawn rate and max spawns are used.	 	 	 	 	 	
 	 	 	 	 	 	
 	 	 	 	 	 	
 	1/20 + per player	1/20 + per player	1/20 + per player 	 	 	
 	 	 	 	 	 	1/
If artificially created. 	1/ 	 	 	 	 	
==== Hardmode ====
Biome/Event/Boss fight	Surface 	Surface 	Surface 	Underground	Cavern	Underworld
Biome/Event/Boss fight	Day 	Night 	 	Underground	Cavern	Underworld
 	1/ 	 	 	 	 	
 	 	 	 	 	 	
 	 	 	 	 	 	
 	 	 	 	 	 	
 	 	 	 	 	 	
 	 	 	 	 	 	
 	 	 	 	 	 	
 	1/20 + per player	1/20 + per player	1/20 + per player	 	 	
 	 	 	 	 	 	1/
 	1/ 	 	 	 	 	
Additionally, the spawn rate will be modified by the number of active enemies in the area. The fewer active enemies there are, the faster new ones will spawn:
* If the sum of the NPC slots of all active enemies is lower than 20% of max spawns, the denominator of the spawn rate will be multiplied by 0.6.
* If the sum of the NPC slots of all active enemies is lower than 40% and greater than or equal to 20% of max spawns, the denominator of the spawn rate will be multiplied by 0.7.
* If the sum of the NPC slots of all active enemies is lower than 60% and greater than or equal to 40% of max spawns, the denominator of the spawn rate will be multiplied by 0.8.
* If the sum of the NPC slots of all active enemies is lower than 80% and greater than or equal to 60% of max spawns, the denominator of the spawn rate will be multiplied by 0.9.
(Note that all products are rounded down to the nearest integer.)
The following table lists some examples for this mechanic. It is to be read in the following way: If the natural spawn rate and max spawns (as determined from the overview above) and sum of the NPC slots of all active enemies are equal to the values in the first three columns, then the spawn rate is altered as shown in the last column.
Spawn rate	Max spawns	NPC slots sum	Altered spawn rate
1/600 	6 	0.8 	1/
1/173 	15 	10 	1/
1/430 	12 	10 	1/430
=== Manual alteration of spawn rates ===
Certain items can be used to change the spawn rate and max spawns when used or placed, as per the following table. All effects are multiplicative rather than additive.
Item	Spawn rate factorThe product of the denominators is rounded down to the nearest integer.	Effective spawn rate	Max spawns
 	1/0.75 	+33% 	+50%
 	1/0.5 	+100% 	+100%
 	1/1.3 	−23% 	−30%
 	1/1.2 	−17% 	−20%
 	1/1.3 	−23% 	−30%
 	1/1.2 	−17% 	−20%
These effects do not stack with themselves, i.e., a single Water Candle, Peace Candle, or Sunflower has the same effect as multiple of each one.
Water and Peace Candles cancel each other out, even though they do not alter spawn rates by the same absolute values. If both are active (e.g. a Peace Candle is held while a Water Candle is placed nearby), none is applied.
There is one exception to this: Water Candles placed (explicitly not held) in SpaceMore precisely, above an altitude of 65% between the surface line and the top of the world, i.e. the same area where Space-exclusive enemies spawn in pre-Hardmode. See Space § Notes. always increase spawn rates, regardless of nearby Peace Candles. Their effectiveness depends on nearby Peace Candles, however:
Item	Spawn rate factor	Effective spawn rate	Max spawns
 	1/0.375 	+167% 	+50%
 	1/0.5 	+100% 	+50%
In addition, nearby have the same effect as Sunflowers and Invisibility Potions, reducing the effective spawn rate by 17% and decreasing the max spawns by 20%.
Up to this point, spawn rates cannot be increased to fractions greater than 1/ and max spawns cannot be increased to values greater than . Both will be cut off there. The spawn rates and max spawns of the invasion events listed in the tables above are exempt from this. There is no lower limit for spawn rates and max spawns.
=== Effect of town NPCs on spawn rates ===
The presence of nearby town NPCs generally reduces spawn rates and max spawns. This effect is adjusted in the Underworld and in .
 Anywhere in the world above the Underworld ( except in a Graveyard)
Number ofnearby town NPCs	Spawn rate factor	Effective spawn rate	Max spawns
1 	1/1.5 	−33% 	−13%
2 	1/(9/7) 	−22% 	−27%
≥3 	 	 	−40%
 In the Underworld
Number ofnearby town NPCs	Spawn rate factor	Effective spawn rate	Max spawns
1 	1/(50/41) 	−18% 	−5%
2 	1/(15/11) 	−27% 	−10%
≥3 	1/1.8 	−44% 	−17%
 In a Graveyard
Number ofnearby town NPCs	Spawn rate factor	Effective spawn rate	Max spawns
1 	1/1.66 	−40% 	−4%
2 	1/2.33 	−57% 	−7%
≥3 	1/3 	−66% 	−13%
