Decals and hit reactions

Both features are opt-in and disabled by default. They cover the hits that do not destroy an object — the majority of hits in a melee-heavy game.

Hit reactions

A hit reaction is a request emitted for a valid hit that leaves the object standing.

Property	Default	Meaning
bEnableHitReactions	false	Master switch
MinimumHitReactionPower	VeryLight	Power gate for reactions
bApplyBuiltInPhysicsReaction	true	Apply a bounded impulse when the intact component already simulates physics
HitReactionImpulseMultiplier	0.2	Fraction of the incoming impulse used
MaximumHitReactionImpulse	250.0	Hard cap on the applied impulse

When the impulse applies

Whether the intact simulates is decided by its physics mode, at BeginPlay:

• New generations use Simulated by default, so their intact prop is already simulating and the built-in impulse can move it;
• Static, Keep Existing and older setups that were not migrated are not changed silently — a static prop stays static, and the impulse is skipped.

See Intact physics and fall damage.

The request is emitted either way. An object that does not simulate physics still reports the reaction, so your own code can play an animation, a shader wobble or a sound instead.

The request payload

FSDFHitReactionRequest:

Field	Meaning
SourceActor	The object that was hit
Location	World-space impact position
Direction	Direction of the incoming hit
RequestedImpulse	Impulse the profile computed, after multiplier and cap
DestructionPower	Power of the causing hit
RandomSeed	Seed for deterministic reaction selection
bPhysicsImpulseApplied	Whether the built-in impulse was actually applied

Listen on OnHitReactionRequested, or read GetLastHitReactionRequest on demand.

bPhysicsImpulseApplied tells you unambiguously whether the object moved because of SDF, which is what your own reaction logic needs in order not to double up.

Typical use

• A hanging lantern that swings when struck — physics simulating, built-in impulse enabled.
• A stone statue that flashes and sparks but does not move — reactions enabled, Physics Static.
• A crate that only reacts to real attacks — raise MinimumHitReactionPower above the value your light attacks carry.

Impact decals

Property	Default	Meaning
bEnableImpactDecals	false	Master switch
bEnableBuiltInDecalSpawning	true	Disable when an external listener owns decal presentation
MinimumDecalPower	VeryLight	Power gate
MaximumImpactDecals	3	Per-object limit (0–32)
ImpactDecals	empty	Weighted variations

Decal variations

FSDFWeightedDecal:

Property	Default	Meaning
Material	none	Soft UMaterialInterface reference
Weight	1.0	Relative selection weight
Size	(8, 16, 16)	Decal projection box
LifeSpan	20.0	Seconds before the decal fades out
FadeScreenSize	0.01	Screen size below which the decal stops rendering
bRandomizeRoll	true	Randomise roll around the projection axis

Deterministic orientation

SelectWeightedDecalVariation(Variations, RandomSeed, ImpactNormal, OutRotation) picks the variation and computes the rotation. Decals are projected along the impact normal; with bRandomizeRoll enabled the roll is derived from the seed, so repeated hits do not produce visibly identical stamps — and the same seed reproduces the same stamp exactly.

The per-object limit

MaximumImpactDecals caps how many decals one object carries. Once the limit is reached, further decal requests are still emitted but marked bSuppressedByObjectLimit, so an external system knows the framework deliberately declined rather than failed.

GetActiveImpactDecalCount returns the current count. Tracked decals are destroyed when the object is cleaned up, so a destroyed crate does not leave its dents floating in the level.

The request payload

FSDFDecalRequest:

Field	Meaning
SourceActor	The object that was hit
Location	World-space impact position
ImpactNormal	Projection normal
Rotation	Resolved rotation
Size	Resolved projection size
SelectedMaterial	The variation picked
RandomSeed	Seed used
SelectedVariationIndex	Index into the profile array
bBuiltInSpawnRequested	Built-in spawning was configured
bBuiltInSpawnAttempted	A spawn was attempted
bBuiltInSpawnSucceeded	The decal component was created
bSuppressedByObjectLimit	The per-object limit declined this request

Listen on OnImpactDecalRequested, or read GetLastImpactDecalRequest.

Cost guidance

Decals are cheap individually and expensive in aggregate. In a room with thirty destructible props, a limit of 3 per object is already up to ninety decal components. Two practical rules:

• keep MaximumImpactDecals at 1 or 2 for common props, and reserve 3+ for hero objects;
• keep LifeSpan short — a decal that outlives the fight adds cost without adding information.