Traces Overview

Overview of the Unreal Engine 4 tracing system.

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Trace by Channel or Object Type

Since Traces use the physics system, you'll be able to define the category of thing you want to trace against. There are two broad categories to choose from: Channels and Object Types. Channels are used for things like visibility and the camera, and almost exclusively have to do with tracing. Object Types are the physics types of Actors with collision in your scene, such as Pawns, Vehicles, Destructible Actors, and so on.

You can add more Channels and Object types as you need them. See Add a Custom Object Type to Your Project for more information on how to do this.

Returning Single or Multiple Hits

When tracing, you can choose to return the first thing that matches the critera hit by the trace, or you can return everything hit by the trace that matches the critera.

Special consideration is given to Multi Trace by Channel versus Multi Trace For Objects . When using Muli Trace by Channel the trace will return all Overlaps up to and including the first Block . Imagine shooting a bullet through some tall grass that then impacts a wall.

Multi Trace For Objects will return everything that matches an object type the trace is looking for, assuming the component is set to return trace queries. This makes it great for counting the number of objects between the start and end of the trace.

Hit Results

When a Trace hits something, it returns a Hit Result struct. Available in Blueprints (and also in C++), this is what the struct looks like:


Member		Definition
Blocking Hit			Whether or not the hit was a blocking hit. This is used when Multi Tracing by Channel , due to the ability to have traces simply overlap and not stop the trace.
Initial Overlap			Whether this is the first overlap in a series of results.
Time			This is the "Time" of impact along the trace direction ranging from [0.0 to 1.0]. If there was no hit, this will return 1.0.
Location			The world space location of the hit that is modified based on the shape of the trace.
Impact Point			The absolute location of the hit. Does not include the shape of the trace, only the point of the hit.
Normal			The direction of the trace.
Impact Normal			The normal of the hit surface.
Phys Mat			The Physical Material of the hit surface.
Hit Actor			The hit Actor .
Hit Component			The specfic Component hit.
Hit Bone Name			If traced against a Skeletal Mesh , this is the name of the bone that was hit.
Hit Item			Primitive-specific data, recording which item in the primitive was hit.
Face Index			If colliding with a trimesh or landscape, this is the index of the face that was hit.
Trace Start			The start location of the trace.
Trace End			The end location of the trace.

Using Shape Traces

When a Line Trace is not enough, you may get the results you are after by using a Shape Trace instead. For example, maybe you are creating a "vision cone" for an enemy, and you want to detect players that enter it. A Line Trace may not be enough as players may be able to avoid detection by ducking under the Line Trace.

In this situation, you could possibly use a Box Trace, Capsule Trace, or Sphere Trace.

Shape Traces function like Line Traces, where you are sweeping and checking for collision from a start point to an end point; however, Shape Traces have an added layer of checking because you are using a shape as a volume (of sorts) in your Raycast. You can use a Shape Trace as a Single Trace, or as a Multi Trace, and each are set up in the same manner as a Line Trace; however, you will need to provide additional details pertaining to the size (or orientation) of the shape you use.

Getting UV Coordinates from Trace

A trace can return the UV Coordinates of the Actor it hit, assuming trace complex is used. As of 4.14 this only works on Static Mesh Components , Procedural Mesh Components , and BSP . It will not work on Skeletal Mesh Components because you trace against the Physics Asset , which doesn't have UV coordinates (even if you choose to trace complex).

Using this feature will increase CPU memory usage, because UE4 needs to keep an additional copy of vertex positions and UV coordinates in main memory.

Enabling UV Coordinates from Trace

To enable this feature, follow these steps:

Access your Project Settings from the Edit Menu .
Enable the Support UV From Hit Results feature in the Physics Section of your Project Settings .
Restart the Editor.

You can inspect the Blueprint Find Collision UV node using this feature prior to restarting the editor; however, the node will only return [0.0, 0.0] when you inspect it. If you want the node to return the correct UV data, you'll have to restart the editor.

Other Features

Tracing also has a number of minor features to limit what they return, making them easier to debug. They have the ability to trace against Complex Collision (if a Static Mesh or Procedural Mesh has it enabled). If they are called from an Actor , they can be told to ignore all attached components by enabling the Actor to trace through itself. Finally, they have the option to show a representation of the trace as red and green lines (with larger boxes representing hits).