Melee Trace

Documentation Unreal Engine Player Combat Melee Combat

Melee trace lands a weapon's hits: the item names its hit volumes, a window on the attack montage opens them, and a sweep fills the gap between frames, so a fast swing connects at 30 FPS and at 144.

This page continues First Combo: Item_MySword is the item you made there.

Set it up

3 minutes

1
Give the item a hit volume
Open Item_MySword, expand its Equippable fragment and find the row in Trace Sockets. An item with a Visuals mesh and no actor class shows that mesh on a component tagged UPC.ItemMesh, so the row names that tag.
Item_MySword
▾Equippable > Trace Sockets > Index [ 0 ]
IDNone
Source Component TagNone
ShapeSphere
Socket or Bone Name and End Socket or Bone Name start on TraceStart and TraceEnd. Add sockets by those names to the weapon mesh, hilt to tip, or name sockets it has.
The UPC Character Base carries the UPC Melee Trace component the row registers on. On a pawn of your own, add that component and send the ready signal as Character Setup shows.
2
Create a damage config
Content Browser
▸ Content
Right-click in the Content Browser, UPC, Damage and Defense, Damage Config.
Set Damage and add UPC.DamageType.Slash to Damage Tags. Damage and Hit Effects covers the rest of the asset.
3
Point the trace windows at it
Open AM_UPC_Sword_Combo. Each swing has a UPC Melee Trace Window with Socket IDs Blade; set Damage Config on each to Damage_MySword in place of the demo's Damage_UPC_Sword. The demo sword plays this montage too; to keep the demo as it ships, duplicate the montage and point your graph's nodes at the copy. On a montage of your own, add the Notify State UPC Melee Trace Window across the strike frames and set both fields.
AM_UPC_Sword_Combo
0.0
0.25
0.5
0.75
1.0
1.25
1.5
1.75
2.0
Notifies
Melee Trace: Blade
Melee Trace: Blade
Melee Trace: Blade
Each window covers the frames where the blade can hurt.
4
Play, and watch the sweep
Press Play, draw the sword and run UPC.Debug.Melee.DrawTracing 1. The swept shape draws while the window is open and changes color on a hit. The item's sockets exist only while it is drawn. A target needs collision that blocks the Pawn channel, the default Trace Filter; the demo dummies have it.

Shape the sweep

Viewport

Interpolation OFF

LVL_UPC_Demo>Melee Trace

Swing Speed60°

SIM_FPS: 10

STATUS: TUNNELING

Successful Hits

Without Interpolation: The fast sword swings 'teleport' over the target between frames, causing a miss.

Shape	Pick it for
Sphere	A fist, a pommel, a small contact point. Radius (15 cm) is the whole setup.
Capsule	A wider path for one contact point. It lies along the way the socket moved, so Capsule Half Height lengthens the sweep, not the weapon.
Capsule Two Point	A blade from hilt to tip, spanning Socket or Bone Name to End Socket or Bone Name.

Give each contact point a row and an ID of its own, so a blade and a pommel can trace in different windows.

Hit volumes from items

Details

Trace Socket

Trace

Blade

Source Component Tag

UPC.ItemMesh

Shape

Capsule Two Point

Socket or Bone Name

TraceStart

End Socket or Bone Name

TraceEnd

Radius

Debug

Debug Color

One row of the Equippable fragment's Trace Sockets. Drawing the item registers it on the character's UPC Melee Trace; putting it away takes it off.

A row registers on the character's UPC Melee Trace as the item is drawn and comes off as it is put away. Source Component Tag picks the mesh it rides:

Empty: the character's own body mesh. Bare hands work this way: the demo's unarmed item, Item_UPC_Unarmed, names the bone hand_r.
A tag: the component carrying it on the item's actor, else on the character and what is attached to it. A weapon Blueprint with a blade mesh tags that mesh component Blade.Mesh and the row names Blade.Mesh.

A character that never holds an item registers its sockets from Blueprint instead (Fists, claws and anything without a weapon).

