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Collision OverviewOn this page:

Interactions

Common Collision Interaction Examples

Collision

Collision and Simulation Generates Hit Events

Overlap and Ignore

Overlap and Generate Overlap Events

Collision Responses and Trace Responses form the basis for how Unreal Engine 4 handles collision and ray casting during run time. Everyobject that can collide gets an Object Type and a series of responses that define how it interacts will all other object types. When a collision or

overlap event occurs, both (or all) objects involved can be set to affect or be affected by blocking, overlapping, or ignoring each other.

Trace Responses work basically the same way, except the trace (ray cast) itself can be defined as one of the Trace Response types, thuslyallowing Actors to either block it or ignore it based on their Trace Responses.

Interactions

There are a few rules to keep in mind with how collisions are handled:

Blocking will naturally occur between two (or more) Actors set to Block. However, Simulation Generates Hit Events needs to beenabled to execute Event Hit, which is used in Blueprints, Destructible Actors, Triggers, etc...Setting Actors to Overlap will often look like they Ignore each other, and without Generate Overlap Events, they are essentially thesame.

For two or more simulating objects to block each other, they both need to be set to block their respective object types.

For two or more simulating objects: if one is set to overlap an object, and the second object is set to block the other, the overlap will occur

but not the block.

Overlap events can be generated even if an object Blocks another, especially if traveling at high speeds.

It is not recommended for an object to have both collision and overlap events. Though possible, there is much that needs manual

handling.

If one object is set to ignore and the other is set to overlap, no overlap events will be fired.

For purposes of testing levels and looking around your worlds:

The default Play In Editor camera is a pawn. Thusly can be blocked by anything set to block pawns.The Simulate in Editor camera, before possessing anything, is not a pawn. It can freely clip through everything and will not create anycollision or overlap events.

Common Collision Interaction Examples

These interactions assume that all the objects have Collision Enabled set to Collision Enabled so they are set to fully collide witheverything. If collision is disabled, it is as if ignore has been set for all Collision Responses.

For the following section, this will be the setup used to explain what is happening:

The sphere is a PhysicsBody and the box is WorldDynamic, and by changing their collision settings we can get a number of behaviors.

Collision

By setting both of their collision settings to block each other, you get a collision, great for having objects interact with each other:

Collision

Physics Bodies

Collision

Collision Overview

Collision How To's

Collision Response Reference

Physical Materials

Physics Constraints

Destructible Properties Matrix

APEX

APEX Types

Physics Asset Tool (PhAT)

Vehicles User Guide

Vehicle Art Setup

How to Build a Double Wishbone

Suspension Vehicle

Walkable Slope

Physics Sub­Stepping

Landscape Outdoor Terrain

Matinee and Cinematics

Localization

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Sphere Collision Setup Wall Collision Setup

In this case, the sphere is a PhysicsBody and it is set to block

WorldDynamic (which is what the wall is).

The wall is a WorldDynamic and is set to block PhysicsBody

Actors (which is what the sphere is).

In this case, the sphere and the wall will simply collide; no further notifications of the collision will take place.

Collision and Simulation Generates Hit Events

Just collision is useful and in general, the bare minimum for physics interactions, but if you want something to report it has collided so aBlueprint or section of code can be triggered:

Sphere Collision Setup Wall Collision Setup

As in the example above, the sphere is a PhysicsBody and it is set to

block WorldDynamic (which is what the wall is). However, the

sphere has also enabled Simulation Generates Hit Events so it will

trigger an event for itself whenever it collides with something.

The wall is a WorldDynamic and is set to block PhysicsBody

Actors (which is what the sphere is). Since the wall is not set to

Simulation Generates Hit Events, it will not generate an event

for itself.

With the sphere set to Simulation Generates Hit Events, the sphere will tell itself that it has had a collision. It will fire off events such asReceiveHit or OnComponentHit in the sphere's blueprint. Now if the box had an event for collision, it would not fire because it will never notifyitself it has happened.

Further, an object that is reporting rigid collisions will report them all and spam reports when it is just sitting on something, so it is best to be

careful to filter what it is colliding within its blueprint or in code.

Overlap and Ignore

For all intents and purposes, Overlap and Ignore work exactly the same assuming Generate Overlap Events is disabled. In this case, thesphere is set to overlap or ignore the box:

Sphere Collision Setup Wall Collision Setup

Here the sphere is set to overlap WorldDynamic Actors (like our

wall), but it does not have Generate Overlap Events enabled. As far

as the sphere is concerned, it has not collided or overlapped

anything, effectively it has ignored the wall.

The wall is a WorldDynamic and is set to block PhysicsBody

Actors (which is what the sphere is). As stated above, both

Actors need to be set to block each other's respective object

types. If they do not, they will not collide.

Or:

Sphere Collision Setup Wall Collision Setup

Here the sphere is set to ignore WorldDynamic

Actors (like our wall), and it will pass through the

wall.

The wall is a WorldDynamic and is set to block PhysicsBody Actors (which is what

the sphere is). As stated above, both Actors need to be set to block each other's

respective object types. If they do not, they will not collide.

Overlap and Generate Overlap Events

Unlike collisions that can fire every frame, the overlap events are ReceiveBeginOverlap and ReceiveEndOverlap, which only fire in thosespecific cases.

Sphere Collision Setup Wall Collision Setup

Here the sphere is set to overlap WorldDynamic

Actors (like our wall), and it will generate an event

for itself when it does overlap something.

The wall is a WorldDynamic and is set to block PhysicsBody Actors (which is what

the sphere is). As stated above, both Actors need to be set to block each other's

respective object types. If they do not, they will not collide. But, an Overlap does

occur here, and an event for the sphere is fired.

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