Filtering Query Results Using Trace Channels

In this lesson, you will learn how to refine your geometric queries by implementing custom trace channels to ignore irrelevant objects and optimize your hit detection performance.

Beyond Default Collision

When you perform a line trace or a sweep, Unreal Engine checks for collisions against every object in the scene that has "Block" or "Overlap" enabled for your query channel. If you use a broad channel like Visibility or Camera for everything, your traces become "noisy"—they might hit particles, trigger volumes, or decorative physics debris that your game logic doesn't care about.

By creating a Custom Trace Channel, you carve out a specific "frequency" for your trace. When you perform a trace using this new channel, the physics engine skips any object that hasn't been explicitly configured to interact with it.

Implementing Custom Channels

You define these in Project Settings > Collision > Trace Channels. Once you create a channel (e.g., InteractionChannel), you can assign it to specific actors.

When executing a query, you pass the channel as the trace parameter. Here is a simple example of a line trace restricted to your custom channel:

FHitResult HitResult;
FCollisionObjectQueryParams ObjectParams;
// We only care about objects set to block our specific channel
FCollisionQueryParams TraceParams(SCENE_QUERY_STAT(MyTrace), true, GetOwner());

bool bHit = GetWorld()->LineTraceSingleByChannel(
    HitResult, 
    StartLocation, 
    EndLocation, 
    ECC_GameTraceChannel1, // Your custom channel
    TraceParams
);

Why This Improves Performance

Filtering at the source is significantly faster than checking hit results in logic. If you perform a trace that hits ten "unimportant" objects before reaching your intended target, the engine calculates physics data for every one of those collisions. By using a custom trace channel, you tell the physics engine to ignore those objects entirely. This reduces the CPU overhead of the query, as the engine doesn't have to resolve collision data for actors that aren't listening to that specific channel.

Key Takeaway: Custom Trace Channels allow you to prune the physics tree before a query even executes, ensuring your traces only interact with the game objects that matter.