Hold Places, on the same Equippable fragment, lists where the item can be held, in order of preference (Equipment). Held in the first place, a row registers under its own ID. Held in another, the ID gains that place's name: in UPC.Place.Main the row is Blade, in UPC.Place.Off it is Blade_Off, which a window naming Blade does not sweep.

The montage window

Details

UPC Melee Trace Window

Melee Trace

Socket IDs

1 Array element

Index [ 0 ]

Blade

Damage Config

Damage_MySword

Extra Damage Tags

0 Gameplay Tags

Debug

Enable Debug Draw

Notify state over the strike frames. Empty Socket IDs sweeps every registered socket.

Socket IDs empty sweeps every socket the character has registered; each name you add must match a socket ID exactly.
Extra Damage Tags describe this swing on top of its damage config's tags, so one config serves a combo whose windows differ: three slashes marked counterable, a finisher marked unblockable.
One trace runs one window at a time; a window opening over another closes the first and logs it.

How often a swing lands

Setting on UPC Melee Trace	Does
Hit Reset Interval (0)	Seconds before one target can take another hit in the same window. 0 lands once per target; 0.15 suits a spin or a drill.
Max Targets Per Swing (0)	Targets one swing cuts through: 0 is everyone, 1 a stab that cannot clip the enemy behind. Only damageable targets count, a blocking one included. Tick Scenery Counts Toward Swing Limit to count walls too.
Skip Record Tags (empty)	Result tags a defense reports, such as `UPC.DamageResult.Blocked` (Defense), that leave the swing unspent on that target. Empty spends the swing on a target whatever it reported; with Blocked listed, a blocked swing stays live on it.

An attack with its own repeat timing or target count sets Multi Hit Interval and Max Targets Per Swing Override on its damage config; -1 keeps the component's values.

Who a swing can hit

Details

UPC Melee Trace

Damage

Team Filter

Enemies Only

Starts on Enemies Only, so a swing spares teammates. Filters who a swing can damage, separate from targeting.

Team Filter starts on Enemies Only: a swing spares teammates and hits anyone hostile or on no team. No Filter allows friendly fire, Friendlies Only suits a healing weapon, and Neutral Only hits actors on no team and those the attacker's team excludes. Teams come from the UPC Team component on the pawn.

You want	Set
A dodge the swing passes through	Hold `UPC.State.Intangible` over the dodge (Animation Integration). Pass Through Tags lists it: the swing lands nothing on that target and carries on behind it.
A mount or a companion the swing ignores	Add Trace Ignored Actor. The sweep already skips the wielder and everything attached to it.
A swing that reaches a ragdoll ignoring the Pawn channel	Trace Filter on Object Types. Nothing stops that sweep, walls included, so keep Channel wherever the level should shield a target.

AdvancedReact to every landed hit

Each landed hit passes through the trace's Confirmed Hit Ability Class before it deals damage; the UPC Character Base grants it on its own. To add logic to each hit, such as a combo counter:

Make a Blueprint child of UPC Confirmed Hit Ability.
Override On Hits Confirmed and call the parent, so the hits still deal damage.
Set the child as Confirmed Hit Ability Class on the character's UPC Melee Trace.

The character receives the class as its ability system comes up (Notify Ability System Ready), unless it already holds that class or a child of it, as a starting ability set can grant. Grant one only: a second deals each hit again, and the trace warns. With none granted, the sweep deals each hit itself in single player and on the server, and a client player's hits are lost.

Debug it

UPC.Debug.Melee.DrawTracing 1   // Every sweep, in its socket's Debug Color, changing color on a hit

Draw Debug on one trace, or Enable Debug Draw on one window, draws that one alone; Debug Draw Duration (0.1 s) is how long a shape stays. The Output Log names a socket a window asked for that nothing registered, a window opened over another, an Object Types filter with no types, a sensor collider blocking the sweep, and an item drawn onto a character with no UPC Melee Trace.

