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Pain in Doom engine games is an actor's reaction to taking damage. When an actor (typically a monster) takes damage, it has a random chance (depending on its PainChance value) of entering into an appropriate Pain state, if it has it defined. Pain chances and pain states can be specific to certain damage types, or generic. Normally, if a damage-specific pain state is missing, it will default to generic Pain state, provided the damage received after any DamageFactor/Armor/PowerProtection post processing is done with a result greater than zero. A source of damage normally causes pain of its damage type, but this can be overridden by setting a PainType.

Pain chances are defined with PainChance [<name>,] <value> where value is a number between 0 and 256. The higher the number, the easier it is for the monster to enter a pain chance based on type. If no name is provided, the generic pain chance is defined. For example:

PainChance "Electricity", 40

In this example, there is a 40/256 (15.625%) chance to react to the Electricity damage type, provided the damage they receive is not zero. If the amount of damage is absorbed (by armor, power-ups, and other similar effects), they will not enter their pain states.

In addition, actors can have a PainThreshold property, preventing them from entering their pain state if the damage received is below the threshold.

Several flags are readily available which can be used to modify different factors for entering pain states based on the actor's damage factors, pain chances, and flags.

The following flags modify a monster (or other vulnerable actor) by itself, and its reaction modifiers when being attacked:

  • NOPAIN - The actor will never enter its pain state under any circumstances.
  • ALLOWPAIN - The actor is always subject to its pain chance, even if actor is invulnerable or calculated damage is zero.
    • Exceptions: - Attacks with a natural damage of zero and attacks with the PAINLESS flag set will not cause pain.

The following flags modify aggressive actors that actually deal the damage to monsters, such as projectiles or puffs:

  • CAUSEPAIN - The victim is always subject to its pain chance, even if actor is invulnerable or damage is zero.
    • Exceptions: If the actor taking damage has the PainThreshold property of 1 or higher, then this flag will be negated if damage is 0.
  • FORCEPAIN - The victim will always enter its pain state (bypassing pain chance).
  • PAINLESS - The actor will never cause the victim to enter its pain state, under any circumstances.

The flags take the following priority, from highest to lowest, in the ZDoom engine: