Damage Scaling

Damage scaling is a mechanic introduced in The Binding of Isaac: Afterbirth to provide a more challenging experience to the players, enabling some enemies to retaliate against high damage attacks by deploying an "armor" of a sort. The more damage dealt to such an enemy in a short period of time, the more it gets reduced, offering an approach to keep the fight challenging and the difficulty high.

Overview
The armor is based on 4 different criteria: how much base HP the entity has, how much damage it has taken in the last 4 seconds (120 game logic frames), the value of its armor (every entity has its unique value), and their starting armor protection (increases from a value of 0 to 120).

Every entity in the game is predisposed to make use of this mechanic: in fact, every entity sports an "starting armor protection" counter and an "armor value" field, but few entities actually use it.

How damage scaling works
When an "armor enabled" entity spawns, it starts off with a separate instance of armor. This instance of armor starts off by reducing incoming damage by 99%, and will steadily decrease over the course of a few seconds at which point it is no longer in effect. This value will increase its "armor deployment" each game logic frame going from 0 (fully armored) to 120 (no longer in effect), and use this value to compute a static degree of damage reduction, separate from this starting armor protection.

The entity keeps track of how much unmodified damage it has taken in the past 4 seconds, which combined with the static degree, the entity's HP and armor effectiveness results in a multiplier to the actual damage dealt, capping out at a multiplier of 0.09x. This can stack with the starting armor protection to reduce damage even further. In practice this is only a significant factor in the battle with Delirium, as each time it transforms into a form that is not its true form, its starting armor protection is reapplied. In the cases of all other armored entities, the starting armor protection becomes irrelevant after a few seconds.

The entity's base HP is divided by its armor value to create a soft cap on damage per second. This soft cap serves as a way to equalize damage coming from Isaac, as if his damage per second is above this cap it will be lowered as to reduce the advantage of having high damage output while DPS values below the DPS soft cap are completely unaffected. The larger Isaac's damage per second, the more his damage will be lowered. As the entity tracks how much unmodified damage it has taken in the past 4 seconds, the damage output will gradually be lowered until it is equalized to the damage per second soft cap, or the damage instance reaches the point that it is reduced to 9% of its original damage.

The damage of Isaac's attacks are also taken into consideration, not just his DPS. If an individual damage instance is equal to more than 4 times the armored entity's designated DPS soft cap, the damage instance will be preemptively lowered to reduce any potential head starts on dealing with the entity, or even the possibility killing the entity in one hit, even if the entity has not been attacked in a period of 4 seconds.

Ultimately this mechanic has its limits, as no matter what, each individual attack cannot have its damage reduced to below 9% of its original damage, allowing the armor to be overwhelmed if the outgoing damage per second is large enough.

Overall this mechanic results in slow, high-powered attacks being less effective than rapid, lower damage ones; the damage reduction formula has difficulty keeping up with rapid, low damage attacks while slow, high-powered attacks will preemptively have their damage lowered, allowing for such attacks to have their damage equalized more easily.

Bypassing Damage Scaling
Repentance added in methods to bypass this mechanic, giving Isaac offensive options to deal full damage completely unhindered. However these ways of dealing damage do contribute to Isaac's damage per second, which can cause the damage dealt from his other attacks to be lowered as a result of running into the DPS soft caps. Though the amount of damage dealt through these methods tend to outweigh the downsides of lowering Isaac's damage in most cases.


 * Bomb damage and the damage boost from fully apply their effects:
 * All Bomb pickups
 * : The initial explosion bypasses damage scaling, however the creep is fully affected by it.
 * : The initial explosion bypasses damage scaling, the poison effect and gas clouds do not.
 * : The explosion and its scaling deal full damage, the poison effect and gas clouds do not.
 * : The initial explosion bypasses damage scaling, the subsequent explosions do not.
 * : The initial explosion bypasses damage scaling, the Brimstone lasers do not.
 * and : The initial explosion bypasses damage scaling, the cloud does not.
 * : The initial explosion bypasses damage scaling, the contact and explosion damage from the ghosts do not.
 * : The initial explosion bypasses damage scaling, the flames do not.
 * : The initial explosion bypasses damage scaling, but it does not allow the tear effects to do the same.
 * : Both the initial and subsequent explosions deal full damage.
 * Necronomicon effects and the damage boosts from A Missing Page and fully apply their effects:
 * Black Hearts
 * Necronomicon effects and the damage boosts from A Missing Page and fully apply their effects:
 * Black Hearts

However, while several explosive attacks bypass damage scaling, there are others in this category that do not and are fully affected by damage scaling:

Entities and Their Armor Values
The game enables the armor for the following entities, one of which is only partial:

Trivia

 * Despite the armor attempting to balance damage dealt and keep fights challenging, it's very possible to overwhelm the mechanic and destroy even strong armored enemies like Hush.
 * will lessen Ultra Greed's armor effectiveness by 25%.