AdvancedWhat a swing lands on

Each contact sorts into one kind:

Kind	What it is	Damage	The swing
Damageable	A live target, one a UPC Invulnerable response protects included.	Sent	Spent on it
Inert	A combatant taking nothing now, such as a pawn with Can be Damaged off.	None	Spent, and reachable again once it takes hits
Scenery	A wall, a floor, a prop.	None	Held for the window, so a wall costs one clang

Respect Can be Damaged (on) makes a target with Can be Damaged off inert, so switching it off on death quiets a corpse.
Require Damageable Target sends no damage to anything without UPC Damageable (a pawn sorts as inert, anything else as scenery). Leave it off while a breakable crate takes hits through Any Damage.
A swing passes through a trigger box: a component with no physics collision that lets pawns through, on an actor that is neither a pawn nor UPC Damageable. An overlap-only hit box on a character still takes the blow.

AdvancedColliders the swing reports instead of hitting

For a parry box or a wake-up volume: tag the collider, list the tag under Sense Component Tags, and set its response to the trace channel to Overlap. A swing crossing it fires On Melee Sensed once per window, spends nothing and reaches the body behind in the same frame. On Block the sweep stops at the sensor.

AlternativeFists, claws and anything without a weapon

A pawn with no item registers its own sockets from Blueprint, usually on Begin Play, and names them on the window as a weapon's are:

Register Sphere Socket registers one point on a bone (a hand, a foot, a horn), Register Capsule Socket one point with a wider swept path, Register Two-Point Capsule a span between two bones or sockets.

A socket registered under an ID in use replaces it; Unregister Trace Socket and Unregister All Sockets remove them. A weapon Blueprint can add sockets from On Drawn and remove them in On Stowed, under IDs its Trace Sockets rows do not use.

AlternativeOpening a trace window without a montage notify

Node	Does
Start Tracing	Opens a window on the socket IDs given, every socket when empty, with a damage config and a debug draw override.
Start Tracing Simple	The same with a damage figure in place of a config; below zero uses Base Damage. With no config it runs no hit effects and sends Damage Type Class.
Start Tracing with Tags, Start Tracing Simple with Tags	The same two, with Extra Damage Tags for this swing.
Stop Tracing	Closes the window and clears its hit records.

Get Registered Socket IDs lists the sockets, and Is Tracing answers whether a window is open.

AdvancedChanging one phase of the pipeline

Override one phase's event on a Blueprint child of UPC Melee Trace. A character built on the UPC Character Base takes the child through a C++ subclass naming it for the MeleeTrace component; your own character carries it in place of UPC Melee Trace.

Event	Controls
Build Damage Info	The damage before the trace sends it, for crits or an elemental pass. It receives the socket that struck.
Handle Damage Result	Whether the trace records a hit. The default records all but a result in Skip Record Tags; an override calls Record Hit for each hit it keeps.
Classify Hit	The Damageable, Inert or Scenery sorting.
Classify Contact	Body to damage, sensor to report, or nothing. Call Is Pass Through Target and Is Trigger Contact to keep the default rules.
Should Detect Hits	Which machine sweeps, by default the one Hit Detection Authority names.

Can Hit Actor, Record Hit and Get Targets Hit This Window (for damage falling off across a cleave) support the overrides. A child of UPC Damage Config can carry more per attack, such as a stagger tier, for the target to read in Handle Incoming Damage.

MultiplayerWhich machine finds a hit

Details

Universal Player Combat

Combat

Multiplayer

Default Hit Detection Authority

Client Authoritative

Hit Stop

Max Hit Stop Seconds

0.30

Rewind

Rewind History Seconds

1.0

Rewind Memory Budget MB

8.0

Project-wide combat defaults, saved to DefaultGame.ini.

Hit Detection Authority on UPC Melee Trace picks the machine that sweeps; Project Default reads Default Hit Detection Authority in Project Settings, Plugins, Universal Player Combat.

