Collision Responses

Collision Responses and Trace Responses form the basis for how Unreal Engine 4 handles collision and ray casting during run time. Every object 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, thusly allowing Actors to either block it or ignore it based on their Trace Responses.

Properties

In the properties list, there are a large number of physics related key words. The two that are not really clear are Simple and Complex Collision. In short, Simple Collision shapes are your boxes, KDOP, spheres, capsules, and convex shapes, all of which can be generated in the Static Mesh editor. Complex Collision is per-poly, and it is never used for an Actor that is simulating physics (it will just fall through the world).

Property	Description
Simulation Generates Hit Events	This flag indicates if Event Hit and On Component Hit should be called for physics collisions involving this PrimitiveComponent. In more simple terms, it lets this object "know" it has actually had a physics collision. However, it is just a notification, and a dynamic object set to collide with the static environment will still do so regardless of this setting; it just will not fire those events.
Generate Overlap Events	If you want an object to generate overlap events, like Event Actor Begin Overlap or Event Actor End Overlap, then this flag needs to be true. Further, for it to be of any use, the object in question will need to be able to overlap another.
Collision Presets	These are a collection of presets for the Collision Responses below. There is an INI file you can edit to change or add profiles. To edit anything below this point, you need to set this to "Custom..."
Collision Enabled	The 3 settings in here are as follows:
	No Collision - No collision is performed against this object, neither trace nor physics.
	Query Only (No Physics Collision) - This body is only used for raycasts, sweeps, and overlaps.
	Physics Only (No Query Collision) - This body is used only for physics collision.
	Collision Enabled (Query and Physics) - This body is used for physics simulations and collision queries.
Object Type	This defines what the object should be considered when it moves. It has more to do with the interactions you see below than it does with defining an object as static or dynamic.
Collision Responses	This sets all the collision responses below to the value checked. So if you check "Block" here, all of the rows below will also check "Block."
Trace Responses	These define how an object should behave when interacting with a trace (usually just done with a ray cast). An object can choose to Block, Overlap, or simply Ignore a trace from the given source. There is no difference in the default trace responses except for their names. In code or Blueprint, you can use either for tracing a ray cast from one thing to another. But, for organization of traces, they should be used as their name implies. The default responses for Trace Responses are:
	Visibility - Trace from one location to another, if an object that blocks this type of trace is in the way, the trace is blocked.
	Camera - Exactly the same as Visibility, but should be used for setting when tracing from the camera.
Object Responses	An object can choose to Block, Overlap, or simply Ignore an object it is interacting with. The default responses for Object Responses are:
	WorldStatic - Volumes and world geometry should be WorldStatic.
	WorldDynamic - Moving Actors outside of Pawns, PhysicsBodies, Vehicles, and Destructible Actors. Like an elevator.
	Pawn - Characters.
	PhysicsBody - Any physics object that is or can be simulated in the world.
	Vehicle - A good response to have so you can have Pawns jump into them.
	Destructible - Destructible Actors.
Use CCD	Whether or not to use Continuous Collision Detection for this component. Increases accuracy of collision detection.
Always Create Physics State	Indicates if you would like to create a physics state (its collision properties, mass, if it is sleeping, etc... all go into a physics state) all the time (for collision and simulation). If set to false, the state will still be generated if collision or simulation are activated on this Actor, say through a Blueprint or code, but there will be some overhead as it has to work the new object into the full physics simulation. Setting this to true can improve performance by removing the overhead for calculating the physics state of the object once it actually needs one (on collision or simulation).
Multi Body Overlap	If true, this component will generate individual overlaps for each overlapping physics body if it is a multi-body component. Think about skeletal physics assets, with all their individual collision shapes. With this enabled, a character's hand would generate its own overlap event. Thus giving more control over what is reported and what to do in that specific case.
Check Async Scene on Move	If this is flagged as true, the component will look for collisions in both of the physics scenes (synchronous and asynchronous). The asynchronous scene is primarily used by the broken pieces of destructible Actors.
Trace Complex on Move	If this is set to true, component sweeps with this component will trace against complex collision during movement. Complex Collision is, in short, per face collision, where as Simple Collision are your spheres, capsules, boxes, and generated convex shapes.
Return Material on Move	Setting this to true will return the Physics Material in the Hit Info.
Can Ever Affect Navigation	Setting this to true can potentially influence navigation.

Some components will have additional or fewer options in the collision category, and many times these very specific options will only be shown in the appropriate editor for that component.

Property	Description
Physics Body in Physics Asset
Collision Response	Whether or not a specific physics body in a Physics Asset will collide at all.

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 be enabled 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 the same.

• 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.

• The Object Responses listed are just the defaults and have no actual bearing on what an object is. See below for more information on setting up custom Object Types and Trace Channels.

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 any collision or overlap events.

Common Collision Interactions

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:

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 a Blueprint 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 as ReceiveHit or OnComponentHit in the sphere's blueprint. Now if the box had an event for collision, it would not fire because it will never notify itself 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, the sphere 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 BeginOverlap and EndOverlap, which only fire in those specific 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 is fired.

Custom Object Channels and Trace Channels

There will be times where 6 Object Response Channels and 2 Trace Response Channels simply are not granular enough for what you want to create. This is what the Collision Editor in your Project Settings is for. You can access them from the Edit Menu -> Project Settings -> Collision:

From here you can add new Object Response Channels and Trace Response Channels. Click the New Object Channel... or New Trace Channel... button, provide a name, select a Default Response, and click Accept.

You can have up to 18 Custom Object Response Channels or Custom Trace Response Channels.

Presets

Custom Presets can also be set here by expanding the Preset Category and clicking the New... button.

From here you can name your preset, enable or disable collision, select your preset's Object Type, and finally define the behavior of each Response Channel for the selected Object Type.

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