UE5-Modular-Game-Framework/docs/blueprints/08-weapons/75_BPC_HitReactionSystem.md

BPC_HitReactionSystem — Hit Reaction System

Blueprint Spec — UE 5.5–5.7

---

Parent Class

ActorComponent

Dependencies

• Requires: BPC_HealthSystem — Receives damage events
• Requires: BPC_CameraStateLayer — Camera trauma effects
• Requires: S_DamageEvent — Damage event struct (E_DamageType, Amount, HitLocation, HitNormal)
• Requires: E_DamageType — Damage type enum for montage selection
• Required By: BPC_AdaptiveEnvironmentDirector — Reads trauma intensity for atmosphere
• Engine/Plugin Requirements: Animation Blueprint (ABP_PlayerBody), GASP animation hooks

Purpose

Drives physical hit reaction behaviour: flinch animations, ragdoll blends, and camera trauma. Processes incoming damage events and triggers appropriate reaction animations based on damage magnitude, direction, and type.

---

1. Enums

Uses E_DamageType from BPC_HealthSystem — Physical, Fire, Psychic, Environmental, InstantKill, Scripted.

---

2. Structs

No new structs defined. Uses S_DamageEvent from BPC_HealthSystem.

---

3. Variables

Configuration (Instance Editable, Expose On Spawn)

Variable	Type	Default	Category	Description
FlinchThreshold	Float	5.0	HitReaction	Minimum damage to trigger flinch animation
RagdollThreshold	Float	50.0	HitReaction	Damage amount that triggers ragdoll blend
TraumaDecayRate	Float	2.0	HitReaction	Camera trauma points recovered per second
HitReactionMontages	Map (E_DamageType → AnimMontage)	Empty	HitReaction	Per-damage-type hit reaction animations

Internal (Private / Protected, No Expose)

Variable	Type	Default	Category	Description
CurrentTrauma	Float	0.0	Internal	Accumulated camera trauma level
bIsReacting	Bool	False	Internal	Reaction animation currently playing
LastDamageDirection	Vector	(0,0,0)	Internal	Direction of most recent damage for animation blend
NextReactionIndex	Integer	0	Internal	Cycles through available montages per type for variety

---

4. Functions

Public Functions

ProcessHitReaction → void

• Description: Main entry point. Called when player receives damage. Determines reaction type based on damage amount vs thresholds.
• Parameters:

  Param	Type	Description
  DamageEvent	S_DamageEvent	Full damage context from HealthSystem

• Blueprint Authority: Any
• Flow: Checks thresholds → selects montage → plays animation → applies trauma → broadcasts dispatchers

ApplyTrauma → void

• Description: Adds camera trauma based on damage amount.
• Parameters:

  Param	Type	Description
  Amount	Float	Trauma points to add

• Blueprint Authority: Any

GetCurrentTrauma → Float

• Description: Returns current accumulated trauma level (0–100).
• Parameters: None
• Blueprint Authority: Any

ResetTrauma → void

• Description: Clears all accumulated camera trauma to zero.
• Parameters: None
• Blueprint Authority: Any

Protected / Private Functions

SelectHitReactionMontage → AnimMontage

• Description: Picks a hit reaction montage based on damage type and cycles through available options for variety.
• Parameters:

  Param	Type	Description
  DamageType	E_DamageType	Type of damage received
  DamageAmount	Float	Magnitude of damage

• Flow: Look up HitReactionMontages map by damage type → cycle through entries → return selected montage

TickTraumaDecay → void

• Description: Called on tick to decay camera trauma toward zero at TraumaDecayRate.
• Parameters:

  Param	Type	Description
  DeltaTime	Float	Frame delta time

• Flow: CurrentTrauma = FMath::FInterpTo(CurrentTrauma, 0.0, DeltaTime, TraumaDecayRate)

---

5. Event Dispatchers

Dispatcher	Parameters	Bind Access	Description
OnFlinchTriggered	DamageType: E_DamageType, Amount: Float	Public	Flinch animation fired (damage below ragdoll threshold)
OnRagdollBlend	Amount: Float, Direction: Vector	Public	Ragdoll blend triggered (damage exceeded ragdoll threshold)
OnTraumaChanged	CurrentTrauma: Float	Public	Trauma level changed (for camera/screen effects)
OnHitReactionComplete	DamageType: E_DamageType	Public	Reaction animation finished playing

---

6. Overridden Events / Custom Events

Event: ReceiveDamage

• Description: Called by BPC_DamageReceptionSystem when player takes damage. Routes to ProcessHitReaction.
• Flow:
  1. Extract damage amount and type from DamageEvent
  2. If DamageAmount >= FlinchThreshold → ProcessHitReaction
  3. If DamageAmount >= RagdollThreshold → trigger ragdoll blend instead
  4. Calculate LastDamageDirection from HitLocation relative to player
  5. ApplyTrauma proportional to damage amount

