Managing Async Traces for High-Performance Environments

Implement asynchronous trace requests to offload heavy geometric calculations and maintain stable frame rates during intensive gameplay scenarios.

In this lesson, you will learn how to offload resource-intensive collision queries to worker threads, preventing frame drops during high-density combat or complex environmental interactions.

The Problem with Synchronous Traces

When you perform a standard LineTraceSingleByChannel or BoxSweep, the game thread stops and waits for the physics engine to calculate the result before proceeding to the next line of code. If you perform a single trace, this is negligible. However, if you are running dozens of traces per frame—such as a complex NPC crowd sensing system or a high-frequency projectile-based combat system—these synchronous calls quickly accumulate, forcing the CPU to stall and causing your frame rate to hitch.

The Asynchronous Solution

Unreal Engine provides an asynchronous alternative: AsyncLineTraceByChannel. Instead of blocking the game thread, this function fires a request into the physics world's task queue. The engine processes this trace in the background, and once finished, it fires a callback (or a LatentAction) to deliver the result.

This approach keeps your game running smoothly because the "heavy lifting" happens outside the main execution loop. Here is how you might structure an async trace request:

FCollisionQueryParams QueryParams(SCENE_QUERY_STAT_NAME(AsyncTrace), false, GetOwner());

// Request the trace asynchronously
World->AsyncLineTraceByChannel(
    EAsyncTraceType::Single,
    StartLocation,
    EndLocation,
    ECC_Visibility,
    QueryParams,
    FCollisionResponseParams::DefaultResponseParam,
    new FTraceDelegate(FTraceDelegate::CreateUObject(this, &AMyActor::OnTraceCompleted))
);

Managing Execution

The FTraceDelegate is crucial because it decouples the request from the response. By handling the hit result inside OnTraceCompleted, you ensure that your main logic loop remains lightweight. You aren't just "firing and forgetting"; you are scheduling work to be handled when the physics engine is ready.

In high-performance environments, this is the difference between a stuttering experience and a fluid, professional-grade simulation. You are effectively parallelizing your physics tasks, maximizing CPU utilization without sacrificing input responsiveness.

Key Takeaway: Use asynchronous traces to move intensive geometric calculations off the main game thread, ensuring that complex physics queries do not interrupt your frame rate.