Back Multiplier

Most enemies in GTFO will experience additional damage when targeted from behind. This extra damage is determined by the Back Multiplier.

Some enemies (generally mini-boss or boss level enemies) do not experience extra damage when targeted from behind and thus will never use the Back Multiplier. The enemies exempt include Mothers, Big Mothers, Tanks, the Immortal, the Mega Mother and the Kraken.

The Back Multiplier acts as a multiplier for the damage that the player deals. By being directly behind an enemy, a player can deal up to double damage to that enemy.

The back of an enemy does not count as a weakspot for the purposes of the Precision Multiplier. However, for enemies that have both a weakspot and can receive bonus back damage it is possible to stack both of these damage bonuses at once. E.g. hitting an enemy in the head from behind can apply both the enemy's Headshot Multiplier, your weapon's Precision Multiplier as well as the Back Multiplier. This is typically referred to as the "occiput" of the enemy.

Formula

The expression used to determine the Back Multiplier depends only on the angle ($\theta$) between two vectors:

• the vector from the player's camera position to the location the hit connected on the enemy, and
• the current direction the enemy is facing.

Given this angle, the formula for the Back Multiplier is given by:

${\text{Back Multiplier}}(\theta )={\begin{cases}2,&{\text{if }}0^{\circ }\leq \theta \leq 41.4^{\circ }\\1.25+\cos(\theta ),&{\text{if }}41.4^{\circ }\leq \theta \leq 104.5^{\circ }\\1,&{\text{if }}104.5^{\circ }\leq \theta \leq 180^{\circ }\end{cases}}$

Image showcasing some of the key angles for the Back Multiplier. The blue region would give the maximum Back Multiplier of 2, while the green region would give the minimum of 1.

A table of some reference values for this formula is given below:

Angle (degrees)	Back Multiplier
0 - 41.4	2
45	1.957
50	1.893
55	1.824
60	1.75
65	1.673
70	1.592
75	1.509
80	1.424
85	1.337
90	1.25
95	1.163
100	1.076
104.5 - 180	1

Notably, the Back Multiplier does not use a hitbox for the enemy. Just because a player hits the visual back of the enemy does not mean they will benefit from the Back Multiplier, similarly, the player can hit the front of the enemy but still benefit from the Back Multiplier.

Furthermore, the formula uses the full 3D vectors for determining the Back Multiplier, thus when a player is above (or below) an enemy, they may lose some of the Back Multiplier despite being directly behind the given enemy. In some cases, it may be advisable for the player to crouch when aiming at nearby crawling enemies, as looking directly down at these enemies will make it unlikely for the player to get the full benefit of the Back Multiplier.

Backstab Multiplier

Currently, the Knife is the only weapon to have an additional Backstab Multiplier, which stacks multiplicatively with the Back Multiplier. Unlike the Back Multiplier, which varies smoothly with the angle, this multiplier is either fully applied or not applied at all. If the vector from the player's camera position to the location the hit connected on the enemy and the facing direction of the enemy's chest/spine are within ~56.6 degrees then the Backstab Multiplier will be applied, otherwise it will not. Furthermore, like other multipliers on melee weapons the Backstab Multiplier also scales with charge time.

Example Calculations

Angle at which the HEL Revolver two-shots Strikers to the body

Two body-shots from the HEL Revolver deal 16.02 damage, so in order to kill the Striker (20HP), we need a Back Multiplier of $20/16.02 \approx 1.248$. I.e. using the angle-dependent case of the Back Multiplier formula:
$20/16.02 = 1.25 + \cos(\theta)$

Solving for $\theta$ gives $\theta \approx 90.09^\circ$. The HEL Revolver can kill a Striker with two body-shots while shooting directly from the Striker's side.

Extra Details

• The Back Multiplier actually starts to give bonus damage while the player is still slightly in front of the enemy.
• Sentries, while they are affected by many of the other multipliers (such as Precision Multipliers and Stagger Multipliers) never benefit from the Back Multiplier. Sentries do not deal additional damage when targeting an enemy from behind.
  • The exception to this is the Rad Labs Meduza HEL Auto Sentry, which does benefit from the Back Multiplier.
• While targeting enemies from behind does deal bonus damage through the Back Multiplier, the backs of enemies do not count as weakpoints for the purpose of the Precision Multiplier being applied.
  • However, targeting an enemy's head while behind them will use the Precision Multiplier (as well as that enemies Head Multiplier) in addition to the Back Multiplier. Often, when targeting an enemy's head from behind players will refer to the Occiput Multiplier (given by the product of that enemy's Head Multiplier and the maximum Back Multiplier). E.g. the Occiput Multiplier for a Shooter would be $5 \cdot 2 = 10$. The Occiput Multiplier (for the enemy) then gets applied in addition to the Precision Multiplier (for the weapon).

• 

  A player directly aligned with a sleeping Hybrid's facing direction. Note their position behind and slightly to the right of the enemy's back.

  Most enemies 'look' slightly to their left during most of their animations, so being directly aligned with the enemy's facing direction often involves being behind and slightly to the right of the enemy's back.
• In the case of weapons that fire bullets, the vector from the player's camera to the location that the bullet hits the enemy will generally align with the direction the player is facing (with the exceptions of shots fired with spread, either from shotguns or hip-fire). This is also true of melee weapons when they use raycasting/hitscan to determine where they hit an enemy. Melee weapons use raycasting/hitscan when used while directly facing an enemy in close proximity (the player's reticle will become a smaller circle when raycasting/hitscan will be used). However, when used far from an enemy or while not directly facing at an enemy, the melee strike will instead use a hitbox to determine if it hits an enemy. In this case, the vector from the player's camera to the location that the melee strike hits the enemy may not align with the player's facing direction.
• The vector for the current direction the enemy is facing is tracked internally by the game, and may not align with the direction that the enemy's visual model would indicate. This is particularly true when enemies undergo certain animations, as their internal facing direction will not update until the animation is complete, even though the animation itself may involve the enemy's model turning around.
  • This effect is very evident for the heavy stagger animations of Giants and Big Chargers. While the animations involve the enemy turning around (and in the case of the Big Charger, wildly pirouetting), the internal facing direction of the enemy does not change until the animation is complete.
  • When dealing with enemies undergoing animations such as these, it may be important for the player to mentally track the direction the enemy was facing at the start of the animation, as this is what will be used for the enemy's facing direction for the purpose of the Back Multiplier.
• Notably, the Back and Backstab multiplier use different vectors for the enemy in their checks. The Back Multiplier uses the enemy's internal facing direction, which as mentioned does not update until enemy animations end, while the Backstab Multiplier uses the facing direction of the enemy's chest/spine, which does update during animations.
  • This means that during particular points of enemy animations where they turn significantly, it is sometimes possible to obtain the full value of the Back Multiplier, while simultaneously not benefiting from the Backstab Multiplier at all.
• The exact expressions for when the expression for the Back Multiplier changes are $\theta = \arccos(3/4) \approx 41.4^\circ$ and $\theta = \arccos(-1/4) \approx 104.5^\circ$ respectively.