When 3 or more town NPCs or the Skeleton Merchant are present nearby, no enemies will spawn. This is only valid for areas outside of the Underworld and Graveyards, and only for Classic worlds – in , there is still a chance an enemy may spawn that tick. Enemies from events as well as bosses (and their servants) will bypass this restriction.
=== Journey Mode ===
Spawn rates can be manually altered in , from 0× (disabling spawns) to 10× the normal rate. Even with the spawn rate set to 0×, bosses may still be summoned, mining up Tombstones can still cause Ghosts to spawn, and certain Celestial enemies can spawn.
== Spawn locations ==
Each player in the game will spawn their own enemies. If two players are in the same area, monsters will appear twice as fast as they would with only one player. Max Spawns rule are still used, which means that even if 2 players are in the same spot, you cannot gather 30 normal monsters near the spot. (Don't forget, the bats count as 1/2, and some special enemies count as 2 or more.)
=== Choosing a spot to spawn the enemy ===
 When the game decides to spawn an enemy, it will first choose a random tile within a rectangle around the player (specifically, the tile containing the top left corner of the player's hitbox). The maximum distance from the player it can choose from is 84 tiles east, 83 west, 46 up, and 45 down. If the picked tile is a solid block, it will try another one. Tiles that are actuated out do not count as solid. If the picked tile has a player-placed wall that blocks enemies from spawning, it will also try another tile.
Once an empty tile is found, it moves down until it finds the ground. (Note that this tile can be more than 45 tiles below from the player. If it finally is outside the "Active Rectangle" it will be instantly despawned, anyway.) If there is enough space for an enemy to be spawned at this location, this will be the tile it chooses. The exception to this is when the player is in an area where Harpies and many other flying enemies can be spawned. In this case, the game does not attempt to find a ground tile.
The chosen tile is then checked to make sure it is not within the "safe" area around the player. It cannot be within 62 tiles east or west, or 34 tiles up or down. Also, the chosen tile space must be completely outside 2088 × 1172 pixel rectangles centered on the center of each player's hitbox. (If the player is not riding a mount, the top edge of this is 34 + 3/16 tiles above the top left corner tile mentioned above, meaning that NPCs can spawn on ground exactly 35 tiles above a player standing on half blocks, but not a player standing on full blocks.)
Using certain items causes monsters to spawn further out than normal. Equipping a Rifle Scope or Sniper Scope does so only when the player has a compatible gun selected, whether or not the player is zoomed out.
Items 	Spawn range	Spawn range	Safe range	Safe range
Items 	Horizontal 	Vertical 	Horizontal	Vertical
Normal 	84 	46 	62 	34
Rifle Scope (or derivatives) 	114 	62 	92 	50
Binoculars 	124 	68 	102 	56
Sniper Rifle 	124 	68 	102 	56
Sniper Rifle with Rifle Scope (or derivatives)	132 	72 	110 	60
On the (pre-1.4 only), the no-spawn area around the player is 68 by 48 tiles, and the spawn area is 112 by 62 tiles. In addition, enemies cannot spawn closer than 34 tiles to the sides and 24 tiles up and down. They also cannot spawn farther than 56 tiles to the sides and 31 tiles up and down.
=== Choosing a type of enemy to spawn ===
The type of enemy spawned will depend on many factors including, but not limited to:
* Time of day
* Weather (e.g., Angry Nimbus and Ice Golem require Rain/Blizzard)
* Depth
* Distance East or West (Ocean enemies, Martian Probes, Goblin Scouts, etc.)
* Player's Current biome (Corruption, Meteor, Jungle, etc., in the case of most biome-specific flying enemies)
* Tile the enemy spawns on (Jungle Grass, Ebonstone, Marble, etc. in the case of most biome-specific grounded enemies)
* Wall behind the player (Lihzahrd Brick Wall, Dungeon Brick Wall, etc.)
*Wall behind the enemy's spawn location (Dungeon Brick Wall variants)
* Special events (Blood Moon, Solar Eclipse, invasion events, etc)
* Randomness
== Notes ==
* When standing on the left or right edge of the world, the enemies can spawn on screen, as long they are at least 62 tiles from the player. The screen is around 85 blocks wide, so in this case the enemies can spawn in the farthest quarter of the screen. (This may not necessarily be the case, some players have larger screens, as big as 120 blocks wide).
* Equipping Binoculars does make the active rectangle bigger.
== History ==
== References ==
Category:NPCs