Setting	Sweeps
Client Authoritative	The default. The controlling player's machine finds that player's hits, so a swing connects on screen the moment it touches; the server finds the hits of characters no player controls.
Server Authoritative	The server finds every hit and refuses hits a player's machine reports. A player sees contact a round trip late.

On Melee Hit and On Melee Sensed fire on the machine that swept; On Melee Hit Response where the hit resolved.
Put the team on the pawn: a team id on a controller reads as no team on another player's machine.
A window started by hand reaches the server with no damage config, so its reported hits land there with the trace's Default Damage Config.

MultiplayerHow the server checks a reported hit

Details

Class Defaults

UPC

Confirmed Hit

Hit Validators

4 Array elements

Index [ 0 ]

Melee Trace Window

Grace Seconds

0.50

Index [ 1 ]

Pass Through

Index [ 2 ]

Plausible Hit

Extra Reach

150

Index [ 3 ]

Rewind Hit

Max Rewind Seconds

0.50

Time Tolerance

0.10

Distance Tolerance

Untied Hit Spacing

0.5

A Blueprint child of the ability, set as the melee trace's Confirmed Hit Ability Class. These are the defaults it inherits.

The confirmed hit ability's Hit Validators run on the server over each hit before it deals damage; the first that refuses drops it.

Check	Refuses a hit when
Melee Trace Window	A player's machine reported it, and the attacker's montage is not playing the window it names, within Grace Seconds of its span.
Pass Through	The target holds one of the attacker's Pass Through Tags.
Plausible Hit	The target is the attacker or something it carries, a teammate its Team Filter spares, or farther than its sockets reach plus Extra Reach and the movement a round trip explains.
Rewind Hit	The target carries a UPC Rewind History and the impact point lay farther than Distance Tolerance from it where the player saw it, about a round trip earlier.

Add UPC Rewind History to each character a remote player can hit, so the server keeps where it stood. Rewind History Seconds (1 second) and Rewind Memory Budget MB (8 MB) in Project Settings, Plugins, Universal Player Combat bound the record.
Every check but Pass Through passes a hit the server swept itself; a check of your own reads Swept Here to do the same.
A hit passing every check lands only as often, and on as many targets, as its window allows. Untied Hit Spacing (0.5 s) spaces reported hits that belong to no montage window.

C++Extending the trace

UUPCCombatTraceComponent is UPC Melee Trace; its base UUPCMeleeTraceComponent is UPC Melee Trace (Base), not offered in Add Component.
DeliverHit carries a swept hit onward, raising UPC.Event.MeleeHit for the confirmed hit ability. KeepsSweepingAfterEnd keeps a window sweeping past its end while a hit stop holds its pose. OpenWindow and CloseWindow see every window.
A hit check subclasses UUPCHitValidator and overrides Validate Hit, in C++ or Blueprint.
UUPCRewindSubsystem (UPC Rewind) answers any hit source, a projectile included, against recorded targets: Sweep (Rewind Sweep) and GetClosestDistance, and the static GetViewDelay, how long ago a player saw a target. RegisterActor and RegisterComponent record without the component.
UUPCAbilitySystemReady::NotifyAbilitySystemReady(Avatar, AbilitySystem) is the Notify Ability System Ready node.

AdvancedTroubleshooting

Nothing lands

Problem	Fix
No hits at all	Match Socket IDs to a socket ID, or clear the list. Check the item is drawn.
A client player's swings deal nothing	Nothing confirms the hit there; the trace warns once. On a pawn not built on the base, send the ready signal (Character Setup).
A wall plays the flesh cue	A wall with Can be Damaged on and no UPC Damageable counts as a live target. Switch the flag off on level geometry, or tick Require Damageable Target.
Two hits from one swing	Hit Reset Interval or the config's Multi Hit Interval is above 0.
A hit lands but nothing takes damage	Implement UPC Damageable on the target, or leave Damage Application on Damageable Interface so Unreal's point damage runs.