---

7. Blueprint Graph Logic Flow

flowchart TD
    A[ReceiveDamage called] --> B{Damage >= RagdollThreshold?}
    B -->|Yes| C[Disable input temporarily]
    B -->|No| D{Damage >= FlinchThreshold?}
    C --> E[Play ragdoll physics blend]
    D -->|Yes| F[SelectHitReactionMontage by DamageType]
    D -->|No| G[Skip reaction, only apply trauma]
    F --> H[Play montage with directional blend]
    H --> I[Broadcast OnFlinchTriggered]
    I --> J[Wait for montage end]
    J --> K[Broadcast OnHitReactionComplete]
    E --> L[Set bIsReacting = true]
    L --> M[Wait for ragdoll timer]
    M --> N[Recover from ragdoll]
    N --> K
    G --> O[ApplyTrauma proportional to damage]
    K --> O
    O --> P[Broadcast OnTraumaChanged]
    P --> Q[Camera system reads trauma]

---

8. Communication Matrix

Who Talks	How	What Is Sent
BPC_DamageReceptionSystem	Direct call	S_DamageEvent
BPC_CameraStateLayer	Dispatcher (OnTraumaChanged)	CurrentTrauma: Float
WBP_ScreenEffectController	Dispatcher (OnTraumaChanged)	CurrentTrauma: Float
ABP_PlayerBody	Direct (anim var write)	LastDamageDirection, reaction montage reference
BPC_AdaptiveEnvironmentDirector	Direct read	CurrentTrauma via getter

---

9. Validation / Testing Checklist

• FlinchThreshold: damage at 5.0+ triggers flinch animation
• RagdollThreshold: damage at 50.0+ triggers ragdoll blend
• TraumaDecayRate: trauma reduces to zero over time when no damage received
• HitReactionMontages map: correct montage plays per E_DamageType
• Edge case: zero damage events do not trigger any reaction
• Edge case: rapid succession hits cycle through montage variations
• Edge case: death event (HealthSystem OnDeath) skips flinch, goes directly to ragdoll

---

10. Reuse Notes

• FlinchThreshold and RagdollThreshold can be tuned per project for desired combat feel
• HitReactionMontages map uses E_DamageType as key — add new damage types without modifying core logic
• Trauma system is separate from camera shake — trauma drives persistent wobble, shake is transient
• For non-combat projects, set FlinchThreshold very high (e.g., 9999) to disable reactions entirely

---

Specification based on Master Section 5.7, line 1821.

---

Manual Implementation Guide

Class Setup

1. Create Blueprint Class: Parent = ActorComponent, Name = BPC_HitReactionSystem
2. Add to Player Character (or EnemyBase)
3. Create HitReactionMontages per damage type: Physical, Fire, Explosive, etc.

Variable Init (BeginPlay)

Event BeginPlay
  ├─ Set CurrentTrauma = 0.0, bIsReacting = false
  ├─ Populate HitReactionMontages map (E_DamageType → AnimMontage)
  ├─ Cache: BPC_HealthSystem (bind OnDamageTaken), BPC_CameraStateLayer
  └─ Start TraumaDecay timer (0.1s loop)

Function Node-by-Node

ProcessHitReaction(DamageEvent: S_DamageEvent) → void

Step 1: If bIsReacting → Return (already playing reaction)
Step 2: Set LastDamageDirection = DamageEvent.HitNormal * -1  ← direction damage came from
Step 3: If DamageEvent.Amount >= RagdollThreshold:
  Play ragdoll blend: ABP → Set bRagdoll=true, physics blend
  Set bIsReacting = true → timer to recover
  Return
Step 4: If DamageEvent.Amount >= FlinchThreshold:
  Select montage: HitReactionMontages[DamageEvent.DamageType]
  If multiple: pick NextReactionIndex, cycle it
  Play Montage with blend from LastDamageDirection
  Set bIsReacting = true
Step 5: Call ApplyTrauma(DamageEvent.Amount * 0.5)
Step 6: Fire OnHitReactionPlayed(DamageEvent)
Step 7: On montage complete → Set bIsReacting = false

ApplyTrauma(Amount: Float) → void

Step 1: CurrentTrauma = Min(100, CurrentTrauma + Amount)
Step 2: Push to camera: BPC_CameraStateLayer.ApplyPostProcessOverride(TraumaVignette, CurrentTrauma/100)
Step 3: Fire OnTraumaChanged(CurrentTrauma)

TraumaDecay Tick → void (Timer 0.1s)

CurrentTrauma = Max(0, CurrentTrauma - TraumaDecayRate * 0.1)
Update camera effect intensity
If zero: clear post-process override

Build Checklist

• Create BPC_HitReactionSystem, add to Player Character
• Create hit reaction montages per damage type
• Implement ProcessHitReaction with threshold branching
• Implement ApplyTrauma with camera effect push
• Set up trauma decay timer
• Bind to BPC_HealthSystem.OnDamageTaken
• Test: take small hit → flinch; take big hit → ragdoll; trauma decays over time