Debugging Complex Traces with Visualizers

In this lesson, you will learn how to leverage built-in visualizers to inspect complex geometric traces and verify that your hit data is accurate within the viewport.

When performing complex queries like SphereSweep or BoxSweep, it is easy to assume the physics engine is "seeing" what you intend, even when the underlying collision volumes aren't aligned as expected. Instead of guessing why a trace missed a target, we use Trace Visualizers to draw the geometric shape directly in the world during runtime.

Visualizing Your Queries

The most powerful tool for this is the DrawDebugType parameter found in trace functions like SweepSingleByChannel. By default, this is set to None, but changing it to ForDuration or Persistent transforms your code into a diagnostic tool.

When you pass EDrawDebugTrace::ForDuration to your trace function, Unreal renders a wireframe of your sweep volume—the box, sphere, or capsule—at the exact moment the query executes. If the trace hits something, the visualizer will typically turn red, showing you the exact point of impact.

FHitResult Hit;
FCollisionShape MyBox = FCollisionShape::MakeBox(FVector(50.f));

bool bHit = GetWorld()->SweepSingleByChannel(
    Hit,
    StartLocation,
    EndLocation,
    FQuat::Identity,
    ECC_Visibility,
    MyBox,
    FCollisionQueryParams::DefaultQueryParam,
    FCollisionResponseParams::DefaultResponseParam
    EDrawDebugTrace::ForDuration // Visualizes the box sweep in the viewport
);

Why This Matters

Visualizers solve the "invisible offset" problem. If your character’s melee box sweep is missing an enemy, the visualizer allows you to see if the box is actually offset from your character's hand or if it is being blocked by a floor collision you didn't account for. By seeing the physical volume of the trace in the editor, you can visually confirm if your Collision Profile or Channel filters are behaving as expected. If the visualizer shows the box hitting the target but your logic isn't triggered, you know the issue is in your FHitResult handling, not the physics query itself.

Key Takeaway: Use EDrawDebugTrace within your sweep functions to render collision volumes in the viewport, allowing you to visually confirm your hit detection logic against the actual game environment.